Abstract Book

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ESTRO 37

radiotherapy technique. Here, we performed the dosimetric comparison using Plan Quality Metric (PQM) among 3-dimensional conformal radiotherapy (3D-CRT) and intensity-modulated radiotherapy (IMRT) plans using deep-inspiration breath hold (DIBH) or free-breathing (FB) techniques to determine the most optimal plan for gastric MALToma. Material and Methods We selected 9 patients with gastric MALToma who had both DIBH and FB planning computed tomography (CT) scans obtained at the same time point. 3D-CRT, step-and- shoot IMRT (SIMRT), volumetric-modulated arc therapy (VMAT), and Tomotherapy plans with the same prescription dose (30 Gy in 20 fractions) were made for each CT scan. Planning target volume (PTV) coverage and mean or maximum doses to the organs at risk (OARs) (heart, lungs, kidneys, bowel, spinal cord, and liver) were calculated for each plan. We generated score functions of the dosimetric goals used in our practice and obtained each scores using the PQM. Mann Whitney U-test and Kruskall Wallis test were used for statistical analysis for group-wise comparison of 3D-CRT, SIMRT, VMAT, and Tomotherapy using DIBH or FB techniques. Results All 8 plans of 9 patients (72 plans) satisfied proper PTV coverages, and all OARs doses met our dosimetric goals. Conformity index (CI) was statistically significantly higher in 3D-CRT plans (median 1.5 vs. 1.0, p<0.001), and the remaining plans showed similarly good conformity. Homogeneity index (HI) was the lowest in Tomo plans (p <0.001). The mean plan quality metric(PQM) scores of each 8 plans (3D-DIBH, 3D-FB, SIMRT-DIBH, SIMRT-FB, VMAT-DIBH, VMAT-FB, Tomo-DIBH, Tomo-FB) were 60.1, 50.9, 75.5, 66.6, 76.8, 69.3, 69.2, and 62.1, respectively (0–100 scale). VMAT-DIBH and SIMRT-DIBH were the best followed by VMAT-FB, Tomo-DIBH, SIMRT-FB, Tomo-FB, 3D-DIBH, and 3D-FB was the worst (p<0.001). 3D-DIBH showed a tendency to be inferior to IMRT-FBs especially in CI, spinal cord and kidney doses. Nevertheless, 3D- DIBH had significantly less irradiated volume compared to 3D-FB and comparable coverage, OAR doses, resulting in a considerably higher PQM score. The mean motor unit (MU) values of each 8 plans were 178, 176, 356, 385, 311, 340, 7567, and 7566, respectively. 3D plans had the advantage of reducing the MU values, however, tomotherapy plans showed exceptionally high MU values. Conclusion Modern radiotherapy plans (VMAT-DIBH, sIMRT-DIBH) were significantly more beneficial than 3D plans for gastric MALToma by saving OARs and conformity. However, we should concern about significantly increased treatment time with IMRT. Tomotherapy plans (3D or DIBH) did not show significant benefit. Although whether DIBH or IMRT is more useful in the clinical setting is unclear, VMAT-FB seems to be better than 3D-DIBH regardless of target margins. The relevance of the most practical plan to a clinical outcome should be confirmed further in a larger patient cohort. EP-1244 Patterns of failure after peri-stem cell transplant consolidation radiotherapy for relapsed lymphoma M.P. Huynh-Le 1 , M. Alhumaid 1 , C. Costello 2 , C. Mulroney 2 , P. Sanghvi 1 1 University of California San Diego, Radiation Medicine and Applied Sciences, San Diego, USA 2 University of California San Diego, Medical Oncology, San Diego, USA Purpose or Objective Lymphoma, both Hodgkin and non-Hodgkin, has become highly curable with advances in chemotherapy and radiotherapy (RT). Unfortunately patients who develop relapsed or refractory disease often require salvage with hematopoietic stem cell transplant (SCT). In these

patients, consolidation RT to sites of relapse and/or bulky disease can be used to improve disease control. The purpose of this study was to define the patterns of failure after consolidation RT in the peri-SCT setting. Material and Methods We conducted an institutional retrospective cohort study of 38 consecutively treated adult patients with Hodgkin lymphoma (HL) or non-Hodgkin lymphoma (NHL) who received consolidation RT in the peri-SCT setting (defined as either immediately pre- or post- SCT) between 2010 and 2017. Patients were included if they were ≥21 at the time of lymphoma diagnosis and were excluded if they received total body irradiation alone. Patients were evaluated for patterns of recurrence after RT, overall survival (OS), and progression free survival (PFS) after SCT using descriptive statistics, Kaplan-Meier curves, and log-rank test/Cox proportional hazard regressions. Results Ten (26%) patients had HL and 28 (74%) had NHL. The most common NHL was diffuse large B cell lymphoma (n=12, 31%). Thirteen patients (34%) were stage IV, 8 (21%) were stage III, 13 (34%) were stage II, and 4 (11%) were stage I at diagnosis. Nineteen (50%) patients had bulky disease. Twenty (52%) patients received RT pre-SCT while 18 (48%) patients received RT post-SCT. Median RT dose was 30.6 Gy (range 24 to 50.4 Gy) and 25 (66%) patients were treated with intensity modulated RT. Median follow-up after SCT was 15 months (range 1 to 59 months). Mean age at transplant was 48.2 years (range 21.7 to 73.7). The majority of patients (n=32, 84%) underwent autologous SCT while 6 (16%) patients underwent allogeneic SCT. Eight (21%) patients had relapsed or recurrent disease after SCT. Of these 8 patients, 3 had distant relapse outside of the radiation field, and 5 had combined in- and out of field failure. No patients had in-field failure alone. Median time to failure after SCT was 5 months (range 1 to 7 months). PFS after SCT was 75% at 2 years and OS after SCT was 80% at 2 years. Seven (18%) patients in the cohort died and all died due to relapse/recurrence of their lymphoma. On univariate analysis, gender, tumor stage, bulky disease, presence of B symptoms, and timing of RT (pre- or post- SCT) did not predict for recurrence after RT. Conclusion The majority of lymphoma patients in our cohort had excellent overall and progression-free survival after peri- SCT radiotherapy. However, there remains a subset of patients who recur and have poor survival outcomes after SCT despite the use of consolidation RT. In our database, disease recurrence after SCT was almost always fatal. Further research should prospectively seek strategies to improve rates of progression-free survival in patients with relapsed/refractory lymphoma. EP-1245 A systematic review of dose-effect relationship in radiotherapy for head and neck plasmacytoma C. Petit 1 , R. Mazeron 1 , A. Boros 1 , S. Rivera 1 , P. Blanchard 1 , E. Deutsch 1 1 Institut Gustave Roussy, Radiation Oncology, Villejuif, France Purpose or Objective Extramedullary plasmacytoma of the head and neck (HNPEM) is a rare disease for which there are uncertainties regarding the optimal dose of radiotherapy. Dose-effect relationship in radiotherapy has been shown with 45 Gy in a retrospective study based on 17 patients. We conducted a review of the literature to assess a dose- effect relationship for radiotherapy in the treatment of

patients with HNPEM. Material and Methods

A review of the literature was performed using different electronic databases to collect data on radiotherapy doses and control of disease (local control and evolution