Automated Analysis of Varian Log Files for Advanced Radiotherapy Treatment Verification: A Multicenter Study

Transcription

1 Automated Analysis of Varian Log Files for Advanced Radiotherapy Treatment Verification: A Multicenter Study by Jeremy Lee Hughes B.Sc. (Hons), University of Western Australia, 2013 This thesis is presented in partial fulfilment of the requirements for the degree of MASTER OF MEDICAL PHYSICS of The University of Western Australia Jeremy Lee Hughes, 2015 School of Physics (Medical Physics) University of Western Australia

2 Abstract Varian linear accelerators store information of their deliveries in log files which can be extracted from the machine and analysed. These files contain a wealth of information, including the actual and expected position of each leaf throughout the treatment. The analysis of Varian log files has been a growing field of interest ever since the turn of the millennia. The high resolution log files allow deep analysis into treatment plans delivered by Varian treatment machines. This thesis undertook an international multicenter study that sought to find trends in the positional error of the MLC when compared to factors such as; the velocity of the leaves, the gantry angle, the age of the machine, the treatment site, the treatment modality, and the treatment machine. A program was devised in MATLAB to perform statistical analysis on the Varian log files. Additionally another MATLAB program was written to clinically assess these Varian log files. By analysing the Varian log files, this thesis found a positive trend between the error of the MLC to the age of the machine. VMAT treatments had greater error than their dimrt and Step and Shoot counterparts. The prostate treatment site had less error than the H&N and PPN treatment sites. And Varian Truebeam machines possessed less error, by a full order of magnitude, than Varian Clinac ix or Varian Trilogy machines. There was a trend towards greater error for leaves with increased velocity. In general there was greater error for leaves at positions more vulnerable to the force of gravity. This trend was not present for Sliding Gap tests delivered at discrete gantry angles. Varian log files were used to assess thousands of deliveries performed by Varian linear accelerators. The information available in each file allowed trends to be drawn between the positional error of the MLC and the age, model, modality, treatment site, and leaf velocity. I

3 Contents Abstract I Acknowledgements VI List of Figures VIII List of Tables XIV Abbreviations and Acronyms XVII 1 INTRODUCTION What is Cancer? Cancer Treatments Surgery Chemotherapy Radiation Therapy Linear Accelerator Basic Overview Treatment Methods Intensity Modulated Radiation Therapy Volumetric Modulated Arc Therapy Introduction to Quality Assurance Quality Assurance of the Multileaf Collimator Varian log files Aim of thesis BACKGROUND AND LITERATURE REVIEW Treatment Machine Multileaf Collimator Gantry Quality Assurance Tests Ling s Test Ling s Test Ling s Test II

4 2.2.4 Sliding Gap Quality Assurance Gamma Map Analysis Literature Review Varian Machine Models Verification of Varian log files Varian log files for Quality Assurance Limitations of Varian log files Effect of Gravity upon deliveries Overshoot Varian log files for analysis MATERIALS AND METHOD Varian Log Files Dynamic Log Files Truebeam trajectory log files Multileaf Collimator model Matlab Software Importing log files Computing Positional Errors Calculating Leaf Velocity Computing Velocity Errors Computation of Fluence Maps Gamma Map Analysis Linear Accelerators Leaf Error Root Mean Squared Deviation Picket Fence (Ling s Test 1) Step and Shoot and dimrt VMAT Comparing Treatment Methods - Clinac ix Positional Errors due to Velocity Inner/Outer Leaves III

5 3.7 Velocity Leaf Error VMAT Sliding Gap Treatment Modalities Gamma Map Analysis Gravity Picket Fence (Ling s Test 1) Sliding Gap VMAT Age Statisical Analysis RESULTS Development of software for automatic Varian log file analysis Confirmation of MATLAB software Leaf Error Picket Fence (Ling s Test 1) Step and Shoot dimrt VMAT Comparing Treatment Methods - Clinac ix Positional Errors due to Velocity Inner/Outer Leaves Velocity Leaf Error VMAT Sliding Gap Treatment Modalities Gamma Map Analysis Gravity Picket Fence (Ling s Test 1) Sliding Gap VMAT IV

6 4.8 Age Errors and Treatment Machine Model Errors and Treatment Site Error Distribution of Prostate Treatments Errors and Different Treatment Modalities Errors and Leaf Velocity Errors and Inner and Outer Leaves Gamma Map Analysis Errors and Gantry Angles Errors and Treatment Machine Age CONCLUSION Future Work Appendix 107 V

