ESTRO 2022 - Abstract Book

S799

Abstract book

ESTRO 2022

M. Bak 1 , N. Jensen 1 , T. Jakobi Nøttrup 1 , H. From Mathiesen 1 , H. Roed 1 , M.E.E. Sjölin 1 , F. Kjær-Kristoffersen 1 , V. Nordmark Hansen 1 , I. Richter Vogelius 1

1 Rigshospitalet, Oncology, Copenhagen, Denmark

Purpose or Objective Vulvar cancer is a rare malignancy that accounts for ~0.6% of all cancers. Radiotherapy for vulvar carcinoma is technically challenging due to the target position combined with common postoperative lymphocele causing anatomical variations. A high burden of radiation sequelae is observed and locoregional failure is the dominant mode of failure. Because of these challenges, vulvar carcinoma became a prioritized patient group for adaptive radiotherapy in our department. We report on the workflow, rate of adaptation and dosimetric results for the first eight patients treated for vulvar carcinoma in our clinic. Materials and Methods The first adaptive treatment was performed on January 22, 2021 and eight consecutive patients were included. This study includes 160 fractions; the adapted plan was chosen for 63 fractions and the scheduled plan for the remainder. Systematic recording on number of adaptations and causes for not adapting was implemented starting with patient 4 in order to improve workflow and increase the rate of adaptation. The recording included reason for not choosing an adaptive plan (if relevant), adaption time and number of slices edited manually per OAR and target structures during the adaptive procedure. DVH data from the adapted and scheduled plans based on setup CBCTs as well as DVH data from the adapted plans based on verification CBCTs from all 8 patients was analyzed and compared to the planned dose. The dose covering the hottest 2% of the bladder and rectum was extracted from DVH. Coverage of the CTV was measured as the minimum dose covering 95% of the CTV relative to prescription dose. All DVH data was normalized to the reference plans. Results The proportions of adapted vs. scheduled plans and the aggregated distributions of the reasons not to adapt are shown in Figure 1A . Dominant reasons for not adapting were “MD not available for treatment” (15 Fx) and “no significant dosimetric gain” (13 Fx). The number of slices where OAR and target structures need editing varied substantially and did not show any clear association with the proportion of fractions adapted. Median adaption time was 14 minutes and 75th percentile was 17 minutes, cf. Figure 1B. We observed an increase in minimum dose covering 95% of the CTV relative to prescription dose (signrank p<0.0001) which was largely retained at the verification step, cf. Figure 2. Data on normal tissue doses was more mixed and didn’t show any gain from adaption. Figure 2 excludes 3 upper outliers on bladder adapted, 2 upper outliers on bladder verification and 3 lower outliers on CTV scheduled to improve visualization. Figure 1:

Figure 2: