The Netherlands Cancer Institute Antoni Van Leeuwenhoek Hospital Amsterdam

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Agnella Franceschi
6 anni fa
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1 Progetto Leonardo The Netherlands Cancer Institute Antoni Van Leeuwenhoek Hospital Amsterdam 1 giugno 31 agosto 2004

2 Activity IMRT treatment planning: I ll tray to define a class solution for IMRT of prostate tumours. Simultaneous integrated boost (SIB) technique to optimize standard conformal plans. Establishing a procedure for routine quality assurance of IMRT treatments: Experimental dosimetric measurements; Methods to evaluate the agreement between measurement and calculation and assess the acceptability of a treatment plan.

3 IMRT treatment planning Treatment planning system: Pinnacle Linac: Elekta 18MV, 80 leafs MLC IMRT technique: Step and shoot

4 Prostate Intensity Modulated Radiotherapy The purpose of this planning study is to define a class solution for IMRT of prostate tumours: the number of beams to use the cost function The goal is to obtain good target coverage (68 Gy to the PTV and 78 Gy to the boost volume) and rectal sparing.

5 Volumes used PTV 68: the target volume that has to be irradiated to 68 Gy Annulus = PTV68 - PTV78 PTV 78: the target volume that has to be irradiated to 78 Gy Shell = ring around the Annulus, obtained by expanding the annulus with 2 cm External Shell = body all other volumes Femural head Rectal wall

6 Pinnacle Inverse Planning modulo

7 Inverse Planning Modulo (IP) set up Optimization Max number of iterations: 25 Stopping tolerance (minimal difference of the cost function between two successive interactions): 1 e -05 Segment extraction Number of intensity levels: 5 Min segment dimension: 3 cm 2 Min number of MUs: 3

8 I tested different cost functions using physical objectives. Volume Type cgy Volume % Weight PTV78 Max dose PTV78 Min dose PTV78 Uniformity Annulus Min dose Shell Max DVH rwall Max dose rwall Max DVH rwall Max DVH rwall Max DVH Left femur Max DVH Right femur Max DVH The rectum cost function is defined using as reference data from Fiorino et al., Huang et al. and Boersma et al. (Int. J. Radiat Oncol Biol Phys 2004; 59; ) Rectal Bleeding Gr 2 V % RB Gr 2 V % RB Gr 2 V % RB = Gr 3 V70 30 % RB = Gr 3 V75 5%

9 Optimization results Pz1 Cf1 Pz2 Cf2 Pz3 Cf3 Pz4 Cf3 I tried to apply the same cost function (Cf) to 4 different patients with different prostaterectum configurations. Proceeding in this way does not allow to obtain an optimal plan for each patient. To obtain a good plan I had to adjust some parameters of the cost function. PTV 78 Dmed Gy n -20 stdev(%) gamma 1.7 EUD D50 70 TCP % Volume receiving 95% % PTV 68 Dmed n -20 stdev(%) gamma 1.7 EUD D % Volume receiving 95% % Rectal wall EUD RB Gr2 NTCP n 0.23 V m 0.19 V D50 80 V V Rectal wall EUD RB Gr3 NTCP n 0.06 V m 0.06 V D50 80 V V Rectum overlap % The same cost function was used for patients 3 and 4 (patients 3 and 4 are standard patients, while patients 1 and 2 have a very extreme rectum-prostate configuration). TCP for the target volume is calculated according to a logit model using D50 = 70 Gy, gamma = 1.7 and n = -20. (Cheung, Int. J. Radiat. Oncol. Biol. Phys. 2003; 56; ) NTCP for rectum is calculated according the Lyman-Kutcher-Burman model (LKB) using n=0.23 and m=0.19 to determine NTCP for RB Gr 2 and n=0.06 and m=0.06 for RB= Gr 3. (Rancati 2004) I calculated the rectum overlap as the portion of the rectal wall that overlaps PTV 68.

10 Patient n.1 Rectal wall Annulus PTV Gy 64,6 Gy 60 Gy 50 Gy

11 Number of beams I optimized plans with different number of beams using the same cost function: 7 beams (Dogan et al. Assessment of different IMRT boost delivery methods on target coverage and normal-tissue sparing, Int J Radiat Oncol Biol Phys 2003, 57, ) gantry angles: 0, 51, 102, 153, 205, 257, 309 degrees 6 beams (Verellen et al. Considerations on treatment efficiency of different conformal radiation therapy techniques for prostate cancer, Radiother. and Oncol , 27-36) gantry angles: 35, 80, 130, 230, 280, 325 degrees 5 beams (NKI) gantry angles: 36, 100, 180, 260, 324 degrees

12 I didn t find a substantial rectal sparing improvement or a better PTV coverage using techniques with 6 or 7 beams. PZ. 3 5 NKI 6 beams 7 beams PTV 78 Dmed Gy n -20 stdev(%) gamma 1.7 EUD D50 70 TCP % Volume receiving 95% % PTV 68 Dmed n -20 stdev(%) gamma 1.7 EUD D % Volume receiving 95% % Rectal wall EUD RB Gr2 NTCP n 0.23 V m 0.19 V D50 80 V V Thick: 5 beams Thin: 6 beams Thin dashed: 7 beams PTV78 Annulus Rectal Wall

