ESTRO 36 Abstract Book

S444 ESTRO 36 _______________________________________________________________________________________________

errors and to blindness relative to their potential degeneracies. The CI for M2 equals 88%, but 90% CI can be achieved for M2 by extending slightly the lateral PTV margin to encompass 92% of possible beam positions and 98% of possible ranges, leading to a 425% volume, thus still better than M3.

The comparison was performed in terms of dose distribution and efficiency by reporting OARs DVH, Baltas’ conformal index (COIN), Paddick’s gradient index (GI), ICRU homogeneity index (HI), integral dose to normal tissue (NTID), number of monitor units (MU) and treatment time. For both collimators, accuracy of dose calculation within heterogeneity was evaluated by delivering a typical lung treatment plan on a QUASAR Respiratory phantom (Modus Medical Inc) including a lung target insert. Calculated dose was compared with delivered dose measured by Gafchromic EBT3 films (Ashland) using a gamma index analysis with a local dose criteria of 3 % and a distance-to- agreement criteria of 2 mm. Results Results are summarized in table 1. Compared to Iris plans, MLC plans did not produce significant differences in terms of OARs sparing and dose conformality except for acoustic neuroma for which COIN was degraded by 20 % with MLC. Dose gradient was improved by using the MLC with a GI mean reduction of 18 %. MLC allowed a slight improvement of PTV dose homogeneity for brain metastasis and liver targets and lead to a NTID reduction for extra-cranial treatments. Except for liver targets, MLC plans delivered less MU than Iris plans with a mean reduction of 25 %. MLC plans lead to a treatment time reduction of 28 % in average compared to Iris plans. The comparison between calculated and measured dose in lung phantom showed a gamma passing rate of 51.6 %, 45.5 % and 98.7 % for FSPB MLC plan, RT Iris plan and MC Iris plan respectively.

Conclusion The best tradeoff between robustness and optimality was achieved through random sampling of all errors limited by the lateral conventional PTV margin and a large margin for the possible proton ranges. PO-0826 Evaluation of the new InCise MLC for Cyberknife stereotactic radiotherapy C. Limoges 1 , J. Bellec 1 , N. Delaby 1 , M. Perdrieux 1 , F. Jouyaux 1 , E. Nouhaud 2 , I. Lecouillard 2 , E. Chajon 2 , R. De Crevoisier 2,3,4 , E. Le Prisé 2 , C. Lafond 1,3,4 1 Centre Eugène Marquis, Medical Physics Department, Rennes, France 2 Centre Eugène Marquis, Radiation Oncology Department, Rennes, France 3 INSERM, U 1099, Rennes, France 4 University of Rennes1, LTSI, Rennes, France Purpose or Objective The aim of this study was to evaluate treatment planning performances of the new InCise multileaf collimator (MLC) with reference to the Iris variable circular aperture collimator for intracranial and extracranial Cyberknife stereotactic radiotherapy. Material and Methods The study was performed on a Cyberknife M6 v10.6 (Accuray). A total of 50 cases including 10 brain metastases, 10 acoustic neuromas, 10 liver targets, 10 spinal metastases and 10 prostate cases were investigated. For each case, two treatment plans were generated with TPS Multiplan v5.3 (Accuray): one plan using the InCise MLC v2 associated with the Finite Size Pencil Beam (FSPB) dose calculation algorithm and one plan using the Iris collimator associated with RayTracing (RT) or MonteCarlo (MC) dose calculation algorithm. Dose was prescribed near the 80 % isodose and normalized to obtain the same PTV coverage at ± 0.5 % for both plans.

Conclusion The use of the InCise MLC for Cyberknife st ereotactic radiotherapy allows a significant reduction of MU and treatment time compared to Iris collimator while maintaining a high degree of conformality and a steep dose gradient. However, circular collimators should be still preferred for treatment of small targets like acoustic neuromas due to their smaller field size capability. The use of the InCise MLC for lung targets treatment should not be recommended currently due to the absence of a type B dose calculation algorithm. PO-0827 Robustness Evaluation of Head and Neck Treatment with Proton Pencil Beam Scanning Technique H. Lin 1 , H. Liu 1 , X. Liang 1 , A. Lin 1 , P. Ahn 1 , H. Zhai 1 , M. Kirk 1 , A. Kassaee 1 , J. McDonough 1 , S. Both 2 1 University of Pennsylvania, Radiation Oncology, Philadelphia, USA 2 Memorial Sloan Kettering Cancer Center, Medical Physics, New York, USA Purpose or Objective To evaluate the treatment robustness of two novel pencil beam scanning proton therapy (PBS PT) beam arrangements relative to volumetric arc therapy (VMAT) for oropharynx head and neck (HN) cancer patients.