ESTRO 36 Abstract Book

S445 ESTRO 36 _______________________________________________________________________________________________

Material and Methods 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: 7-field 6MV IMRT for a conventional Elekta Agility linac (Elekta AB, Stockholm, Sweden), 6MV FFF single 360° arc VMAT using Pinnacle 9.10 (Philips Radiation Oncology Systems, Fitchburg, WI) for a non-MRL and CyberKnife treatment using Multiplan (Accuray inc, Sunnyvale, CA). Plans were acceptable if the 16 dose constraints of the PACE trial (NCT01584258) were achieved, without a major variation to the protocol. Results

Clinically acceptable 7-field IMRT MRL plans (see Figure 1) were achieved in all ten patients. Clinically acceptable plans were also achieved for all ten patients using 9-field IMRT, non-MRL 7-field IMRT, non-MRL VMAT and CyberKnife treatment. Clinically acceptable 5-field IMRT MRL plans were only possible in seven patients. Table 1 summarises the number of exceeded constraints, mean rectal doses and mean bladder V37Gy for each plan type. Given the small patient group, exploratory ANOVA analyses were undertaken for the number of co nstraints missed, the rectum D1cc and the two most challenging constraints to achieve- rectum V36Gy and bladder V37Gy. For the MRL, 5-field IMRT MRL plans performed significantly worse in all these analyses compared to 7- field IMRT. 7-field IMRT MRL plans had significantly lower rectal doses compared to CyberKnife plans. No significant differences were seen between 9-field IMRT MRL plans and non-MRL VMAT plans compared to 7-field IMRT. Conclusion MRL IMRT plans for prostate SBRT achieved the PACE trial constraints in all patients with 9-field appearing similar to 7-field IMRT. 5-field IMRT in this set-up appears inferior for the MRL. All platforms could produce clinically acceptable plans. Further work is needed for dosimetric validation and feasibility of MRL delivery. PO-0829 Robustness of IMRT and VMAT for interfraction motion in locoregional breast irradiation R. Canters 1 , M. Kunze-Busch 1 , P. Van Kollenburg 1 , M. Kusters 1 , P. Poortmans 1 , R. Monshouwer 1 1 Radboud University Medical Center, Radiation oncology, Nijmegen, The Netherlands Purpose or Objective Conventional techniques for locoregional breast irradiation using field abutment are challenging, even more in combination with breath-hold irradiation and with hypofractionation, since over- or underdosage may occur more consistently in the abutment region in these circumstances. IMRT and VMAT techniques are likely to