Radiotherapy Treatment Planning: Objectives, Formulations and Clinical Implications

Transcription

1 Radiotherapy Treatment Planning: Objectives, Formulations and Clinical Implications Michael J Zelefsky M.D Memorial Sloan-Kettering Cancer Center New York, N.Y

2 New Challenges for RT Treatment Planning New treatment delivery systems such as IMRT have compelled physicians and physicists to more carefully consider the dose distribution over the normal tissues. 3D-CRT and IMRT have facilitated dose escalation strategies, placing greater demands on treatment planning.

3 Conventional Forward Planning and 3D-CRT Define PTV dose, treatment beams, and specify parameters of each beam Dose Calculation PTV Organ at risk PTV Assess dose distribution and adjust beam parameters until satisfactory plan is derived

4 Specify beam directions and dose distribution Inverse Planning PTV Computes intensitymodulated beams Organ at risk PTV

5 Intensity Modulated Radiation Therapy Inverse treatment planning Organ at risk PTV Computer-aided optimization derives desired treatment plan Organ at risk PTV Delivery of intensity modulated beams a n a..+..n..+..z z

6 Comparison of 3D-CRT and IMRT Prostate Cancer Treatment Plans 3D-CRT IMRT Bladder Prostate PTV Rectum Rectum Prostate PTV Bladder

7 Incidence of Grade 2-3* Rectal Toxicity in Prostate Cancer Patients Treated to 81 Gy by 3D-CRT and IMRT Percent Grade 2-3 Rectal Toxicity p = Gy IMRT (189) 81 Gy 3D-CRT (61) Grade /772 (98%) Grade 2 11/772 (1.5%) Grade 3 4/772 (0.5%) Months *One case of grade 3 rectal bleeding in each treatment group

8 Normal Tissue Considerations Dose-volume constraints for each normal organ need to be established to minimize treatment-related toxicities. These constraints ultimately need to be incorporated into the treatment plan and balanced with target coverage parameters.

9 Correlation of Mean DVH With Rectal Bleeding at >30 Months After 3D-CRT 100 Rectal Volume (%) Gy No Rectal Bleeding (82) Rectal Bleeding (36) p= Prescription Dose (%)

10 Incorporating Rectal Constraints in the Treatment Plan 100 Volume (%) % 47 Gy Dose (cgy) PTV Rectal Wall Bladder Wall

11 Target Dose Constraints Homogeneity factors considered Penalties assigned for overdosage (lower penalty) and underdosage (greater penalty) Dose painting reported by the UCSF group to selectively intensify dose to regions of the target Inhomogeneity may be more preferable!

12 MSKCC OBJECTIVE FUNCTION Beamlets target w l (D-P l ) 2 w (D-P ) 2 u u organ at risk w c (D-D c ) 2 Greater penalty applied for target underdosage P l P u D c

13 Dose-Volume Constraint Templates

14 Prostate Cancer IMRT Planning at MSKCC A coplanar, non-collinear, 5-field arrangement is used Dose constraints and penalties for 81 Gy plan: PTV minus rectum overlap: Prescription dose = 100% Minimum dose = 98%, penalty = 50 Maximum dose = 102%, penalty = 50 PTV plus rectum overlap: Prescription dose = 95% Minimum dose = 93%, penalty = 10 Maximum dose = 96%, penalty = 20 Rectal wall: Maximum dose = 95%, penalty = 20 70% of rectal volume receives < 40% maximum dose, penalty = 20 Bladder wall: Maximum dose = 98%, penalty = 35 70% of bladder volume receives < 40% maximum dose, penalty = 20

15 Nasopharynx Cancer - Comparison of Conventional and IMRT Treatment Plans Conventional IMRT e - e - Laterals to 70 Gy Posterior e - strips Prescription: Gross disease: 70 Gy Micro. disease: 54 Gy Constraints: Min PTV isodose - 100% Max PTV isodose - 120% Max cord dose - 40 Gy Max brainstem dose - 45 Gy Prescription: Gross disease: 70 Gy Micro. disease: 54 Gy BID after 36 Gy < 50 Gy Gy > 70Gy

