ESTRO 36 Abstract Book

S557 ESTRO 36 2017 _______________________________________________________________________________________________

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.

0.5 Gy higher (0.7 Gy maximum) using DGART compared to standard IGRT. Conclusion DGART with only one replanning applied to a selected sub- group of patients may reduce the rectum and bladder overdose in prostate IGRT.

Poster: RTT track: Patient care, side effects and communication

PO-1018 Improvement of radiation-induced late toxicity after hyperbaric oxygen treatment R. Roncero 1 , J. Pardo 1 , E. Jimenez-Jimenez 1 , D. Morera 2 , N. Aymar 1 , I. Ortiz 1 , M. Vidal 1 1 Hospital Universitari Son Espases, Radiation Oncology, palma de mallorca, Spain 2 Hospital Universitari Son Espases, Medical Physics, palma de mallorca, Spain Purpose or Objective To assess the efficacy of hyperbaric oxygen therapy (HBOT) in the management of patients with radiation- induced late effects, in which more conservative We retrospectively reviewed the clinical records of 33 patients treated at our Department, from 2012 to 2016, who developed late toxicity (Grade IV CTCAE4.0) and which did not respond to conservative treatment, and recorded the variation, if occurred, in the degree of toxicity after hyperbaric treatment. The average age of the patients was 61 +/- 12 years and the mean dose delivered during the radiotherapy treatment was 52 +/- 12Gy with standard fractionation. Regarding HBOT, they received an average of 61 sessions. The patients presented the following toxicities: enteritis/proctitis in 33%, bone necrosis and sacroileitis in 30%, skin injury 9%, Cystitis 6% and others 9% (neurocognitive impairment, dysphagia and xerostomy). In order to its evaluation, responses were classified into three groups according the CTCAE4.0 scoring: Major Response Group : Improvement of toxicity from Grade IV to Grade I or 0 (without toxicity, or minor toxicity not requiring medical treatment), Minor Response Group : Improvement from Grade IV to Grade III/II (permanent toxicity controlled with medical treatment) and No Response Group . The statistical study was carried out by using SPSS_22. Results Ninety-one percent of the patients (30) completed the treatment sessions with hyperbaric chamber scheduled (2 patients didn´t start the treatment and 1 patient stopped after 4 sessions). Statistical significant toxicity improvement (p<0.05) was observed after the hyperbaric oxygen treatment. 60% of the patients presented a Major Response, and 18% presented a Minor Response. 9% (3) of our patients were no responders. In our patients, no relationship was founded between the response and the age, the number of sessions of HBOT, or the time relapsed since radiation treatment to the indication of the HBOT. Table 1 presents the patients outcomes according the toxicity. treatments have failed Material and Methods

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.

Results For the four patients, the DGART resulted to only one replanning during the first week of treatment. For the rectum wall V 72 , the overdose was on average reduced of 50% (100% maximum) and the mean dose reduced of 4.5 Gy compared to standard IGRT. For the bladder wall V 70 , the overdose was on average reduced of 19% (37% maximum) and the mean dose reduced of 6.6 Gy compared to standard IGRT. For the prostate, the D 99 was on average