ESTRO 2020 Abstract book

S274 ESTRO 2020

that might be used as an alternative to other local treatments in highly selected patients.

Proffered Papers: Proffered papers 24 : Lymphoma

OC-0458 Local tumour control after mediastinal radiotherapy with DIBH and small CTV-PTV margin O. Hafez 1,2 , J. Brady 1 , H. Attallah 3 , N.G. Mikhaeel 1,4 1 Guy’s & St. Thomas’ NHS Foundation Trust, Department of Clinical Oncology, London, United Kingdom ; 2 Helwan University, Department of Clinical oncology, Cairo, Egypt ; 3 International Medical Centre, Departement of Radiation Oncology, Cairo, Egypt ; 4 School of Cancer and Pharmaceutical Sciences- Faculty of Life Sciences and Medicine, King’s College London, London, United Kingdom Purpose or Objective Mediastinal Radiotherapy with deep-inspiration breath hold (DIBH) improves the sparing of normal tissues particularly the heart and lungs 1 . Two factors contribute to the sparing effect; the displacement of organs-at-risk (OARs) from the target and the ability to reduce the margin which is normally added to CTV to account for breathing motion. The aim of the study was to evaluate local control rate and sites of relapse with the use of smaller margin to ascertain if the smaller margin is safe. Material and Methods We conducted a retrospective review of mediastinal lymphoma cases treated with DIBH in our centre. Inclusion criteria were histologically confirmed lymphoma, treatment with DIBH and 0.5 cm CTV-PTV marginas previously published 1 . Treatment techniques included Butterfly-VMAT (mostly), full-arc VMAT and 3D conformal. Daily CBCT was routinely performed in all cases. We collected data on occurrence of relapse, its time and sites. We reviewed scans showing relapse and correlated sites of relapse to CTV & PTV. Relapse was classified as a) in-volume relapse : defined as relapsed disease entirely within treated PTV, b) out-of- volume relapse : defined as relapsed disease which is at least 1 cm away from treated PTV, c) marginal relapse : defined as local relapse which is not entirely within treated PTV but within ≤1 cm from PTV edge. Results We found 91 cases of thoracic lymphoma who were treated with DIBH.3 patients were excluded due to larger CTV to PTV margin (0.8cm in 2 cases and 1cm in 1 case). We included 88 patients (56 females) with a median age 34 years (range 21-73). The median follow up of patients without relapse was 17 months (1.2 – 49.5). 4 cases relapsed with time to relapse of 0, 0.4, 3.0, 8.8 months. Table 1 shows disease and treatment characteristics. Only 1 patient (with chemotherapy refractory double-hit lymphoma) relapsed “in-volume” giving a local control rate of 98.8%. There were no “marginal” relapses (within 1 cm from PTV margin). 3 cases relapsed “out-of-volume” in the following sites: supraclavicular fossa (HL), abdomen (MZL)and leg (DLBCL). 2 year PFS is 95.0% and 2-year OS is 98.9%.

Conclusion Using a small CTV-PTV margin with DIBH and daily CBCT was safe and did not result in any marginal recurrences in this series. Reference: 1 Starke A et al. Radiother oncol 2018;129(3):449 OC-0459 Clinical outcomes in Hodgkin Lymphoma patients treated with IMRT-VMAT according to ISRT principles. M. Levis 1 , V. De Luca 1 , S. Bartoncini 1 , E. Orlandi 1 , G.C. Iorio 1 , R. Parise 1 , C. Palladino 1 , C. Cavallin 1 , B. Botto 2 , F.R. Giglioli 3 , C. Fiandra 1 , U. Ricardi 1 1 University of Torino, Oncology, Torino, Italy ; 2 A.O.U. Città della Salute e della Scienza, Haematology, Torino, Italy ; 3 A.O.U. Città della Salute e della Scienza, Medical Physics Unit, Torino, Italy Purpose or Objective Hodgkin Lymphoma (HL) is a highly curable hematologic malignancy, with long term survival rates >80%. Late effects as second cancers or cardiotoxicity represent an important issue in such long-surviving patients. Modern radiation therapy (RT) techniques allow to irradiate progressively smaller volumes with highly conformal dose distribution, thus obtaining a better sparing of the organs at risk located in the close proximity of the target. On the other hand, there is still a lack of evidence on the long- term efficacy of these strategies, for the potentially higher risk of relapse related to the reduction of radiated volumes. In this study, we aimed to evaluate the long term clinical outcomes in terms of overall survival (OS) and relapse free survival (RFS) in a cohort of HL patients treated with modern RT. Planning was thus pursued with high dose gradient technical solutions (such as intensity modulated radiotherapy –IMRT– or volumetric modulated arc therapy –VMAT), and target volumes were contoured according to Involved Site RT (ISRT) or Involved Node RT (INRT) principles, as described by the ILROG guidelines for HL. Material and Methods We retrospectively evaluated 213 consecutive HL patients (age 31, range 15-77) treated at our Institution between 2010 and 2018. Most patients had Stage I (13%) or stage II disease (74.6%), with a high prevalence of bulky lesions at diagnosis in the overall population (109 patients, 51%). Forty-three patients (20%) were staged early favorable (EF), 120 (56%) early unfavorable (EU) and 50 (24%) advanced (ADV) according to the German Hodgkin Study Group (GHSG) classification system. All patients received a combined-modality treatment, with chemotherapy (CT) followed by RT. Median prescription dose to the PTV was 30 Gy (range 20-40). The target was delineated according