7 Acknowledgements As I sit down to write my Acknowledgements a bittersweet feeling washes over me. On one hand I am happy to be finally done with this thesis that was the cause of many tea fuelled sleepless nights but on the other, a part of me is sad to see it go. But I suppose that s Stockholm Syndrome for you. Of course this thesis would never have been completed if it was just me tapping away at the keyboard. I had a lot of help in order to complete this project. A lot of help. I would like to thank all my supervisors, Dr Pejman Rowshanfarzad, Prof Martin Ebert, and Prof Mike House, for all the hard work they put into this project so that it was an otherwise rewarding and enjoyable experience. I would like to especially thank Dr Pejman Rowshanfarzad for all the advice he had to give and all the insights he gave. Thank you for working tirelessly away and giving me direction whenever my biannual freak out, What am I even doing? sessions came and went. You were a lot more help than you realise. I would also like to thank the wonderful people over in Belfast, Dr Conor McGarry and Christina Agnew, for their massive windfall of treatment files without which this thesis would be severely diminished. My thanks also goes out to Dr Mahsheed Sabet and Nikki Caswell (Chief Medical Physicist) for their immense help in supplying me with treatment files from all around the country without which my thesis would be non existent. Also a thank you to Dr Alison Scott, Dr Sivakumar Somangili, Anthony Walsh and Garry Grogan for all your help and input. The clinical point of view was very much welcomed and appreciated. VI

8 And it would be remiss of me not to mention the wonderful people that made up the research group I was a part of for two years. It was a privilege to belong to such an incredible bunch of people who always inspired me to work harder and get smarter. I not only thank my family but offer my apologises for the long hours spent tapping away at this infernal machine (especially near the end). Fear not though, after this thesis is submitted I shall return to my duties with only the solace of long term unemployment to look forward to. Penultimate thanks goes to Blake Segler who printed out the final copy of this thesis while I was in Japan and to Mike and Pejman who sorted out the rest. And finally, my thanks to the WA Department of Health whose funding allowed me to undertake this Master s degree, without which I would not know what I would be doing. And as this sentence culminates two years worth of work, to you dear reader, it is just the beginning. So enjoy as much as humanely possible otherwise, what s the point? VII

9 List of Figures 1.1 Healthy cells undergo apoptosis and die off whereas malignant cells bypass apoptosis and grow in number. Figure courtesy from the National Cancer Institute [1] A megavoltage linear accelerator. Image courtesy of NELCO Worldwide A basic schematic of a linear accelerator. Taken from Rowshanfarzad [5] A Varian MLC plus carriage Dose distribution for IMRT (a,b) and VMAT (c,d) for prostate radiotherapy. Figure courtesy of Department of Medical Physics, Royal Surrey County Hospital, UK A head on schematic of adjacent MLC leaves from Siemens, Elekta, and Varian showing the tongue and grooves of each leaf. The protrusions minimise interleaf leakage. Image modified from [29] Varian s M80 MLC. The leaves move independently to collimate the beam into different shapes for treatment. Image courtesy of Varian Basic schematic of M120 MLC. Individual leaves extend and retract from opposing banks A picture of two different leaf motors, one for the inner leaves and one for the outer leaves of the M120 MLC. Image taken from Institute of Radio Oncology, KFJ Hospital Vienna [34] The encoder quadrature channels on each leaf motor. Image taken from Varian [33] Radiation penetration: (a) Penetration dependence upon leaf position is minimised. (b) Penetration through square leaves is dependent on leaf positioning which is why Varian MLC leaves are curved The gantry rotates in the horizontal axis and the collimator rotates in the vertical axis. The point in space where the radiation beams intersect when the Gantry is rotated is called the radiation isocenter Dose distribution for Picket Fence QA recorded by an EPID VIII

10 3.1 Different Position Readout Scale Conventions. Modified from High Energy C-Series Clinac, Installation Product Acceptance Booklet Schematic of a typical 120 leaf MLC showing the relative positions of the leaf banks and upper and lower jaws. Leaf 54 of Bank B and leaf 8 of Bank A are both extended across the centreline A basic workflow schematic of the importation of Varian log files The Expected and Actual positions from Varian log files of a leaf during a step and shoot treatment. When the beam is off the expected position jumps to the next segment whereas the actual position slowly changes A basic workflow schematic of how the Fluence Maps are created Basic structure of how the program works The velocity of the leaves with respect to time for a Varying Sliding Gap QA computed by a) Varian log file analysis, and b) EPID analysis. The white circles highlight the discrepancy of leaf number The RMSD of 32, 25, and 1805 Picket Fence deliveries were computed for Trilogy, Clinac ix, and Truebeam machines respectively. The box and whisker plot represents the data as follows: The top line (or whisker) is the maximum value, the next line is the 3rd Quartile, the red line is the median, the next line is the 1st Quartile, and the last line is the minimum value. The red crosses are outliers Error histogram of 1643 Step and Shoot prostate treatments delivered on machine ix 6. In total points were assessed. A double gaussian distribution was fitted with R 2 = Errors of 7 different dimrt prostate treatments from Clinac ix machines. A double gaussian distribution was fitted with R 2 = IX

