Abstract Book

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ESTRO 37

stratification was performed by dichotomizing patients into high- and low-risk groups based on mid-treatment response as well as p-16-status and smoking history, and compared in terms of loco-regional control, progression- free survival and overall survival. Results Out of the 96 patients, 74% had stage IVa or IVb (AJCC 7th ed) and 57% had p16 positive disease. With a median follow-up of 34 months, 14 patients experienced LRR. The median (inter-quartile range) reduction in total tumor volume was 18.7% (8.4% - 30.9%). Reduction in total tumor volume > median is an independent predictor of LRR (HR 0.22, 95% CI: 0.05 – 0.89, p=0.020), and the reduction in primary tumor volume is an even stronger predictor (HR 0.11, 95% CI: 0.02 – 0.57, p=0.002). For patients with p-16 negative disease, the three-year locoregional control rate for patients with total tumor reduction ≤ median was 55.3%, compared to 90.9% for >median reduction (p=0.035). For those with p-16 positive disease, the three-year locoregional control rate for patients with tumor reduction ≤ median (21.5%) for that group was 85.6%, compared to 95.0 % for p-16 positive > median reduction (p=0.30. Stratifying patients into a high-risk group with reduction in total tumor volume at mid-treatment ≤median, p-16 negative status, and smoking status >10 pack years, compared to a low- risk group without these factors showed a clear separation in Kaplan-Meier curves with actuarial 3-year loco-regional control, progression-free survival and overall survival rates for the high-risk patients of 45.7%, 38.2%, 71.8% compared to 90.7%, 70.6%, 89.8% for the low-risk patients, respectively (p≤0.021 for all).

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-

Conclusion Our study shows that early response assessment based on mid-treatment CT is an independent predictor of LRR, especially for those with HPV negative disease and can be used to effectively distinguish high-risk and low-risk patients, allowing for risk-adaptive treatment stratification at the midway point.

OC-0178 Optimal timing for tumor response assessment to nCRT with MRI in patients with esophageal cancer A.S. Borggreve 1 , S.E. Heethuis 2 , L. Goense 1 , P.S.N. Van Rossum 2 , A.L.H.M.W. Van Lier 2 , R. Van Hillegersberg 3 , J.P. Ruurda 3 , J.J.W. Lagendijk 2 , S. Mook 2 , G.J. Meijer 2 1 UMC Utrecht, Radiation Oncology and Surgical Oncology, Utrecht, The Netherlands 2 UMC Utrecht, Radiation Oncology, Utrecht, The Netherlands