ESTRO 37 Abstract book

S1066

ESTRO 37

Material and Methods We reviewed a series of 16 CP patients treated with PT to a median dose of 54 GyRBE, for progressive disease after surgical resection (n=13) or postoperatively after partial resection (n=3). Two (12.5%) patients presented RICVs 14 and 24 months (median,19) after PT. CTs and MRIs pre- and post-PT were used to contour organs at risks (OARs) as bilateral ICAs and circle of Willis. A detailed analysis of the vascular toxicity was performed for the patient’s cohort including number of fields, PTV volume, prescribed dose, dose per fraction and delivery technique (SFUD or IMPT). A preliminary quantitative analysis, in terms of dose volume histogram and dose metric, was performed computing the dose metric of each contoured volume of interests (VOIs: OARs and PTV) for all the patients. The LET distribution has been then computed for all the patients of this study, with in-house dedicated software. LET analysis was performed similarly to the dosimetric (DVH based) analysis, evaluating the LET metric (minimum, mean and maximum) of the PTV and the circle of Willis’structures (Tab. 1). Finally, evaluation of LET changes due to range uncertainties has been also computed, introducing a +/-3% variation on the nominal HU values of the planning CT.

Conclusion Quantitative dosimetric analysis was not able to show significant correlations between delivered dose, LET and RICV. However, no contralateral (asymmetric) fields’ arrangement approach seems to have an impact in the presence RICV toxicity. Asymmetric LET distributions, with high LET values at the contralateral distal edge of the PTV may contribute to RICV. EP-1959 Does library sub-categorisation improve auto- outlining accuracy in breast radiotherapy planning? C. Welgemoed 1 , E. Spezi 2 , M. Gooding 3 , D. Peressutti 3 , E. Aboagye 4 , R. McLauchlan 5 , D. Gujral 1 1 Imperial College Healthcare NHS Trust, Department of Radiotherapy, London, United Kingdom 2 Cardiff University, School of Engineering, Cardiff, United Kingdom 3 Mirada Medical Ltd, Research Department, Oxford, United Kingdom 4 Imperial College London, Department of Surgery and Cancer, London, United Kingdom 5 Imperial College Healthcare NHS Trust, Department of Medical Physics, London, United Kingdom Purpose or Objective Accurate definition of clinical treatment volumes (CTVs) and organs at risk (OARs) is essential to optimise breast radiotherapy outcomes and minimise radiation side- effects. Commercial software products have been developed for auto-outlining and have been shown to save time. In atlas-based systems, outlining accuracy has been show to improve when using atlas sub- categorisation. The aim of this project was to determine the difference in outlining accuracy, using two main atlas sub-groups (libraries) in breast radiotherapy. Material and Methods 22 Computed tomography (CT) data sets were used to retrospectively outline CTVs and OARs for patients who received radiotherapy to the left breast and nodes. Standard outlining guidelines were used for the brachial plexus, breast CTV and nodal volumes. Creation of one generic library consisting of 22 atlases and 2 main sub- groups with 3 atlas sub-categories each was performed by means of commercial software (WorkflowBox 1.4 Mirada Medical Ltd., UK). Breast separations (BS) from 22 atlases were divided into three atlas sub-categories: 0-19 cm, 20-23 cm and 24-30 cm, comprising of 6, 10 and 6 atlases

Results Quantitative dosimetric results, based on the DVH metric related with the PTV and the circle of Willis’structures, did not reveal any evident dose metric/toxicity correlation. Both patients presenting with RICV toxicities presents max LET values up to 7.9 keV/µm in the vascular structures. The fields arrangement for those two patients did not include contralateral fields (asymmetric fields arrangement). In case of this fields arrangement, the LET hot region (>60%) is localized asymmetrically in the distal range of the fields, involving (based on the size of the PTV) the side of the Willis’ structure with toxicity. Instead, a symmetric fields arrangement, shows how the highest LET regions (smaller in comparison with the asymmetric field arrangement LET distribution) are localized in the anterior/posterior region outside the PTV (Fig.1). Despite the highest max LET values, no significant differences were found the RICV between patients with and without toxicities. LET variations due to range uncertainties was estimated and up to 1.2 keV/µm, especially for patients treated with asymmetric fields’ arrangement.