ESTRO 36 Abstract Book

S185 ESTRO 36 _______________________________________________________________________________________________

The uni- and multi-variate analysis showed statistical significance for all parameters except lesion location (table 1).

The high coefficient associated to the peripheral character is probably due to the EPL un-modelled lateral electron equilibrium caused by the prevalent presence of low density lung tissues surrounding the peripheral lesion, thus greatly impacting dose calculation differences. Based on the multivariate analysis predictive nomogram was generated (R²=0.58, Figure 1).

Dose to OARs calculated with EPL and MC showed strong linear correlation (R²=0.99-1.00). The dose constraints decreased by 1% in the heart (D10cc), great vessels (D10cc) and spinal cord (D0.25cc), 2% in the oesophagus (D5cc), 5% in the ribs (D5cc) and 16% in the trachea (D4cc). . Conclusion The differences between MC and EPL are significantly impacted by dose, tracking, location and the volume of the lesion. Predictive nomogram helps to estimate the differences on GTV D50. EPL to MC OAR dose tolerance limit proved to have a strong linear correlation with conversion factors ranging from 0.84 to 0.99. Based on our model, re-prescription value can be estimated and, if required, used to further restrict the constraints on the OARs during EPL optimization. OC-0350 ExacTrac®-based Fractionated Radiosurgery (fSRS) of Choroidal Melanoma (CM) M. Wösle 1 , P. Goldschmidt 1 , G. Lohm 1 , L. Grajewski 2 , L. Krause 2 , I. Ciernik 1 1 Dessau City Hospital, Radiation Oncology, Dessau, Germany 2 Dessau City Hospital, Ophthalmology, Dessau, Germany Purpose or Objective Proton therapy (PT) has been a standard for treating choroidal melanomas for the last three decades. However, PT is not easily available for the majority of patients. Advances in photon therapy allow highly conformal dose delivery while sparing normal tissue. However, fSRS for small moving target volumes, such as CM, has remained a challenge. Material and Methods Since December 2014, we treated 40 patients with central choroidal tumors with fSRS. Radiotherapy plans were obtained with iPlan® RT (Version 4.5.3, Brainlab, Feldkirchen, D) after placement of 4 Tantalum clips (Altomed Ltd., U.K.) a week prior the planning CT/MRT. Thirteen cases have been reanalysed: ten treated with 10 Gy x 5 for CM, one case of haemangioma treated with 14,5 Gy in one fraction, and 2 cases of breast cancer metastasis treated with 6 Gy x 5 and 4. iPlan® RT uses HybridArc™ as field configuration with three to six dynamic conformal arcs complemented with five to seven dynamic IMRT

OC-0349 Prediction of GTV median dose differences benefit Monte Carlo re-prescription in lung SBRT D. Dechambre 1 , Z.L. Janvary 1 , N. Jansen 1 , C. Mievis 1 , P. Berkovic 1 , S. Cucchiaro 1 , V. Baart 1 , C. Ernst 1 , P. Coucke 1 , A. Gulyban 1 1 C.H.U. - Sart Tilman, Radiotherapy department, Liège, Belgium Purpose or Objective The use of Monte Carlo (MC) dose calculation algorithm for lung patients treated with stereotactic body radiotherapy (SBRT) can be challenging. Prescription in low density media and time-consuming optimization conducted CyberKnife centers to propose an equivalent path length (EPL)-to-Monte Carlo re-prescription method, for example on GTV median dose (Lacornerie T, et al. Radiat Oncol 2014;9:223). The aim of this study was to evaluate the differences between the two calculation algorithms and their impact on organs at risk (OAR) and to create a predictive model for the re-prescription. Material and Methods One hundred and twenty seven patients (with 149 lesions) were treated with CyberKnife (CK; Accuray, Sunnyvale, US) between 2010 and 2012. A high-resolution grid (512³) was used for the EPL and MC calculations (2% variance). All re-calculation from EPL to MC maintained the number of beams and their monitor units. Relative differences in GTV D50 between the two algorithms were assessed and uni/multivariate linear regression was performed using prescription dose (Gy), tracking (ITV concept if not available), location (peripheral or central) and volume (in cc) of the lesion as input parameters. Statistical significance was determined using F-test at p-value<0.05. OARs volumetric dose constraints were applied from Timmerman RD et al. (Semin Radiat Oncol 2008;18:215- 22). As tolerance limits were defined based on simple heterogeneity correction algorithm (e.g. EPL), correlation between EPL and MC OARs dose values was assessed following the work from the Rotterdam team (van der Voort van Zyp NC , et al. Radiother Oncol 2010;96:55–60). Results The observed difference (MC compared to EPL) varied from 0 % to 48% (median = 10%, standard deviation = 9%).