ESTRO 37 Abstract book

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ESTRO 37

outlined using b =800 s/mm 2 DW-MRI and the apparent diffusion coefficient (ADC) was estimated using four b - values ( b =400-800 s/mm 2 ) in a mono-exponential model. Treatment outcome was evaluated by volume criteria using T1W scans and thresholds from RECIST. The prognostic capacity of ADC was evaluated by the area under the curve (AUC) of receiver operating curves using Matlab (α=0.05, H1: AUC > 0.5). Results Non-responding (progressive disease (PD) or stable disease (SD)) metastases (n=10) showed no mean volume change during the first six fractions and a mean reduction of about 25 % during the remaining 4 fractions, stabilizing at fraction 9 (Fig. 1a). Responding (partial response (PR) and complete remission (CR)) metastases (n=19) showed a large variation in volume change and no overall trends (Fig. 1b). The overall composition of the tumor i.e. volume fraction of viable tumor remained constant regardless of treatment outcome (data not shown). The mean ADC increased almost monotonously in responding metastases, and decreased in nonresponding metastases, leading to a statistical significant capacity to stratify treatment response at fraction 3, and more permanently from fraction 7 and on (Fig. 2a). Further, neither tumor histology nor baseline tumor volume was correlated to treatment outcome (Fig. 2b).

Fig. 2 a. Relative ADC of responding (PR and CR) and non- responding (SD and PD) metastases. Asterisks indicate statistical significance. b. Primary cancer and baseline tumor volume plotted against follow-up volume. Conclusion During RT, neither volume change nor tumor composition change were reliable predictors of treatment outcome. In fact a (T2W based) volume reduction during RT may falsely categorize the tumor as responder . These findings were uncorrelated to baseline tumor volume and histology. On the other hand, tumor diffusivity change showed capacity to stratify treatment outcome, suggesting that MRI with functional information should be included in longitudinal early-response studies. OC-0177 Mid-Treatment Response Assessment is predictive in HPV negative Oropharyngeal Cancers undergoing RT R. Kabarriti 1 , N.P. Brodin 1 , G. Lundgren 1 , N. Ohri 1 , W.A. Tomé 1 , S. Kalnicki 1 , M.K. Garg 1 1 Montefiore Medical Center- Albert Einstein College, Radiation Oncology, New York, USA Purpose or Objective To evaluate if response assessment based on mid- treatment computed tomography (CT) scans can predict loco-regional recurrence (LRR) for patients receiving definitive intensity-modulated radiation therapy (IMRT) for oropharyngeal head and neck cancer (HNC). Material and Methods HNC patients treated at our institution with RT undergo CT rescans at 15th RT fraction and are re-planned in case of inadequate dose to gross disease or increased dose to organs at risk. A retrospective cohort analysis was performed on 96 consecutive patients with oropharyngeal cancer treated in 2007-2015 with mid-treatment rescan. The volume of primary GTV and involved lymph nodes were delineated on pre- and mid-treatment CT and volumes were compared to compute volumetric change as an early response assessment. Univariable and multivariable Cox proportional hazards regression analysis were used to evaluate the efficacy of mid-treatment reduction in tumor volume as a predictor of LRR. Risk-

Fig. 1 Relative volume change in a. Non-responding metastases, b. responding metastases. Error bar = 1 SD.