ESTRO 36 Abstract Book

S440 ESTRO 36 2017 _______________________________________________________________________________________________

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.

10 HN oropharynx consecutive patients treated with PBS PT underwent prospective evaluation computer tomography scans (eCTs) during their course of treatment (average 4 eCTs per patient). The robustness of the treatment plans containing two-posterior oblique(PO) PBS fields (2PBS), contingency VMAT plans (2 arcs) and retrospectively generated 3-field PBS plans (3PBS) was evaluated against anatomy changes and residual setup uncertainties via evaluation plans generated on eCTs. 3PBS plan matched two PO fields (same to 2PBS) with an anterior field at thyroid notch level in order to treat the lower neck nodal target. Plan robustness was assessed based on the accumulated dose through deformable image and dose registration between treatment and evaluation plans using VelocityAI. The D98% dosimetric indicator of target coverage and OARs planning constraints were used to evaluate the plan robustness. Changes over 5% in target coverage, excessive cord dose and/or clinical decision triggered proton replan or the use of the VMAT contingency plan. Results The average change of D98% in the accumulated plans for 2PBS, 3PBS and VMAT were:4.1%±4.8%, -0.1%±0.8% and - 2%±3.2% for low risk CTV, -1.7±1.8%, -0.5±0.8% and - 0.73±1.2% for high risk CTV, -0.2%±0.2%, -0.1%±0.1% and -0.4%±1.3% for gross CTV respectively. The main source of coverage loss at low risk CTV level for 2PBS was found to be due to variable soft tissue deformation of the posterior neck for elderly or short neck patients leading to replanning for 2 out10 patients. OARs robustness was maintained within planning constraints. Conclusion 2PBS plans were not consistently robust relative to target coverage due to variable folding neck tissue, and therefore it should be cautiously employed for elderly and short neck patients. 3PBS was proved to be consistently robust, similarly with VMAT. PO-0828 Stereotactic body radiotherapy (SBRT) for localised prostate cancer on the MR-Linac A. Pathmanathan 1 , A. Mitchell 2 , K. Thomas 3 , D. Henderson 2 , S. Nill 1 , U. Oelfke 1 , R. Huddart 1 , N. Van As 2 , A. Tree 2 1 Institute of Cancer Research, Radiotherapy and Imaging, London, United Kingdom 2 The Royal Marsden NHS Foundation Trust, Department of Radiotherapy, London, United Kingdom 3 The Royal Marsden NHS Foundation Trust, Statistics Unit, London, United Kingdom Purpose or Objective As the estimated alpha-beta ratio for prostate cancer is low (1), moderate hypofractionation has been shown to be isoeffective (2). The MR-Linac (MRL) combines an MR scanner and linac allowing intrafractional tracking of the target (3). However, dose distributions are affected by the magnetic field (4). The first Elekta system MRL (1.5T/ 7MV) will deliver step-and-shoot intensity modulated radiotherapy (IMRT), a technique rarely used for stereotactic body radiotherapy (SBRT). This planning study assesses whether adequate dose distributions for MRL- based prostate SBRT are possible with comparison to non- MRL based planning techniques: IMRT, volumetric modulated arc therapy (VMAT) and CyberKnife. Material and Methods Using planning CT scans acquired for ten patients with localised prostate cancer, clinical target volume (CTV) was defined as prostate plus proximal 1cm of seminal vesicles. The planning target volume (PTV) was created by addition of a 5mm isotropic margin, except 3mm posteriorly. For the MRL, 5, 7 and 9-field step-and-shoot IMRT plans were created to deliver 36.25Gy in 5 fractions to the PTV with an integrated dose of 40Gy in 5 fractions to the CTV using Monaco 5.19 (research version, Elekta AB, Stockholm, Sweden). Non-MRL comparison plans included:

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. Material and Methods