Dosimetry Requirements for IMRT

Transcription

1 Dosimetry Requirements for IMRT Peter Williams North Western Medical Physics Foundation Trust Manchester Trust

2 An IMRT Plan Trust

3 The IMRT Patient pathway Imaging Plan Verification In-vivo Dosimetry Planning Delivery Trust

4 The IMRT Patient pathway Imaging Plan Verification In-vivo Dosimetry Planning Delivery Commissioning Trust

5 The IMRT Patient pathway Imaging Plan Verification In-vivo Dosimetry Planning Delivery Depends on dosimetry at most stages Commissioning Trust

6 Measurements requirements Point Measurements Dose Distributions Single field Complete treatment Trust

7 QUASIMODO: QUality ASsurance in Intensity MODulated RadiOtherapy. ESTRO Team: B. Perrin, J. Bohsung, S. Gillis, R. Arrans, A. Bakai, C. De Wagter, T. Knöös, B. Mijnheer, M. Paiusco, J. Welleweerd, P. Williams. EUROPEAN COMMISSION Directorate General Health and Consumer Protection - Europe Against Cancer Programme ESQUIRE Project: Education, Science and QUality assurance In Radiotherapy in Europe Grant Agreement (2001CVG2-005) Trust

8 Aims of the QUASIMODO project To develop Quality Assurance methodologies for IMRT To develop test procedures for QA of TPS s To develop test procedures for IMRT planning and delivery systems. Trust

9 Aims of the QUASIMODO project To develop test procedures for QA of TPS s Trust

10 CarPet phantom Z 75 EDR2 films Y X 20cm (cut) x 12 inch film 16 cm Cranial Caudal Sagittal view 37 cm Transversal cut plane Ionisation chamber measurements Clamping rod Ion chamber/dummy disk Film prick pin / dummy disk Trust

11 CarPet: Dosimetic Verification Trust

12 CarPet phantom Trust

13 CarPet dosimetry (1) Ion chamber measurements in OAR and PTV (traceable to national standard) Transverse EDR2 0, +2, +4cm Film irradiated for calibration curve on site Films processed and read out to analysis centre. RTOG dosegrids & DVHs sent to analysis centre Trust

14 CarPet dosimetry: cdah - PTV No of treatment verification Mean plan PTV dose (cgy) Mean verification Deviation [%] a* b* * Trust

15 CarPet dosimetry: cdah - OAR No of treatment verification Mean plan OAR dose (cgy) Mean verification Deviation [%] a* b* * Trust

16 CarPet dosimetry (3) Perform Gamma evaluation (in 3D) Established 4%/3mm tolerance For each pixel if γ 1 it is OK Gamma map with isodose display for each film Calculate the 95percentile for OAR and PTV for each film Trust

17 CarPet dosimetry: γ analysis (1) Trust

18 CarPet dosimetry: γ analysis (2) PTV - 95th percentile ID No. Z = 40 mm Z = 20 mm Z = 0 mm Z = -20 mm Z = -40 mm a* b* * Trust

19 CarPet dosimetry: γ analysis (3) OAR - 95th percentile ID No. Z = 40 mm Z = 20 mm Z = 0 mm Z = -20 mm Z = -40 mm a* b* * Trust

20 CarPet dosimetry: Summary DAH is a useful approach. 7/9 plans had mean PTV doses within +2.5% of prescribed Larger variations in mean OAR dose γ-map and 95 percentile Identify significant areas of deviation Allows investigation of local points of concern Trust

21 IMRT Specific Challenges Intensity modulation by definition requires measurements in non uniform fields Spatial and dosimetric accuracy Relative Absolute Reference point might be in high gradient Influence of delivery method Trust

22 Dosimetric Considerations for Uniform Fields Tolerance (2% or 2mm) Accurate Dose Accurate Position Low enough not to matter Trust

23 Dosimetric Considerations for Modulated Fields Trust

24 Gamma metric dose Reference dose Measured dose acceptance criteria: D max (e.g. 3 %) DTA (e.g. 3mm) D max DTA γ < 1 => γ > 1 => position Trust D. A. Low et al. Med. Phys. 25, (1998)

25 Dosimetric Considerations for Modulated Fields Intensity Modulation implies: Small Fields Dimensions Monitor Units Trust

26 Dosimetric Considerations for Modulated Fields Intensity Modulation implies: S&S Dyn Small Fields Dimensions Monitor Units Trust

27 Dosimetric Considerations for Modulated Fields Intensity Modulation implies: S&S Dyn Small Fields Dimensions Monitor Units Leaf speed accuracy Trust

28 Output Factor Variation with Field Size. 1,2 1,1 1 0,9 0,8 0,7 0,6 0,5 OF (15MV open field) SSD=95 cm OF (15MV) field size Issues: Collimator calibration Partial volume effects Lateral CPE Perturbations Trust

29 Small fields Detectors Small dimensions Tissue equivalent Low perturbation factors Diamond detectors Pin point Ionisation Chambers (dose rate dependence) (sensitivity/leakage) Trust

30 Leakage, Transmission and related effects. Significant dose from leakage within PTV Higher dose from leakage to OARs Depends on efficiency of delivery Requires accurate beam mode within TPS Trust

31 Leakage, Transmission and related effects. Origin of dose outside beam aperture: Transmission through collimators Typically 1% Leakage between collimators Typically 3% Scatter from Collimators and Phantom Variable with distance from beam edge Trust

32 Leakage, Transmission and related effects. Dose rate Spectrum Spatial variation Low Uncertain High Trust

33 The Low Monitor Unit Problem Is the dose delivery linear for small beams? Dose rate Variable? Beam energy Variable? Measurement Uncertain? Trust

34 The Low Monitor Unit Problem Is the dose delivery linear for small beams? Dose rate Variable? Beam energy Variable? Measurement Uncertain? Trust

35 Mapping dose distributions 2D Photographic film Gafchromic film CR phosphor plates Diode and ion chamber arrays EPIDs 3D Gels Trust

36 In vivo dosimetry Measurement of entrance and exit dose on the patient, at the Linac Reconstruction of actual tumor dose in reference point Trust

37 Entrance and exit diode in portal image Trust

38 In-vivo Dosimetry Diode Results IMRT field Patient Patient & Beam Dose Measured expected Trust

39 Conclusions Dosimetry for IMRT Requirements are more demanding than for conventional treatments Minimum requirements can be met with careful application of existing systems Uncertainties are higher than for convention treatments There are some interesting problems to be solved Trust