ESTRO 38 Abstract book

S22 ESTRO 38

area under tumor growth curve STF IR=2060±204 vs Full IR=2095±343, P=0.99). Reduction in the irradiated volume resulted in a decreased response (Fig. 2A, Half IR). CTVM (Fig. 2B) measured overall significantly smaller starting volumes with an increased dispersion compared to CALVM (V 0 CTVM=238±53mm 3 vs CALVM=288±14mm 3 , P<0.001), while the relative tumor growth curves (each point normalized to corresponding V 0 ) did not differ between the two methods.

with the colony stimulating factor 1 receptor (CSF1R) antagonist PLX5662 and with inhibitors of adenosine kinase. Interestingly, more recent data implementing FLASH radiotherapy has achieved similar promising results in terms of sparing normal tissue complications in the irradiated brain without compromising tumor cure. Conclusion Progress in each of these research areas holds significant potential for translation to the clinic with the hope of improving the quality of life of millions of cancer survivors. These will be highlighted in further detail in our presentation. OC-0057 Probing spatiotemporal fractionation on the preclinical level I. Telarovic 1 , M. Pruschy 1 , I. Grgic 1 , J. Krayenbuehl 1 , M. Guckenberger 1 , J. Unkelbach 1 1 Universitätsspital Zürich, Radiation Oncology, Zürich, Switzerland radiotherapy, spatiotemporally fractionated (STF) treatments deliver a distinct dose distribution in each fraction. The aim is to increase the therapeutic window by simultaneously achieving partial hypofractionation in the tumor along with near uniform fractionation in normal tissues. To that end, each fraction delivers high single fraction doses to alternating parts of the tumor while creating a similar dose bath in the surrounding normal tissue. This approach has been studied in silico (Unkelbach, 2015) based on the working hypothesis that different parts of the tumor can be treated in different fractions as long as each part receives the prescribed cumulative biological dose in the end. Here, we do an initial step towards verifying this assumption in an experimental animal model. Material and Methods We compare the tumor growth rate of conventionally treated xenografts derived from mouse colon cancer cells to a reductionistic preclinical model of STF. Upon reaching the tumor volume of 300 mm 3 ±10%, animals are randomly assigned to 4 treatment groups (Fig. 1). No irradiation (IR) group is sham irradiated. Half IR group is irradiated with a partial 12 Gy IR covering the cranial half of the tumor. STF IR group receives two partial 12 Gy IRs given on day 0 and day 1, covering the cranial and caudal half of the tumor, respectively. Full IR group receives two partial 12 Gy IRs successively. All animals undergo CT imaging on days 0, 1, 4 and 7 post IR and are euthanized on day 7. To perform such fractionated IR of varying target volumes in small animals, we develop a pipeline that combines a dedicated small animal image-guided research platform (X-RAD 225Cx / SmART-Plan) with the clinical software MIM (MIM Software Inc.). Finally, we compare caliper-based (CALVM) growth rate curves with CT-volumetry (CTVM) in the context of tumor growth rate studies. Purpose or Objective In contrast to conventional

Conclusion The assumption of spatiotemporal fractionation that the tumor response can be predicted by locally adding up biological doses from each fraction is confirmed in a reductionistic preclinical model whereby xenografts irradiated either immediately or with a 24 hour delay between two partial irradiations exhibit no difference in growth delay. The study suggests caliper- and CT- volumetry to be interchangeable when considering relative, but not absolute volumes in tumor growth studies. OC-0058 Dose-volume effects in the central nervous system and sparing in microbeam/minibeam radiation E. Bahn 1,2 , M. Alber 1 1 Heidelberg University Hospital, Radiation Oncology, Heidelberg, Germany; 2 Clinical Cooperation Unit Radiation Oncology, German Cancer Research Center DKFZ, Heidelberg, Germany Purpose or Objective Animal experiments with narrow proton beams in the mm- range and bath/shower dose distributions have produced evidence of an abnormally high volume effect in rat spinal cord more than a decade ago. Whether these results have

Results Tumors irradiated to the same dose, either immediately (Full IR) or with a 24 hour delay between two partial IRs (STF IR), exhibited no difference in growth rate (Fig. 2A,