ESTRO 36 Abstract Book

S16 ESTRO 36 _______________________________________________________________________________________________

The Film, TLD, Alanine, and ion-chamber measurements showed a similar delivered dose (10 Gy), regardless of the dose rate setting. The TLD measurements in the brain of the sacrificed mouse verified that the prescribed 10 Gy dose was delivered to the mouse brain, for a 10 Gy in 1.8 µs and a 0.1 Gy/s dose rate delivery (10.06 and 9.90 ± 8.2%, k = 2, respectively). Our results showed spatial memory preservation in mice after 10 Gy WBI delivered in a single 1.8 µs electron pulse, whereas 10 Gy WBI delivered with a dose rate similar to what is conventionally used in radiotherapy (0.1 Gy/s) impaired mice mid-term spatial memory. Using systematic dose rate escalation, 100 Gy/s was found to be the lower limit for full preservation of spatial memory functions after 10 Gy WBI. Conclusion This study shows for the first time that normal brain tissue toxicities after WBI can be reduced with increased dose rate. Spatial memory is preserved after WBI with mean dose rates above 100 Gy/s, whereas 10 Gy WBI at a conventional radiotherapy dose rate (0.1 Gy/s) totally impairs spatial memory. This radiobiological advantage, together with other practical considerations that benefit from rapid radiotherapy treatment delivery, such as minimizing intra-fractional motion, increased patient comfort, and improved treatment efficiency, makes Flash- RT a promising treatment modality. OC-0040 Validation of prospective electronic toxicity registration to audit dose constraints T.M. Janssen 1 , A. Dikstaal 1 , M. Kwint 1 , S. Marshall 1 , A.L. Wolf 1 , J. Knegjens 1 , L. Moonen 1 , J. Belderbos 1 , J.J. Sonke 1 , M. Verheij 1 , C. Van Vliet-Vroegindeweij 1 1 Netherlands Cancer Institute Antoni van Leeuwenhoek Hospital, Radiotherapy, Amsterdam, The Netherlands Purpose or Objective In 2012 we started with the prospective, electronic registration by the treating physician of all grade ≥2 toxicities (CTCAE v4.0) for all patients irradiated at our department. Simultaneously we set up an infrastructure to couple this data to dose and treatment parameters. The aim of this work is to assess the validity of the data and to show the feasibility of such an infrastructure to audit toxicity prediction models and dose constraints in daily clinical practice. Material and Methods As a showcase we consider the relation between the esophagus V50Gy and grade ≥2 esophagitis in locally advanced NSCLC patients receiving concurrent chemoradiotherapy (CCRT; 24 x 2.75Gy, daily 6mg/m 2 cisplatin). Clinically we use V50Gy<50% as a dose constraint based on a previously developed NTCP model [1]. The applicability of this model to current clinical practice is not evident since dose criteria changed after publication and patients currently receive intravenous pre-hydration (1L, NaCl 0.9%) which was shown to decrease esophagitis [2]. For all CCRT patients (excluding adaptive RT and re- irradiations) treated in 2014/2015, the planned V50Gy and the registered esophagitis ≥grade 2 were retrieved. Furthermore, a single observer retrospectively scored toxicity, based on the electronic health records. We calculated the incidence of grade ≥2 esophagitis per V50Gy and compared this with the expected incidence based upon the model by Kwint et al. using a χ 2 test. ROC analysis was performed to assess the predictive value of For 82 patients, 558 consultations were performed. Median follow up was 3.5 months and grade ≥2 esophagitis was prospectively (retrospectively) scored for 41 (46) patients. For 74 patients, retrospective and prospective scoring was identical and for 3 patients esophagitis was scored in both cases, but grades differed. V50Gy. Results

A comparison of the observed and predicted grade ≥2 esophagitis is shown in Figure 1a. The prospective (retrospective) observed incidence of grade ≥2 esophagitis was 50% (56%) while the model predicts 54%. Neither registrations differ from the model prediction (prospective p=0.78 , retrospective p= 0.91 ). ROC analysis (figure 1b) of prospective registrations result in an area under the curve of 0.71.

Conclusion Retrospective and prospective toxicity registration showed overlap in 90% of cases. Moreover, the observed dose-effect of grade ≥2 esophagitis was almost identical to the NTCP model. This implies 1) that the V50Gy accurately predicts toxicity for our current treatment protocol and 2) that our prospective toxicity registration results in valid data. Esophagitis incidence was expected to decrease due to pre-hydration. While this discrepancy requires further investigation, it does show that the electronic toxicity registration and connection to dose parameters appears to be a valuable tool to audit the applicability of dose constraints in daily clinical practice. OC-0041 Predictors of asymptomatic radiation induced vascular damage to infradiaphragmatic vessels L. Cella 1 , R. Liuzzi 1 , P. Romanelli 2 , M. Conson 2 , V. D'Avino 1 , M. Ottaviano 3 , V. Damiano 3 , G. Palmieri 3 , R. Pacelli 2 , M. Mancini 1 1 Institute of Biostructure and Bioimaging-CNR, National Council of Research, Napoli, Italy 2 Federico II University School of Medicine, Department of Advanced Biomedical Sciences, Napoli, Italy 3 Federico II University School of Medicine, Centro Riferimento Tumori Rari Regione Campania, Napoli, Italy Purpose or Objective Blood vessels may be damaged by radiation exposure and radiation therapy (RT) may have a negative impact on the vascular system by promoting accelerated atherosclerotic vascular complications. Aim of the present study is to identify predictors of asymptomatic radiation-induced [1] Kwint et al. IJROBP 2012 [2] Uyterlinde et al. R&O 2014