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introducing electronic toxicity registration (ETR) within our institute, a more systematic and accurate recording of toxicity was implemented in clinical practice. Using ETR, toxicity predictions and dose constraints can be audited. Hence, the aim of this work is to assess the validity of the RWE toxicity registration and to show the feasibility of ETR to audit toxicity prediction models and dose constraints. As a showcase, the NTCP-models of acute esophagus toxicity (AET) for concurrent chemoradiation (CCRT) for NSCLC patients were used to validate the ETR of AET before/after 1 st June 2015. At this date, dose on the irradiated mediastinal lymph nodes was de-escalated from 66 Gy to 58,08 Gy (60Gy BED) in 24 fractions. Material and Methods To validate the NTCP-models, clinical data of 219 (N=101 before 1 st June 2015) inoperable NSCLC patients receiving CCRT using IMRT was used. The planned V50Gy and V60Gy and the prospectively scored grade ≥2 AET were retrieved. The grade ≥2 AET probability for the V50Gy and V60Gy was calculated as; [1/1+ exp [ -0.515 + (0.027*V50 ]] and [1/1+ exp [ -0.701 + (0.029*V60 ]]. Validity of the model was assessed with the ability to predict the number of grade ≥2 AET events (calibration) and the ability to distinguish between those who develop grade ≥2 AET from those who do not (discrimination, area under the curve (AUC)). Furthermore, sensitivity and specificity for different cut- off points were determined. Results The incidence of grade ≥2 and grade ≥3 AET, decreased between the 2 periods from 51% to 37,6% and 8.8% to 3.4% respectively. Calibration showed that the V50 overestimated the risk of grade ≥2 AET in low-risk patients while the V60 underestimated the risk of grade ≥2 AET in high-risk patients. Discrimination of both algorithms demonstrated a similar moderate fit (AUC 0.70 and AUC 0.68 for the V50 and V60, respectively). Cut-off points at 40% and 75% probability of grade ≥2 AET resulted in most favorable sensibility of 95.8% (specificity 30.1%) and 99.2% (specificity 2.1%) for V50 and V60, respectively. For the V50-model an almost similar model- fit was found (AUC before 1 st June 2015: 0.69, and AUC after: 0.70. For the V60-model the model fit decreased after dose de-escalation; AUC= 0.726 compared to an AUC = 0.616, respectively (Figure1). Conclusion RWE is a useful method to audit NTCP-models in clinical practice. De-escalation of the mediastinal dose decreased the AET. The NTCP-models to predict AET in NSCLC patients both showed good predictive accuracy. For clinical practice, the V50Gy seems to be the most stable to the dose de-escalation and sensitive without compromising safety and efficacy. EP-1384 The role of SABR for eradicative treatment of thymoma recurrences C. Menichelli 1 , C. Casamassima 2 , G. Pastore 3 , A. Fanelli 4 , V. Mazzotti 5 , E. Lombardo 6 , M. Cantarella 4 , A. Chella 7 , J. Petrini 8 1 Centro di Radioterapia - Ecomedica, Resarch Institute- Ecomedica, Empoli, Italy 2 Resarch Institute Ecomedica, Radiation Oncology, empoli, Italy 3 Research Institute, Radiation Physics, Empoli, Italy 4 Research Institute, Radiation oncology, empoli, Italy 5 Reserach Institute, Radiation Oncology, Empoli, Italy 6 Research Institute, radiaton Oncology, Empoli, Italy 7 University of Pisa, Thoracic Surgery, pisa, Italy 8 Univversity of Pisa, Medical Oncology, pisa, Italy Purpose or Objective Surgery usually represents the standard treatment of Thymoma with neoadjuvant or adjuvant chemoradiotherapy in locally advanced stages. Frequently the recurrences of disease appears on the

pleura. The relative radiosensitivity of thymomas and the small tendency to systemic spread offer curative possibility to local eradicative therapies. We have evaluated in terms to local control, OS and toxicity the SABR directed to all sites of relapses. Material and Methods Between April 2014 and February 2017, 17 pts with thymoma underwent SABR for pleural relapses Median age was 49 ys ,with KPS >90. 12 pts presented myasthenia gravis at diagnosis, at the onset all patients were treated by surgery .Thymic epithelial tumours were staged according to the Masaoka-Koga staging system, 8/17 pts were stage B3,6/17 pts were stage B2, 2/17 was B1, 1/14 was stage AB. Relapses in pleural regions were detected by CT in 100% pts, and PET-CT in 82% pts. 53% pts were treated with somatostatin analogue and all pts received corticosteroid . 53 lesions were treated with SABR. SBRT was delivered by 6MV Linac using beam modulator (ELEKTA S ynergy S)equipped with 4 mm MLC, through two coplanar arcs with VMAT optimization. Median GTV volume was 8.8 cc (2-173 cc). The prescription dose was 30Gy/3fx , 26 Gy /1fx , 24 Gy/3fx to the 80% isodose, in 42/53, 4/53,7/53 lesions respectively. Set-up and isocenter position was controlled before each fx using CBCT .The response was evaluated 60 days after SBRT by CT and PET scan and every 4 months successively. Toxicity was assessed by CTCAE score. Results All pts and 50/53 lesions have been evaluated for response. Median FU is 6.5 (range6-45) months the mean survival is 42.09 months(median not reached), with median progression free survival of 12.91 months.All Pts are still alive and all lesions treated are in complete response. 10/17 pts had relapses outside of treatment sites, these recurrences were treated by SABR. Only grade 2 pneumonitis occurred in 1 pts , who underwent to SBRT for three ipsilateral pleural lesions simulataneously. Conclusion All pts and 50/53 lesions have been evaluated for response. Median FU is 6.5 (range6-45) months the mean survival is 42.09 months(median not reached), with median progression free survival of 12.91 months.All Pts are still alive and all lesions treated are in complete response. 10/17 pts had relapses outside of treatment sites, these recurrences were treated by SABR. Only grade 2 pneumonitis occurred in 1 pts , who underwent to SBRT for three ipsilateral pleural lesions simulataneously. EP-1385 Carbon-ion radiotherapy for peripheral lung tumors in patients with severe pulmonary dysfunction. K. Terashima 1 , K. Matsumoto 2 , A. Matsunobu 3 , H. Suefuji 1 , M. Shinoto 1 , S. Toyama 1 , Y. Shioyama 1 1 SAGA HIMAT Foundation, Radiation Oncology Ion Beam Therapy Center, Tosu, Japan 2 Kyushu University Beppu Hospital, Department of Radiology, Beppu, Japan 3 Fukuoka University Hospital, Department of Radiology, Fukuoka, Japan Purpose or Objective Treatment of lung tumors for patients with severe pulmonary dysfunction is challenging. Carbon-ion radiotherapy (CIRT) possesses possibility to be one of the curative modalities for lung tumor with less irradiated volume of normal lung compared to stereotactic radiotherapy. In this study, we retrospectively analyzed safety and effectiveness of CIRT for peripheral lung tumors in patients with severe pulmonary dysfunction. Material and Methods Between April 2014 and December 2016, total of 115 peripheral lung tumors, including primary lung tumors and solitary metastatic lung tumors, were treated with CIRT. Of these patients, those who were severely