11 4.5 RMSD of VMAT treatments at treatment sites Head and Neck (H&N), Prostate Pelvic Node (PPN), and Prostate (PROS), for treatment machines a) True 1 and b) True 2. A total of 76 H&N, 182 PPN, and 203 PROS treatments were analysed for True 1 and 444 H&N, 570 PPN, and 193 PROS treatments were analysed for True 2. The box and whisker plot represents the data as follows: The top line (or whisker) is the maximum value, the next line is the 3rd Quartile, the red line is the median, the next line is the 1st Quartile, and the last line is the minimum value. The red crosses are outliers The RMSD of VMAT Head and Neck treatments for treatment machines Clinac ix (32 files) and Truebeam (520 files) RMSD of different treatment modalities for Clinac ix Prostate Step and Shoot, 7 Prostate dimrt, and 32 Head and Neck VMAT treatments were analysed. The box and whisker plot represents the data as follows: The top line (or whisker) is the maximum value, the next line is the 3rd Quartile, the red line is the median, the next line is the 1st Quartile, and the last line is the minimum value. The red crosses are outliers Absolute errors from three different Sliding Gap QA. The Sliding Gap QA was performed 3 times for 30mm/s Sliding Gap QA and 4 times for 10mm/s, and 20mm/s Sliding Gap QA. The errors of each delivery were: a) uncorrected, b) corrected for the 55+ms delay present in older Varian systems. The box and whisker plot represents the data as follows: The top line (or whisker) is the maximum value, the next line is the 3rd Quartile, the red line is the median, the next line is the 1st Quartile, and the last line is the minimum value. The red crosses are outliers RMSD of inner and outer leaves for Bank A and Bank B of: a) Clinac ix and Trilogy machines, and b) Truebeam machines. 13, 19, 19, 6, 732, 704, and 369 picket fence deliveries were analysed for ix 1, ix 2, Tri 1, Tri 2, True 1, True 2, and True 3 respectively X

12 4.10 The Velocity RMSD for VMAT treatments delivered at different treatment sites on True 1 (left) and True 2 (right). A total of 76 H&N, 182 PPN, and 203 PROS treatments were analysed for True 1 and 444 H&N, 570 PPN, and 193 PROS treatments were analysed for True 2. The box and whisker plot represents the data as follows: The top line (or whisker) is the maximum value, the next line is the 3rd Quartile, the red line is the median, the next line is the 1st Quartile, and the last line is the minimum value. The red crosses are outliers The error of velocity in different Sliding Gap QA deliveries on ix 3. Four 10mm/s, four 20mm/s, and three 30mm/s sliding gap deliveries were analysed. The box and whisker plot represents the data as follows: The top line (or whisker) is the maximum value, the next line is the 3rd Quartile, the red line is the median, the next line is the 1st Quartile, and the last line is the minimum value. The red crosses are outliers Velocity RMSD of different treatment modalities for Clinac ix. 7 dimrt (PROS) and 32 VMAT (H&N) treatments were analysed. The box and whisker plot represents the data as follows: The top line (or whisker) is the maximum value, the next line is the 3rd Quartile, the red line is the median, the next line is the 1st Quartile, and the last line is the minimum value. The red crosses are outliers Gamma map analysis analysing the expected and actual fluence of a H&N treatment from ix 6 using 3%/3mm. Colour intensity details how similar the two fluence maps are. The formula for calculating these values was presented in section Polar plot of the error (mm) in the radial component that has been averaged over 2 degree control points, vs the gantry angle (IEC convention) for a) ix 1 (13 tests) b) Tri 1 (14 tests) and c) True 1 (732 tests). This was further split into the error present on i) Bank A, ii) Bank B, and iii) the leaf gap width XI

13 4.15 The error (mm) for Sliding Gap QA delivered at four different gantry angles on ix 3 for a) Bank A, and b) Bank B. Leaves moved with velocity 10mm/s. The box and whisker plot represents the data as follows: The top line (or whisker) is the maximum value, the next line is the 3rd Quartile, the red line is the median, the next line is the 1st Quartile, and the last line is the minimum value. The red crosses are outliers The error (mm) for Sliding Gap QA delivered at four different gantry angles on ix 3 for a) Bank A, and b) Bank B. Leaves moved with velocity of 20mm/s. The box and whisker plot represents the data as follows: The top line (or whisker) is the maximum value, the next line is the 3rd Quartile, the red line is the median, the next line is the 1st Quartile, and the last line is the minimum value. The red crosses are outliers A histogram of the collimator angles (IEC convention, degrees) of 1451 VMAT treaments delivered on Truebeam machines Polar plot of the error (mm), averaged over 2 degree control points, in the radial component vs the gantry angle (IEC convention) for treatments with collimator angle (IEC convention) i) 30, ii) 330, and iii) all collimator angles. These were also split into a) Bank A and b) Bank B. There were 540, 398, and 1451 treatments for collimator angles 30, 330, and all collimator angles respectively RMS error for of routine Picket Fence deliveries vs date delivered for: a) True 1, b) True 2, c) True 3, and d) all of the above. Results were averaged on a monthly basis. A linear trend was fitted to each of the machines with R-value: a) R = , b) R = , and c) R = Error histogram of 1643 Step and Shoot prostate treatments delivered on machine ix 6. In total points were assessed.a double Gaussian was also fitted Errors of 7 different dimrt prostate treatments from Clinac ix machines. A double Gaussian was also fitted Errors of 7 different dimrt prostate treatments from Clinac ix machines split into Bank A and Bank B errors XII

