ESTRO 2021 Abstract Book

S483

ESTRO 2021

evaluating and adjusting the contours, creating the new plan and verifying the CTV on the CBCT scan took on average 20 minutes. Including delivery and post treatment verification this was 26 minutes. The system- generated target volumes needed user adjustments in 50% of fractions. Target volumes on second pretreatment CBCT were all covered with the 95% isodose line. The number of monitor units (MU) of the adapted plan never deviated more than 15% from the number of MU of the reference plan. Patient compliance with respect to bladder filling and total treatment time was excellent. Conclusion First clinical experience with CBCT-based online adaptive radiotherapy shows it is feasible and suitable for hypo fractionated rectal cancer treatment. OC-0619 Plasma HPV elimination patterns as strong predictor of outcome in primary Anal Cancer treatment A.C. Lefèvre 1 , N. Pallisgaard 2 , C. Kronborg 3 , K.L. Wind 1 , S.N. Krag 4 , K.G. Spindler 5 1 Aarhus University Hospital, Department of Experimental Clinical Oncology, Aarhus , Denmark; 2 Zealand University Hospital, Department of Pathology, Næstved, Denmark; 3 Danish Centre for Particle Therapy, Danish Centre for Particle Therapy, Aarhus, Denmark; 4 Aarhus University Hospital , Department of Pathology, Aarhus , Denmark; 5 Aarhus University Hospital, Department of Experimental Clinical Oncology / Department of Oncology, Aarhus, Denmark Purpose or Objective We have developed of a highly sensitive method for detection of human papilloma virus circulating tumor DNA in plasma (pHPV). Here, we present data on pHPV from patients with primary squamous cell carcinoma of the anus (SCCA) and the predictive value of pHPV elimination during primary chemoradiotherapy treatment (CRT). Materials and Methods From 2016 to 2020, prospective collection of samples prior to CRT (PT), mid treatment (MT), end of therapy (EOT) and 1-3 years after CRT were retrospectively analysed from patients treated at Aarhus University Hospital, Denmark. A sensitive in-house, multiplex digital droplet PCR method measured six relevant HPV subtypes 16, 18, 31, 33, 51 and 58 from a single 4 mL plasma sample. The level of pHPV was measured as fraction of total DNA in %. Comparison analysis used Wilcoxon Mann-Whitney test with median values and 95% confidence intervals (CI). Fisher’s exact test compared elimination patterns and outcome, and outcome analyses used log-rank and cox-regression models. Results Plasma HPV was detectable in 52 patients of all stages, including T1N0 tumors. The level increased with increasing tumor stages (T1: 0.35%, T2: 1.26%, T3: 2.29%, T4: 13.46%), p =0.39, and lymph node positive tumors had a significantly higher pHPV level compared to node negative tumors, 6.09% (95% CI 2.08-34.54) and 0.39% (95% CI 0.18-2.59) respectively, p =0.02. During CRT, three elimination patterns were observed with strong correlation to outcome: “ Fast responders” (n=12) were patients eliminating pHPV from PT to MT. None of the " fast responders" had treatment failure. “ Slow responders” (n=20) eliminated pHPV by the time of EOT. These patients had a significant risk of local treatment failure of 20%. Finally, the “ Persistent” molecular disease group (n=13) included patients with detectable pHPV in the EOT sample. In this group, 31% of patients were later diagnosed with distant treatment failure. The three patterns were significantly different p <0.01, and there were no difference between groups regarding tumor- or node status, p =0.81 and p =0.37, respectively. Using the median PT pHPV level as cut-off, patients were divided into two groups showing a trend for difference in overall survival (HR= 2.42 (CI 0.44-13.44) p =0.31) and disease free survival (HR= 4.07 (CI 0.84-19.64) p =0.08). The number of events was however low. Conclusion Measurement of pHPV is possible even in low stage SCCA and repeated measurements during CRT can divide patients into groups with significantly different risk of failure. The use of pHPV can potentially assist in future clinical treatment decision, with modified treatment according to pHPV pattern, i.e. decreased dose for fast responders, thereby avoiding unnecessary side effects, EOT boost for slow responders , increasing the local control, and post-CRT systemic treatment for patients in the Persistent group . OC-0620 A multicenter study on Intensity Modulated Radiotherapy techniques for anal carcinoma (RAINSTORM). L. Caravatta 1 , G. Mantello 2 , F. Valvo 3 , F. Pierfrancesco 4 , G. Lucrezia 5 , C. Rosa 5 , N. Slim 6 , P. Pittoni 7 , F. De Felice 8 , M.A. Gerardi 9 , S. Vagge 10 , M. Krengli 11 , E. Palazzari 12 , M.F. Osti 13 , G. Catalano 14 , G.B. Ivaldi 15 , A. Galardi 16 , M. Lupattelli 17 , R.M. Niespolo 18 , A. Guido 19 , G. Macchia 20 , M.A. Gambacorta 21 , M. Di Nicola 22 , V. Donato 23 , D. Genovesi 24 1 SS Annunziata" Hospital, "G. d'Annunzio" University of Chieti, Radiation Oncology Unit, Chieti, Italy; 2 Azienda Ospedaliero Universitaria Ospedali Riuniti, Department of Oncology and Radiotherapy, , Ancona, Italy; 3 CNAO National Center for Oncological Hadrontherapy, Radiotherapy Unit, Clinical Department, Pavia, Italy; 4 University of Turin, Department of Oncology, Radiation Oncology, Torino, Italy; 5 "SS Annunziata" Hospital, "G. d'Annunzio" University of Chieti, Radiation Oncology Unit, Chieti, Italy; 6 IRCCS San Raffaele Scientific Institute, Department of Radiotherapy, Milano, Italy; 7 Asst Lariana, Ospedale di Como , Radiation Oncology Unit, Como, Italy; 8 Policlinico Umberto I, "Sapienza" University of Rome, Department of Radiotherapy, Roma, Italy; 9 IEO European Institute of Oncology, IRCCS, Department of Radiotherapy, Milano, Italy; 10 IRCCS Ospedale Policlinico San Martino, Department of Radiation Oncology, Genova, Italy; 11 University of Piemonte Orientale, and University Hospital “Maggiore della Carità” , Division of Radiation Oncology, Department of Translational Medicine, Novara, Italy; 12 Oncological Referral Center, Radiation Oncology Department, Aviano, Italy; 13 Sant'Andrea Hospital, Sapienza University of Rome, Unit of Radiation Oncology, Roma, Italy; 14 IRCCS Proffered papers: Proffered papers 36: Lower GI