ESTRO 36 Abstract Book

S561 ESTRO 36 _______________________________________________________________________________________________

interfractional vaginal motion in the LR and AP direction and a moderate agreement in the CC direction (see figure 1), which we in all directions significant (p<0.00) . Considering only interfractional vagina motion, applying a BA based image guidance strategy requires CTV to PTV margins of 0.3 cm, 0.8 cm and 1.0 cm in the LR, CC and AP direction. When applying a FM or ST registration based imaging strategy the residual LN variability (which move with the BA) will be larger, and needs to be considered in the CTV to PTV margins, leading to LN margins of 0.3, 1.1 and 1.3 cm in the LR, CC and AP direction.

motion. We are currently investigating an offline adaptive workflow to address this. PO-1017 Dose guided adaptive radiotherapy based on cumulated dose in OAR for prostate cancer M. Nassef 1 , A. Simon 1 , B. Rigaud 1 , L. Duvergé 2 , C. Lafond 2 , J.Y. Giraud 3 , P. Haigron 1 , R. De Crevoisier 2 1 LTSI, INSERM U1099, Rennes, France 2 Centre Eugène Marquis, Radiothérapie, Rennes, France 3 CHU Grenoble, Radiothérapie, Grenoble, France Purpose or Objective Large dose differences between planned and delivered doses may be observed in the rectum and in the bladder, resulting from anatomical variation in the course of prostate IMRT. The objective of this study was to compare dosimetrically an original approach of Dose Guided Adaptive Radiotherapy (DGART) to the standard IGRT (CBCT daily repositioning). Material and Methods Based on a series of 24 patients with daily CBCT, planned and delivered dose were compared in manually delineated structures (prostate, rectum and bladder), using dose accumulation process after estimation of the fraction dose [Nassef et al, Radiother Oncol 2016]. The four patients with the most important overdose in the rectum wall and the bladder wall were selected to estimate the DGART benefit compared to the standard IGRT. The DGART strategy (Figure 1) was based on replanning(s) triggered by monitoring the cumulated doses to the prostate, the rectum wall and the bladder wall. Thereby, the first step consisted in estimating the relative excess of the cumulated dose compared to the planned dose after every fraction for the prostate D 99 , the rectum wall V 72 and the bladder wall V 70 . After an observation phase of 5 fractions, the adaptation was triggered (i.e. a replanning was performed), if a 2 % underdose of D 99 for prostate or an overdose of 10 % on V 72 for the rectum wall or V 70 for the bladder wall occurred.

If a replanning was triggered at the fraction n, the CBCT chosen for the replanning corresponded to the anatomy leading to the highest dose drift compared to the planned dose. For that, for every fraction x (x=1..n), an index (see figure 2) was calculated to select the morphology leading to the highest dose drift compared to the planned dose. If the relative excess was compensated by the replanning, no other adaption was needed and the new replanning was used for the rest of the fractions. If the relative excess was not compensated, the replanning process was repeated in case of a new CBCT leading to a higher index value. An example of DGART implementation is provided in Figure 2, showing the benefit of DGART to decrease the dose to the bladder.

Conclusion FM registrations can be applied as an IGRT strategy to measure and correct the vagina motion. However applying FM registration increases the LN interfractional position variability, subsequently increasing the CTV to PTV margins for the LN regions even more in comparison to the margins needed to encompass the interfractional vagina