14 4.23 The position of the MLC at gantry angles 90 and 270 for Ling s Test 1. The collimator angle is at Polar plot of the error (mm), averaged over 2 degree control points, in the radial component vs the gantry angle (IEC convention) for a) Bank A, b) Bank B, and c) the error in the leaf gap width. 6 tests were analysed Polar plot of the error (mm), averaged over 2 degree control points, in the radial component vs the gantry angle (IEC convention) for a) Bank A, b) Bank B, and c) the error in the leaf gap width. 19 tests were analysed Polar plot of the error (mm), averaged over 2 degree control points, in the radial component vs the gantry angle (IEC convention) for a) Bank A, b) Bank B, and c) the error in the leaf gap width. 704 Picket Fence tests of True 2 were analysed Polar plot of the error (mm), averaged over 2 degree control points, in the radial component vs the gantry angle (IEC convention) for a) Bank A, b) Bank B, and c) the error in the leaf gap width. 369 Picket Fence tests of True 3 were analysed XIII

15 List of Tables 1.1 Verification and delivery parameters for a VMAT treatment planned on Pinnacle SmartArc Module. Higher Γ values indicate a more accurately delivered treatment. Table taken from Rowbottom et al [11] Structure of the header in a DynaLog file. Modified from DynaLog File Viewer Reference Guide Structure of the contents in a DynaLog file. Modified from DynaLog File Viewer Reference Guide Structure of the header in a Truebeam trajectory log file. Modified from Truebeam Trajectory Log File Specification [89] Structure of the subbeam in a Truebeam trajectory log file. Modified from Truebeam Trajectory Log File Specification [89] Summary of linear accelerators used in this study Comparison between Argus QA and Timberwolf RMSD of Picket Fence deliveries. Results are presented as the mean of the RMSD and standard deviation RMSD of VMAT deliveries at different treatment sites for True 1 and True 2. Results are presented as the mean of the RMSD and standard deviation RMSD of H&N VMAT deliveries for Truebeam and Clinac ix machines. Results are presented as the mean of the RMSD and standard deviation The RMSD of Sliding Gap QA moving at different velocity presented as mean and standard deviation. The corrected deliveries were corrected for the 55ms+ delay present in older Varian systems a) RMSD and b)std of inner and outer leaves of Bank A and Bank B for different machines. 13, 19, 19, 6, 732, 704, and 369 picket fence deliveries were analysed for ix 1, ix 2, Tri 1, Tri 2, True 1, True 2, and True 3 respectively XIV

16 4.7 Velocity RMSD of VMAT deliveries at different treatment sites for True 1 and True 2. Results are presented as the mean of the Velocity RMSD and standard deviation The Velocity RMSD of Sliding Gap QA moving at different velocity presented as mean and standard deviation Gamma passing rates calculated through Varian log file analysis for Head and Neck (H&N), Prostate Pelvic Node (PPN), and Prostate (PROS) treatments. Results are presented as mean and STD Gamma passing rates calculated through Varian Log file analysis for 20 LT1, 44 LT2, and 17 LT3 QA tests. All tests were delivered on Clinac ix machines. Results are presented as mean and STD P-values of Student T-tests between the averaged Control Point error located in 40 degree arcs centered on gantry angles 0, 90, 180, and 270. Colour coded such that the value is red if it is <0.05. Values of 0 represent P-values < Picket Fence tests from ix 1 were analysed and data was only taken into account if the leaves were stationary P-values of Student T-tests between the averaged Control Point error located in 40 degree arcs centered on gantry angles 0, 90, 180, and 270. Colour coded such that the value is red if it is <0.05. Values of 0 represent P-values < Picket Fence tests from Tri 1 were analysed and data was only taken into account if the leaves were stationary P-values of Student T-tests between the averaged Control Point error located in 40 degree arcs centered on gantry angles 0, 90, 180, and 270. Colour coded such that the value is red if it is <0.05. Values of 0 represent P-values < Picket Fence tests from True 1 were analysed and data was only taken into account if the leaves were stationary The error for Sliding Gap QA (10mm/s) delivered at four different gantry angles on ix 3 on Bank A and Bank B. Results are displayed as mean and STD XV

17 4.15 The error for Sliding Gap QA (20mm/s) delivered at four different gantry angles on ix 3 on Bank A and Bank B. Results are displayed as mean and STD P-values of Student T-tests between the error of Sliding Gap treatments (20mm/s) delivered at gantry angles 0, 90, 180, and 270. Colour coded such that the value is red if it is <0.05. Values of 0 represent P-values < Comparison of treatment modalities by two different groups XVI

