ESTRO 36 Abstract Book

S15 ESTRO 36 2017 _______________________________________________________________________________________________

1 Karolinska University Hospital, Medical Radiation Physics and Nuclear Medicine, Stockholm, Sweden 2 San Raffaele Scientific Institute, Medical Physics, Milano, Italy 3 Fondazione IRCCS Istituto Nazionale dei Tumori, Prostate Cancer Program, Milano, Italy 4 San Raffaele Scientific Institute, Radiotherapy, Milano, Italy 5 Ospedale Bellaria, Radiotherapy, Bologna, Italy 6 Istituto di Candiolo- Fondazione del Piemonte per l'Oncologia IRCCS, Radiotherapy, Candiolo, Italy 7 Università degli Studi di Milano, Oncology and Hemato- Oncology, Milano, Italy Purpose or Objective To explore which features of the dose distribution in the ano-rectal wall determine the risk of late rectal bleeding and late faecal incontinence following prostate cancer radiotherapy (RT). Material and Methods Patients from the DUE-01 study with available 3D dose distributions and follow-up data at 24 months after RT were included in this study. Patients with pre-treatment symptoms were excluded. An incidence of 22 in 152 was observed for a maximum grade ≥ 2 rectal bleeding, while 12 patients in 110 experienced a mean grade > 1 faecal incontinence, calculated from at least 3 occasions from 6 to 24 months after RT. Dose surface maps were extracted and converted to EQD2; structures considered were the rectum, anal canal and the combination of the two. For each endpoint, the mean of the dose surface maps in the group of patients with and without toxicity respectively were calculated. A t-test was performed on the mean values of each pixel to identify regions where the dose differed between patients with and without toxicity (i.e. with low p-value). The lateral and longitudinal extent, and eccentricity, of EQD2 isodoses from 5 to 73 Gy were extracted from the dose maps. Univariate NTCP models using each parameter were fitted to the outcome data and the performance evaluated using AUC. Results The patients who experienced rectal bleeding received higher dose to the posterior part of the rectal wall (see figure; dose map unfolded along anterior axis); the greatest difference was found when aligning all dose maps at the inferior border of the rectum. Patients with faecal incontinence had a higher dose in the posterior wall of the anal canal compared to patients without; here a greater difference was found when aligning the dose maps according to the centre of mass of the dose maps. For rectal bleeding, the highest AUC was found for the lateral extent of the 31-Gy isodose; this is in agreement with the difference in dose to the posterior wall in the toxicity vs. non-toxicity groups. For faecel incontinence, on the other hand, the model based on the lateral extent of the highest isodose (73 Gy) had the highest AUC.

Conclusion The dose received by the posterior part of the rectal wall is related to the risk of late rectal bleeding, and the lateral extent of the 31-Gy isodose is the best spatial dose- parameter to include in an NTCP model. The risk of causing late faecal incontinence is related to the dose to the anal canal. OC-0039 Unique sparing of spatial memory in mice after whole brain irradiation with dose rates above 100Gy/s K. Petersson 1 , P. Montay-Gruel 2 , M. Jaccard 1 , G. Boivin 2 , J. Germond 1 , B. Petit 2 , F. Bochud 1 , C. Bailat 1 , J. Bourhis 2 , M. Vozenin 2 1 Lausanne University Hospital, Institute of Radiation Physics IRA, Lausanne, Switzerland 2 Lausanne University Hospital, Department of Radiation Oncology, Lausanne, Switzerland Purpose or Objective Radiotherapy at ultra high dose rate (Flas h-RT) has been suggested to increase the differential response between normal and tumor tissue compared to conventional radiotherapy. In order to further explore Flash-RT and to validate its protective effect on normal tissues, we decided to investigate brain response to Flash-RT as it is a well-defined and robust model in radiobiology. Material and Methods 10 Gy was used as the prescription dose for the whole brain irradiations (WBI). The irradiation settings, corresponding to the prescription dose, were defined according to film (Gafchromic™ EBT3), TLD (LiF-100), Alanine pellets, and ion-chamber (Advanced Markus, corrected for ion recombination) measurements at the surface of a solid water phantom, positioned behind a 1.7 cm in diameter aperture of a graphite applicator. The measurements and the subsequent mice WBI were performed for different dose rates, ranging from a conventional radiotherapy dose rate of 0.1 Gy/s to 10 Gy delivered in a single 1.8 µs electron pulse. TLD were positioned inside the skull of a sacrificed mouse to validate the dose delivered to the brain during WBI for the highest and lowest dose rate setting. 75 Female C57BL/6J mice were used in the study. Dose rate effect on neuroprotection was evaluated by 'Novel Object Recognition test” two months post-irradiation. All the experiments were video-recorded. Analysis was performed blindly and the time the mice spent investigating each object was measured in order to calculate the Recognition Ratio (RR) such as: RR= (time spent investigating the novel object / time spent investigating the two objects). Results