16 Nasopharynx IMRT Plan Goals versus Constraints Structure Max/Pen. Min/Pen. Vol. Max. Dose Vol. PTV 105%/50 95%/ % (84 Gy) D 95 > 95% Cord 40%/ % (40 Gy) Brain Stem 50%/ % (45 Gy) Cochlea 45% 20 77% (54 Gy) Parotid 70%/ Parotid 23% 20 30% Mean Dose 37% (26 Gy)

17 Strategy for Determining Optimization Parameters Set Constraints Optimize Both PTV & NT acceptable NT dose Too high PTV Too high Both PTV & NT unacceptable No significant Change Change NT Constraint NT Constraint Or Penalty Change NT penalty STOP

18 Five Fields, Parameter Set 1 % Volume Bladder V 47Gy : 53% PTV V 95 : 90% V 100 : 65% Dose (% of Prescription) Bladder Wall Optimization Parameters: Maximum Dose: 95%, Penalty: 50 Dose Volume: 30% Vol. to 34% of Rx., Penalty: 20

19 Five Fields, Parameter Set 2 % Volume Bladder: V 47Gy : 61% PTV V 95 : 96% V 100 : 83% Dose (% of Prescription) Bladder Wall Optimization Parameters: Maximum Dose: 95%, Penalty: 50 Dose Volume: 30% Vol. to 55% of Rx., Penalty: 20

20 Seven Fields, Parameter Set 3 % Volume Bladder V 47Gy : 53% PTV V 95 : 93% V 100 : 81% Dose (% of Prescription) Bladder Wall Optimization Parameters: Maximum Dose: 95%, Penalty: 50 Dose Volume: 30% Vol. to 38% of Rx., Penalty: 20

21 Optimization of Treatment Plans Current approach balances dose constraints and limitations applied to the normal tissues and target coverage Penalties applied for plans where constraints are exceeded Not routine for rewards to be applied for discriminating and selecting plans that achieve lower doses to normal tissues.

22 Optimization Parameters and Target-Normal Tissue Proximity PTV % Volume Gy Lt. Cochlea Dose (Gy) PTV GR Cochlea GTV

23 Optimization Parameters and Target-Normal Tissue Proximity % Volume Gy Dose (Gy) PTV Lt. Cochlea PTV GR Cochlea PTV EL

24 Optimization Parameters and Target-Normal Tissue Proximity Gy % Volume PTV Lt. Cochlea Dose (Gy) PTV GR Cochlea PTV EL

25 Intraoperative Conformal Planning for Prostate Brachytherapy at MSKCC using a Genetic Algorithm Software captures and stack the axial ultrasound images and the position of the needles. Prostate and normal organs are reconstructed in 3-dimensions. Genetic algorithm determines the optimal seed coordinates to satisfy dose-volumes constraints for urethra, rectum and target.

26 Genetic Algorithm-I Optimization code with operating mechanism that relies on natural selection. Member of the population represents a specific seed-loading pattern Each seed represents a chromosome Algorithm evaluates each seed arrangement according to an objective function using dose constraints and weighting factors for normal tissues and target

27 Genetic Algorithm-II Members of the population evaluated to how the seed-loading pattern meet the criteria of the objective function. Best fit individual preferentially selected to serve as parents for next generation. Iterative process continues until best fit solution found after 6000 generations

28 Intraoperative Conformal Planning for Seed Implants at MSKCC Inverse planning system which incorporates a genetic algorithm for optimization Target Constraints Minimum dose of 144 Gy Urethra <125% of prescription dose Rectum < 100% of prescription dose

29 Target Coverage According to Technique 100 % CT Pre-Plan Ultrasound Manual Intraoperative 3D P < %V100 %V90 D90 (Zelefsky et al IJROBP -2000)

30 Urethral Dose According to Technique % of prescription CT Pre-Plan Ultrasound Manual Intraoperative 3D P < ave urethral dose max urethral dose (Zelefsky et al IJROBP -2000)

31 Conclusions New treatment delivery systems have placed increased challenges for radiotherapy treatment planning Optimization strategies currently rely on dose constraints and penalties for exceeding pre-determined dose limitations. In the future, new paradigms such as biologic-based variables will need to incorporated into such strategies.

32 Conclusions Because of variations in the proximity of normal tissues to target, the same constraints for each patient will not consistently identify the best plans. While these dose-volume constraints are not exact and will not lead to the optimal solutions, new optimization strategies which select the most feasible solution will likely impact upon improving conformality and treatment outcome.