18 Abbreviations and Acronyms AAPM American Association of Physicists in Medicine ACMP American College of Medical Physics CBCT Cone-beam computed tomography D Dose Tolerance DC Direct current DICOM Digital Imaging and Communications in Medicine dimrt Dynamic Intensity Modulated Radiation Therapy DTA Distance to agreement DVH Dose volume histogram DynaLog Dynamic Log EBRT External Beam Radiation Therapy EMF Electromotive force EPID Electronic Portal Imaging Device GUI Graphical User Interface HD MLC High Definition MLC H&N Head and Neck IAEA International Atomic Energy Agency ICRP International Commission on Radiological Protection ICRU International Commission on Radiation Units and Measurements IEC International Electrotechnical Commission IMRT Intensity Modulated Radiation Therapy Linac Linear Accelerator LT1 Ling s Test 1 LT2 Ling s Test 2 XVII

19 LT3 Ling s Test 3 MLC Multileaf Collimator M52 MLC 52-leaf Millennium MLC M80 MLC 80-leaf Millennium MLC M120 MLC 120-leaf Millennium MLC MU Monitor unit NCRP National Council on Radiation Protection and Measurements OAR Organs at risk PCB Printed Circuit Board PPN Prostate Pelvic Node PROS Prostate PTV Planning treatment volume PWM Pulse Width Modulation QA Quality Assurance RMS root mean squared RMSD root mean squared deviation RMSE root mean squared error TLD Thermoluminescent Dosimeter TPS Treatment planning system VMAT Volumetric Modulated Arc Therapy XVIII

20 CHAPTER 1 INTRODUCTION 1.1 What is Cancer? Cancer is a serious disease of the body s cells. Cells normally divide and grow in a regulated manner. Healthy cells have preprogrammed cell death or apoptosis ingrained into their genetic code so that there is never an over-abundance of cells. Instead of an overcrowded environment a percentage of cells die off before they can subdivide further ensuring an environment suitable for cell growth. However the genetic code of a cell can be altered and damaged whether simply by random mutations or from external factors, one of which is radiation. If these mutations survive the multiple checks put in place by the body to stop such a thing occurring then their mutations grow and spread to their daughter cells. Eventually the mutation gets to a point of uncontrolled cell growth, this is known as cancer. The cells now no longer perform their original task and instead focus on dividing and growing. Additionally they mutate so that they no longer undergo preprogrammed cell death meaning they effectively live for however long the body can sustain them. The main difference between healthy and cancerous cells is represented graphically in figure 1.1. The cancer cells grow and multiply such that the healthy cells have restricted access to the blood supply causing them to die off. During the later stages of cancer, the primary tumour spreads microscopic cancer cells to the rest of the body, this is known as metastasis. They travel mainly through the lymphatic or circulatory system to infect other areas. The spread and growth of the tumours eventually leads to the death of the patient. Cancer is a leading cause of death in Australia accounting for approximately 3 in 10 1

21 Figure 1.1: Healthy cells undergo apoptosis and die off whereas malignant cells bypass apoptosis and grow in number. Figure courtesy from the National Cancer Institute [1]. deaths [2]. It is a serious disease and there are multiple treatments available that try to cure the patient of cancer. These fall under three main categories; surgery, chemotherapy, and radiation therapy. These treatments are often performed in conjunction with each other for maximum effect. 1.2 Cancer Treatments Surgery Surgery is an effective measure against cancer in which the tumour is simply excised from the patient. This method is most effective during the early stages of cancer growth when the tumour is still localised to a small area. During the later stages of cancer development the tumour grows and metastasises meaning the cancer spreads throughout the body via microscopic cancer cells. Only one microscopic cancer cell would have to survive and regrow for the treatment to be considered a failure [3] which is why surgery is normally confined to the early stages of tumour development. That being said surgery is still used in conjunction with other treatment options in the later stages of cancer growth. 2

22 1.2.2 Chemotherapy Chemotherapy uses drugs to combat cancer. Traditional chemotherapy drugs are chosen so that they target and kill rapidly dividing cells, which is one of the main properties of cancer cells. Unfortunately this means that healthy cells that divide at a rapid rate are also targeted and killed. These include cells in the digestive tract, hair follicles, and bone marrow and as such the patient may experience a whole raft of side effects including nausea and vomiting, loss of hair, and myelosuppression. Fortunately these healthy cells repair themselves at a greater rate than the cancer cells so although the patient will experience negative side effects to the treatment the majority are only temporary. Chemotherapy is often used in conjunction with radiation therapy [4] Radiation Therapy Radiation Therapy, or Radiotherapy, is an effective yet potentially hazardous treatment for cancer. Radiotherapy works on the basis of the ionisation and destruction of malignant cells by bombarding the tumour with ionising radiation. Ideally all malignant cells will be eradicated from the patient during treatment. However, as radiation does not discriminate between healthy and malignant cells, healthy cells will also be affected and harmed. Targeting the cancer cells whilst sparing the healthy cells then becomes a top priority and this is achieved in many different ways that differ depending upon if the radiation source is internal or external to the body. Internal radiation therapy, also known as brachytherapy, is the placement of sealed radioactive sources near or in the cancerous tumour to deliver high doses to the tumour with a rapid dose falloff to the surrounding healthy tissue. Whilst effective, internal radiation therapy is an invasive treatment. External beam radiation therapy (EBRT) is a subset of radiation therapy that delivers high energy radiation to a targeted area from an external source, usually a linear accelerator. The patient lies on a treatment couch and the linear accelerator delivers the treatment as ascribed by a Radiation Therapist. The aim is to destroy the malignant cells whilst doing as little damage as possible to the surrounding healthy tissue. This is achieved by the careful planning of the treatment as well as ensuring 3

