ESTRO 2022 - Abstract Book

S702

Abstract book

ESTRO 2022

tissue (tumor)match and bone match). The IF motion was investigated for dependence on: 1) tumor localization (lobe) 2) tumor size (GTV) (group 1: volume 0-5 cm 3 , 2: volume 5-10 cm 3 , 3: volume 10-15 cm 3 , 4: ≥ 15 cm 3 ) 3) single fraction dose (which serves as surrogate for the time between scan B and C).

Results IF motion patient

The average IF motion amounted to 0.0 ± 1.1 mm, 0.0 ± 1.3 mm and -0.2 ± 1.3 mm respectively for left-right (LR), craniocaudal (CC) and anteroposterior (AP) direction (vector 1.6 ± 1.5 mm). For short fractions (5 Gy/fr), the IF motion was slightly less than for longer fractions (18 Gy/fr) (1.4 ± 1.6 mm (vector) and 1.8 ± 1.5 mm for 3 fractions. The translations and rotations of the IF motion did not show systematic effects. IF motion tumor The average IF motion amounted to 0.0 ± 1.3 mm, 0.2 ± 1.6 mm and -0.5 ± 1.7 mm respectively for LR, CC and AP direction (vector 2.1 ± 1.7 mm). Note the systematic 0.5 mm motion in the posterior direction. The average IF motion vector amounted to 2.0 ± 1.4, 1.8 ± 1.1 mm and 2.3 ± 2.1 mm respectively for upper, middle and lower lobe. For 5Gy/fr, the IF motion was slightly less than for 18Gy/fr, 1.4 ± 1.6 mm vs 1.8 ± 1.5 mm (vector). The IF motion did not depend on tumor size. Tumormatch post CBCT The soft-tissue (tumor)match of scan C revealed that in only 7% of the fractions the tumor had partially moved outside of the PTV margin. A systematic average motion of the tumor in cranial (0.51 mm) and dorsal (1.3 mm) direction was observed. Conclusion The applied PTV margin of 5 mm is appropriate. The IF tumor motion shows systematic components in cranial and dorsal directions. The IF motion slightly depended on the lobe the tumor is located in, as well as on the applied dose- scheme/treatment duration. The differences are too small and the variation between patients is too large for clinical consequences. J. Dang 1 , W. Li 2,3 , I. Navarro 4,5 , J.D. Winter 6,3 , A. Berlin 7,5 , P. Chung 7,3 , R. Glicksman 4,3 , J. Helou 4,5 , V. Malkov 8,5 , J. Padayachee 5,7 , S. Raman 5,7 , V. Kong 1,5 1 Princess Margaret Cancer Centre, Radiation Medicine Program, Toronto, Canada; 2 Princess Margaret Cancer Centre, Radiation Medicine Program, Toronto , Canada; 3 University of Toronto, Radiation Oncology, Toronto , Canada; 4 Princess Margaret Cancer Centre, Radiation Oncology, Toronto , Canada; 5 University of Toronto, Radiation Oncology, Toronto, Canada; 6 Princess Margaret Cancer Centre, Medical Physics, Toronto , Canada; 7 Princess Margaret Cancer Centre, Radiation Oncology, Toronto, Canada; 8 Princess Margaret Cancer Centre, Medical Physics, Toronto, Canada Purpose or Objective The feasibility of using an MR-linac to fully adapt to anatomical changes observed prior to radiation delivery has been established. There is a need to assess the value of the additional resources invested for re-contouring and re- optimization. This study compared the estimated dose delivered using full adaptation versus three degrees of freedom (3DOF) correction for MR-guided prostate stereotactic body radiotherapy (SBRT). Materials and Methods Twenty-five patients treated with focal HDR-brachytherapy and MR-guided SBRT with full adaptation were included. A reference plan was generated to deliver 3000cGy/5 to the prostate with a 5mm expansion for the planning target volume. Images acquired for localization and during radiation delivery from each of the 5 fractions were rigidly registered to the reference image for every patient using a soft tissue prostate match. Prostate (CTV), solid bladder and rectum were contoured on all images for either generation of an adapted plan or estimation of delivered dose. To compare the delivered dose between 3DOF and full adaptation, dose clouds of the reference plan and the 5 adapted plans were overlaid onto the corresponding images acquired during delivery for each patient. Pairwise comparisons were performed for dose/fraction to 95% of the CTV, 5cm 3 of the bladder and 50%, 20% and 1cm 3 of the rectum with values of ±10% being considered clinically significant. Results A total of 125 fractions were analyzed. Median time for treatment sessions was 50 minutes (range 36-81 minutes). Dose to 95% of the CTV was not significantly different between the two methods. There was a large range of variation in the bladder and rectum volume at the time of treatment from reference planning (Figure 1). Despite the large changes in bladder volume, dose to 5cm 3 of bladder was comparable between full adaptation and 3DOF, with 81% of fractions having a difference of <10% (Figure 2). However, mean delivered dose (cGy) to 20% of rectum was significantly reduced from 327 (SD: 62) to 310 (SD: 77) (p = 0.0082), and from 587 (SD: 31) to 554 (SD: 79) to 1cm 3 (p = 0.003), when a full adaptation was executed. A higher number of fractions benefited from full adaptation with respect to rectal sparing when compared to 3DOF (Figure 2). OC-0783 Is full adaptation needed for prostate stereotactic body radiotherapy? A dosimetric comparison.