13 IMRT pre-treatment verification The aim of this work is to define a routine procedure to evaluate the agreement between measurement and calculation and assess the acceptability of a IMRT treatment plan

14 Verification procedure The IMRT plan is imported onto a cubic phantom and the dose distribution in the phantom is calculated. The dose distribution in the phantom is calculated also for each single beam. The dose calculated is extracted from the phantom Film measurement: complet plan and single beam (coronal plane) Comparison of the dose distributions of the plan and the exposed films. (MatLab-based software)

15 Activity: Software debug Film dosimetric calibration (grey level dose) Test with simple distribution (square beam) Scanner spatial calibration verification Plan verification

16 Film calibration Livelli di girgio - DO DO Kodak EDR-2: linear dose response region extending to 500 cgy Livelli di grigio scanner DO - Dose DO dose cgy

17 Scanner 12-bit Lumiscan Spatial Lineariti lutscanner 0.5 correction [mm] position [mm]

18 Plan Verification Film Plan Plan Film Gamma Gamma Dose % 1 gamma

19 Gamma evaluation

20 Gamma evaluation Γ 1 Γ = 2 D 2 + M D r r 2 2 M Γ 2 Γ 3 γ = min( Γ i ) Low et al. (1998)

21 Gamma evaluation γ 1, accepted γ > 1, not accepted Low et al. (1998)

22 Gamma evaluation Above 20 % of the prescribed dose D M = 3 % local dose difference d M = 3 mm shift Below 20 % of the prescribed dose D M = 30 % local dose difference

23 Simultaneous Integrated Boost The purpose of the second part of this work was to verify the possibility to use the simultaneous integrated boost (SIB) technique to optimize standard conformal plans. 1. For each patient I calculated a conformal plan using a boost technique: one plan for the large-field (68 Gy) and one for the boost (10 Gy). I used a technique with 6 beams, gantry angles: 45, 90, 135, 225, 270, 315 degrees. (In order to define the beam shape 7 mm are added as margin to the PTV) 2. Then I tried to reduce the rectum dose using simple SIB techniques: Using the Inverse Planning (IP) option optimize beam weight (conf W) Using the IP option optimize beams weight without penumbra margin for the boost fields (conf W Nm) Using the IP option DMPO (this IP module would optimize also the beam shape) (conf DMPO)

24 Patient 3 PTV78 Annulus Rectal wall Conf: medium lines, Conf W: Thin dashed lines, Conf W Nm: Thin lines, Conf DMPO: Medium dashed lines

25 Patient 3 I have obtained similar results using the IP option optimize beams weight and reducing the penumbra margin of the boost fields (Conf W Nm) and using the IP option DMPO (Conf DMPO). conf Conf W Conf W Nm Conf DMPO PTV 78 Dmed Gy n -20 stdev(%) gamma 1.7 EUD D50 70 TCP % Volume receiving 95% % PTV 68 Dmed n -20 stdev(%) gamma 1.7 EUD D % Volume receiving 95% % Rectal wall EUD RB Gr2 NTCP n 0.23 V m 0.19 V D50 80 V V Rectal wall EUD RB Gr3 NTCP n 0.06 V m 0.06 V D50 80 V V These settings allow to reduce high doses to the rectum (V75), compared to the conformal plan, so the probability of grade 3 rectal bleeding decreases.

26 conformazionale Fascio modulato in intensità IMRT

27 IMRT tecnique

28 Modulazione del fascio La tecnica Step and Shoot si ottiene erogando, in sequenza, campi multipli statici con il fascio radiante OFF nelle fasi di movimentazione del MLC.

29 Pianificazione inversa RT e 3D-CRT: pianificazione di tipo diretto (modifico ripetutamente il set-up del piano fino a trovare la soluzione che meglio si conforma al tumore) IMRT: numero di gradi di libertà troppo elevato (e possibile variare la fluenza di ogni bixel) è impossibile un approccio diretto. PIANIFICAZIONE INVERSA Si utilizzano tecniche matematiche che permettono di passare dalla distribuzione di dose voluta alla fluenza dei fasci radianti. E necessario esprimere in formato matematico gli obbiettivi clinici.

30 Funzione obbiettivo (tipo fisico) Sono espresse in funzione di quantità ben misurabili quali DOSE e VOLUME. Si definiscono vincoli di dose e volume per PTV e OAR. Possono essere HARD o SOFT

31 Invers planning Il modulo IP ricerca la soluzione ottimale al problema inverso preimpostato (attraverso la definizione delle funzioni obbiettivo). Procedimento: Determinazione per ogni voxel del volume di trattamento dei coefficienti di dose assorbita per unità di fluenza del singolo fascio. Utilizzando metodo interattivo il modulo IP modifica la fluenza di ogni fascio e verifica il valore assunto dalle funzioni obbiettivo fino a minimizzare la differenza fra DVH desiderato e DVH calcolato. Il sequencer converte la fluenza teorica in fluenza reale (tenendo conto delle caratteristiche meccaniche e dosimetriche dell MLC) Utilizzando la fluenza reale il modulo IP calcola la distribuzione di dose su tutto il volume CT

32 Sequencer FUNZIONE: convertire fluenza teorica in fluenzareale, definendo la sequenza di movimenti che deve essere effettuata dal MLC.

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