23 Figure 1.2: A megavoltage linear accelerator. Image courtesy of NELCO Worldwide. that this treatment is accurately delivered to the patient. The linear accelerator must perform within certain specifications to provide a clinically safe treatment. 1.3 Linear Accelerator Linear accelerators, or more colloquially linacs, are an integral component of EBRT due to their ability to produce and deliver X-rays or high energy electrons. Linacs work by accelerating charged particles, such as electrons, to high energies along a linear tube. This electron beam can be used to treat superficial tumours or alternatively be used to hit a metal target thus producing bremsstrahlung radiation and characteristic X-rays. The radiation is then delivered to the patient in such a way to maximise the damage to the tumour, but minimise the impact on the healthy cells. Although there are variations to the linear accelerator s design, those used in EBRT are fundamentally similar. For the purposes of this thesis we will only be focusing on linear accelerators that produce radiation Basic Overview Figure 1.3 showcases a basic schematic diagram of a linear accelerator. The DC power supply is injected into the pulsed modulator which converts the continuous input into 4

24 Figure 1.3: A basic schematic of a linear accelerator. Taken from Rowshanfarzad [5]. pulse form. The high voltage pulsed power is then injected into the RF power source, either a Klystron or Magnetron, which causes the production of pulsed electromagnetic waves that are then injected into the accelerating waveguide. The pulsed power from the modulator is also injected into the electron gun resulting in a pulsed stream of electrons also entering the accelerating waveguide. The electrons enter the accelerating waveguide with an initial energy of approximately 50keV [6] where they interact with the pulsed electromagnetic waves and emerge from the accelerating waveguide with a vastly increased energy, on the order of a mega electron volt. The electron beam then travels through the treatment head where it strikes a metal target to produce bremsstrahlung radiation and characteristic X-rays, with energy on the order of mega voltage. The radiationis then collimated by the primary collimator and in Varian (Varian Medical Systems, Palo Alto, CA) linacs the radiation is further collimated through slabs of tungsten known as Jaws and more recently a component known as a Multileaf Collimator (MLC). The radiation is then used to treat the patient. It is important to note that the treatment head resides upon a gantry which can rotate around the patient throughout the duration of the treatment. The added degree of freedom of the gantry rotation allows more conformal and technical treatment options. The MLC is affixed to the treatment head to further collimate the radiation beam during treatment. The idea behind the MLC is relatively simple. Affixed to two opposing parallel carriages, also known as bank A and bank B, are rows of tungsten 5

25 Figure 1.4: A Varian MLC plus carriage. leaves that can move one dimensionally towards or away from the opposite carriage. These leaves can move independently from each other and as such a multitude of shapes may be formed to collimate the radiation. An image of the MLC and subsequent carriages can be found in figure 1.4. The MLC itself can also rotate about the axis of the radiation beam allowing for a greater degree of freedom. This is known as the collimator rotation. The angle of the collimator, or MLC, generally remains constant throughout delivery. More detail pertaining to the physical aspects of the MLC can be found in section Treatment Methods Intensity Modulated Radiation Therapy In traditional EBRT the intensity profile of the radiation beam normal to the beam axis is relatively uniform. Wedges or compensators are sometimes used to modulate the field such that more radiation is delivered to the planned treatment volume (PTV) and such that the organs at risk (OARs) are spared the brunt of the radiation which reduces the acute and late toxicity [7]. This is known as intensity modulated radiation therapy (IMRT). The invention of the MLC made IMRT easier to perform due to the reduction of the wedges and compensators that were needed to be manufactured. Instead shapes can simply be made with the MLC that emulate the wedges and compensators. Modern 6

