ESTRO 2022 - Abstract Book

S1267

Abstract book

ESTRO 2022

Conclusion Patient specific dose plan QA shows in general good agreement within the deform-registered structures and radiographer contoured structures. The rather large HD for some regions of structures may be explained by variation in inter radiographer variations. However, large dose difference in some OAR (aorta and bronchial tree), indicates that inter fractional patient specific QA may be required to avoid exceeding dose constrains.

PO-1491 Dosimetric impact of bladder changes in prostate VMAT based on dose deformable image registration

A. Gonzalez Pose 1 , B. Vázquez Barreiro 2 , J.S. Vázquez Rodríguez 1 , B. Andrade Álvarez 1 , A. Teijeiro García 1 , A.B. Blanco Ledo 2 , M. Riveira Martin 3 , R. Dorado Dorado 1 , A. López Medina 1 , M. Salgado Fernández 4 1 Hospital Meixoeiro, Medical Physics, Vigo, Spain; 2 Hospital Meixoeiro, Radiation Oncology, Vigo, Spain; 3 Galicia Sur Biomedical Foundation, Health Research Institute, Vigo, Spain; 4 Hospital Meixoeiro, Medical Phyisics, Vigo, Spain Purpose or Objective To investigate dosimetric impact of prostate radiotherapy sessions delivered with a bladder volume smaller than the planned. Based on CBCT dose deformable registration, it is possible to establish a criteria about how many sessions with non-optimal bladder filling are possible to avoid CBCT repetitions. This would be helpful to avoid overdoses and to optimize treatment schedules and resources, resulting in a specific offline protocol for prostate cancer VMAT EBRT. Materials and Methods We evaluated daily CBCTs of 23 patients treated with EBRT (VMAT 6MV ClinacIX). The prescribed doses were 57.6 ± 7.5 Gy to the prostate or prostate bed and 44.74 ± 0.58 Gy to the CTV encompassing prostate or prostate bed and regional pelvic lymphatics nodes. All patients were classified in 6 groups of 4 patients depending on the number of sessions in which CBCT repetition due to different bladder filling was necessary: from none to five sessions. A dose deformable image registration computed with Velocity software (Varian) was used to obtain full treatment dosimetry. All non-optimal bladder sessions calculated (Eclipse v15.6.06 TPS) on their unfavourable CBCTs were added. The optimal bladder sessions were grouped every five and calculated on one CBCT chosen as representative. Bladder, rectum , prostate CTV and pelvis CTV were contoured in each one. The mean dose (Dmean) and V40 of bladder and rectum, as well as the D95 of both CTVs were assessed. The dose differences between planned and registered treatment volumes were calculated (difference = registered - planned). Finally, this difference was evaluated for every patient group with the mean and standard deviation. Dose constraints were based on RTOG 0415 dose limits. Results The mean ratio in bladder volume between planned CT and CBCT (bladder planned/bladder CBCT) was 1.2 ± 0.4. Differences between calculated and planned Dmean for rectum and bladder were always less than 5 Gy. Admissible variations in V40 in rectum (-1.8 ± 1.7 %) and bladder (0.8 ± 1 %), also a better CTVs coverage have been reached in no- repeated CBCT and CBCT repeated once. Groups in where 2 and 3 CBCTs were repeated, a slight increase of the V40 in bladder was found (0.9 ± 1.1 %) along with an underdose of pelvic CTV (D95 -0.2 ± 0.4 % prescribed dose) and prostate CTV (D95 -1.1 ± 1.9 % prescribed dose) . Finally, for groups of 4 or more non-optimal CBCT, a significant increased was observed in V40 of bladder (8.9+-6.3 %) and rectum (6.3 +-2%), underdosing both CTVs (D95 CTV prostate -1.3 +-2 % prescribed dose) and CTV pelvic /-0.67 +- 0.35 % prescribed dose).