26 linacs generally do not use physiacl wedges which removes the problem of manufacture however MLCs are still an easier, more practical implementation. The most basic form of IMRT is known as the Step and Shoot method. The Treatment Planning System (TPS) optimises the treatment through the design of optimal field apertures, appropriate beam directions, the number of fields, beam weights, and MLC leaf positions. The linac rotates to the first gantry angle (normally there are around 7 different fields in total), moves the MLC leaves into position, and delivers radiation until the dose for that segment has been reached. The beam is then held and the linac rotates to the second gantry angle whilst moving the MLC leaves into their new position. The beam then stops being held and the appropriate dose is delivered. This process repeats until the treatment is completed. The discrete radiation beams of the treatment is further split into subfields that can have different intensity from each other (hence intensity modulated radiation therapy). This aids the sparing of OARs and increases the conformity to the targeted area [7]. A slightly more complex version of IMRT is known as dynamic IMRT (dimrt). It is similar to the Step and Shoot method in many ways however when the beam is on and delivering radiation, the MLC leaves are not constrained to a static location. During delivery the MLC leaves move unidirectionally and are optimised to minimise treatment time [6] Volumetric Modulated Arc Therapy Volumetric modulated arc therapy (VMAT) is an even more complex form of treatment. Instead of limiting the treatment to a discrete number of gantry angles the beam continuously delivers radiation as the gantry rotates around the patient. The MLC leaves are also moving bidirectionally to continuously shape the radiation. The continuous nature of this treatment not only increases the conformity to the PTV but it also reduces patient treatment time [8]. The treatment varies the gantry rotation speed, dose rate, and the collimation to deliver a highly conformal dose [7]. A comparison between the dose distribution as a result of VMAT treatments and IMRT treatments can be seen in figure 1.5. This method allows for greater control in treatment planning and dose distribution 7

27 Figure 1.5: Dose distribution for IMRT (a,b) and VMAT (c,d) for prostate radiotherapy. Figure courtesy of Department of Medical Physics, Royal Surrey County Hospital, UK. but is computationally more expensive to produce than the other treatment types. Additionally due to its continuous delivery the dose gradient at the edge of the field is not as sharp as treatments with discrete gantry rotations [9]. Treatments that deliver from a discrete number of gantry angles are split into segments however as there are a continuous number of gantry angles that are used for delivery in VMAT a separate method to divide the treatment into smaller components for treatment planning is introduced. The VMAT treatment is divided into subarcs that are known as control points which outline the MLC shape, MLC segment dose, and a gantry-angle window across which each shape sweeps dynamically [10]. VMAT treatments typically consist of 177 control points with each control point roughly spanning a subarc of 2 degrees. Of course the number of control points in the total treatment can be changed. If the total number of control points increases then the angle that the subarcs span decrease and you can hypothetically create a better treatment plan due to the higher number of control points allowing for greater control over the evolution of MLC shape. However this has limitations in the physical velocity of the leaves as well as the downside of increased computational hours and treatment time. A table showing the different delivery times and their accuracy, in the form of gamma map analysis can be found in table 1.1 for Pinnacle TPS. As the number of control points increases the accuracy of the treatment increases but so does the delivery time of the treatment. For 8

28 Table 1.1: Verification and delivery parameters for a VMAT treatment planned on Pinnacle SmartArc Module. Higher Γ values indicate a more accurately delivered treatment. Table taken from Rowbottom et al [11]. this system a compromise of 121 control points would be optimal [11]. As a consequence of the continuous treatment beam delivery, the treatment delivery time in VMAT treatments is used much more efficiently than Step and Shoot techniques. Therefore the treatment delivery time for VMAT treatments are much shorter when compared to Step and Shoot methods [12]. A shorter treatment delivery time is beneficial to both the patient and Radiation Therapist as an increased treatment delivery time can impact on the patient s comfort on the treatment couch, the reproducibility of the treatment position and if the delivery uses discrete gantry angles then there may be some intrafraction motion [7]. Also with a reduced treatment delivery time there can be a higher throughput of patients. Abbas et al. recorded VMAT treatment times 55% less than their IMRT counterpart (median of 120s versus median of 254s [12]). Additionally Varian records a VMAT treatment time of 1.5min compared to the equivalent IMRT treatment time of 5.5min but as they rely on hospitals upgrading to their new systems these numbers should be viewed sceptically. [13]. In addition to the reduced treatment delivery time, VMAT treatments also have a reduction in monitor unit (MU) usage when compared to IMRT techniques[7]. MUs are used to measure the amount of radiation delivered and hence VMAT treatments reduces the potential for secondary malignancies arising due to the treatment. There is still concern that the greater amount of healthy tissue being irradiated may offset this reduction but more research still has to be done on the matter. VMAT is a relatively new treatment technique that has greater conformity and greater sparing of the OARs when compared to basic conformal radiation therapy tech- 9

29 niques [7]. There is still debate on the whether this is also true when VMAT is compared to IMRT treatments. Many different studies have come to the conclusion that for the most part the conformity of the dose distribution is largely the same, as is the sparing of the OARs [7]. However nearly all studies have reached the same conclusion that VMAT treatments provide highly conformal treatments with a reduced treatment delivery time and reduced MUs when compared to IMRT [7], [14] [22]. 1.5 Introduction to Quality Assurance Quality Assurance (QA) is a set of policies and procedures implemented to objectively monitor the quality and appropriateness of patient care. This is achieved by ensuring the mechanical and dosimetric characteristics of a linac lie within an acceptable range of a baseline value which in turn ensures that the patient treatments are delivered within specified spatial and dosimetric tolerances. Without QA patients may receive harmful or irrelevant treatments through either complacency or neglect. It is for this reason that many professional organisations have proposed their own QA guidelines for centres to follow. These organisations include the International Atomic Energy Agency (IAEA), the American Association of Physicists in Medicine (AAPM), and the American College of Medical Physics (ACMP). These guidelines in turn utilise data published by the International Commission on Radiological Protection (ICRP), the International Commission on Radiation Units and Measurements (ICRU), the International Electrotechnical Commission (IEC), and the National Council on Radiation Protection and Measurements (NCRP). Although these guidelines exist it is ultimately the choice of the individual centres as to what QA program they wish to implement, as long as international or national standards are fulfilled. A QA program encompasses many aspects ranging from how the equipment is performing to pre and post treatment evaluation. For the sake of relevancy this thesis will discuss QA focusing on the quality assurance of the equipment as well as treatment evaluations. Guidelines published by AAPM [23] suggest different checks are to be performed on linear accelerators annually, monthly, or daily depending on how often the part that 10

30 is being assessed is prone to failure, its clinical impact, and how long it will take to perform the test. This periodic QA is mainly in place to assure the staff that their measurements are consistent with the machine s performance from a mechanical and dosimetric standpoint. It ensures that the linear accelerator is delivering radiation and moving as per the staff s instructions, within certain tolerance percentages. It is for this reason that periodic QA is also known as linac QA. Some examples of these tests include checking the functionality of the multileaf collimator, the collimator size indicator, and the electron output constancy. Whilst periodic QA is implemented to keep the machine s performance in check, a different form of QA, patient-specific QA is implemented to verify that the treatment being delivered to the patient is the treatment that was planned by the physicists. If the machine is not physically capable of delivering the prescribed treatment, or the machine is incorrectly calibrated, patient-specific QA is designed to pick up on these inconsistencies. Patient-specific QA normally takes the form of a dry run, that is the treatment is delivered where the patient is replaced by a dosimeter and the resulting dose distribution is compared against the desired outcome. This comparison is usually performed by gamma map analysis, first suggested by Low et al. [24] in Gamma map analysis measures how well two different dose distributions agree with each other. To function effectively as a radiation dosimeter the dosimeter in question must possess a physical property that is dependent upon the measured dose quantity. They then can be calibrated to give results that are useful in a clinical setting. Additionally the properties of accuracy and precision, linearity, dose or dose rate independence, a flat energy response (although in reality corrections have to be made), directional dependence and high spatial resolution are desirable for a radiation dosimeter [9]. 1.6 Quality Assurance of the Multileaf Collimator The MLC is a complicated instrument that is responsible for correctly collimating the radiation beam. Therefore it is important that it performs to the best of its abilities and does not fall out of calibration otherwise a hazardous delivery could be made. Therefore the alignment of the MLC leaves is an area of concern as misalignment of the leaves results in treatments being delivered incorrectly. Different quality assurance 11

31 tests are performed to ensure this does not occur. One such test is performed by a quality assurance tool built into the MLCs that are produced by the company Varian. This exists as a collimated beam of light that is directed across the paths of the leaf ends. Each leaf is moved individually up until they block the beam of light and their default position is reset. This ensures the leaf positions are calibrated to the same position every time this process is run, which is every time the machine is turned on [25]. There exists independent methods to verify the leaves positions. These take the form of dosimetric quality assurance. Two such examples are Ling s Test 1, also known as the Picket Fence test, and Sliding Gap QA. The leaves are moved in a predetermined fashion while radiation is being continuously delivered to a detector. When viewing the resulting dose map from film, EPID, or ion chamber array, the movement of the MLC leaves make it obvious when a leaf is misaligned. If a leaf is out of alignment then the dose will not be uniform with the surrounding leaves. Corrections can then be made. 1.7 Varian log files The analysis of the quality assurance tests of the MLC have traditionally been performed dosimetrically however there exists an alternative method. Linear accelerators produced by Varian log information at regular intervals during treatment. These are exported to dynamic log files (DynaLog files) or Truebeam trajectory log files depending on the machine model. Each log file contains useful information such as the normalised MUs delivered, whether the radiation beam was delivering or not, the angle of the gantry, and most importantly, the expected and actual location of each individual leaf of the MLC, for both bank A and bank B at each time step. By reading these logs into appropriate software the errors of each individual leaf can easily be obtained. By assessing the Varian log files of QA tests instead of assessing the QA tests dosimetrically, the user introduces the variable of time. Although analysing the QA tests dosimetrically can tell you whether a leaf does fail, it cannot tell you when this occurs as well as what else the machine is doing at the time. There exists multiple commercial treatment software that analyse Varian log files. 12