ESTRO 36 Abstract Book

S63 ESTRO 36 2017 _______________________________________________________________________________________________

We plan to proceed with a clinical trial to evaluate long- term outcomes. PV-0135 Short tangential arcs in VMAT based breast and chest wall radiotherapy planning A. Munshi 1 , B. Sarkar 1 , S. Roy 1 , T. Ganesh 1 , B.K. Mohanti 1 1 Fortis Memorial Research institute, Radiation Oncology, Haryana, India Purpose or Objective The study aimed to analyze partial tangential arc Volumetric Modulated Arc Therapy (VMAT) treatment planning and delivery, including analyzing the cardiac and contralateral breast doses resulting from this technique. Material and Methods A total of 104 consecutively treated breast cancer (conservation as well as mastectomy) patients were taken for this dosimetric study. All patients were planned using partial arc volumetric modulated arc therapy (VMAT) in the Monaco treatment planning system (TPS) using two partial arc beams. All patients were divided into seven different categories: 1) All the patients in the study (OVERALL), 2) Left sided whole breast and chest wall patients (LWBCW), 3) Left chest wall patients (LCW), 4) Left whole breast patients (LWB), 5) Right sided whole breast and chest wall patients (RWBCW), 6) Right chest wall (RCW) patients, and 7) Right whole breast (RWB) patients. We evaluated each treatment plan for PTV coverage and doses to OARs. SPSSversion 16.0 software was used for statistical analysis. Results There were 62 left sided and 42 right sided breast cancer patients in the overall analysis. The percentage of PTV volume receiving 95% of the prescription dose (PTV V95%, mean±SD) varied in the range of 91.2±5.2% to 94.8±2.1 with mean dose of 92.4±5.2% for all cases. The (mean ±SD) cardiac dose for all the patients was 289±23 cGy. The (mean±SD) cardiac doses were higher for left sided patients (424±33.8 cGy) as compared to right sided patients (123.9 ± 80 cGy) (p<0.001). Cardiac mean doses were higher with arc angles> 30 degrees versus 30 degrees (324.5±247.1 cGy versus 234.4±188.4 cGy) (p= 0.001). Similarly contralateral breast mean dose was higher with arc angles > 30 degrees versus 30 degrees (126±115 cGy vs 88.6±76.1 cGy) (p =0.001). However cardiac V20Gy, V30Gy and V40Gy did not exhibit any statistical difference between the two groups (p= 0.26, 0.057 and 0.054 respectively). Conclusion This is the first large study of its kind that assesses the dosimetric outcome of tangential partial arc VMAT treatments in a large group of mastectomy and breast conservation patients. Our study demonstrates the efficacy of this technique in dose coverage of PTV as well as in minimizing dose to OARs. Further, based on our results, we conclude that the arc length for the bi- tangential arcs should be 30 ⁰ since it helps to achieve the most optimal balance between target coverage and acceptable OAR doses. PV-0136 Linear energy transfer in normal tissues in spot scanning proton therapy of pro state cancer J. Pedersen 1 , J. BB Petersen 1 , C. H. Stokkevåg 2 , K. S. Ytre-Hauge 3 , O. Casares -Magaz 1 , N. Mendenhall 4 , L. P. Muren 1 1 Aarhus University Hospital, Department of Medical Physics, Aarhus C, Denmark 2 Haukeland University Hospital, Department of Oncology and Medical Physics, Bergen, Norway 3 University of Bergen, Department of Physics and Technology, Bergen, Norway 4 University of Florida Proton Therapy Institute, Department of Radiation Oncology, Gainesville- FL, USA

Conclusion The CTV and/or surrogate target structures in recurrent rectal cancer are visible on all CBCTs from our on-board imaging system, enabling volumetric image-guided adaptive strategies. A 5 mm margin was found to be sufficient to account for the deformations of the target in the majority of treatment fractions; there is therefore a considerable potential for reduction of the treated (normal tissue) volume compared to current wide margins. PV-0134 Isotoxic stereotactic radiotherapy for central pelvic recurrence in gynecological cancer M. Llewelyn 1 , A. Taylor 1 1 The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, Clinical Oncology, Sutton, United Kingdom Purpose or Objective Radical radiotherapy is the treatment of choice for central pelvic recurrence in gynaecological cancer. Following whole pelvic radiotherapy, a high dose boost is given to macroscopic disease. When brachytherapy is not feasible, local control with EBRT alone is only 30-50%. Stereotactic radiotherapy offers potential for dose escalation to improve outcomes. Cumulative OAR dose tolerances are internationally established for intrauterine brachytherapy, and similar principles can be applied with SBRT. This can be delivered with Cyberknife or linear accelerator VMAT, and the GTV-PTV margin depends on whether real-time motion tracking of tumour is utilised. The aims were to compare dose escalation options with the two stereotactic techniques and the impact of variable GTV-PTV margins. Material and Methods The scans of 10 patients with central pelvic recurrence were used for comparison of techniques, delivering EBRT 45 Gy in 25 fractions to pelvis followed by a SBRT boost. Cumulative dose limits for bowel, bladder and rectum were developed using GEC-ESTRO guidelines. Cyberknife and VMAT SBRT plans were produced: initially 20 Gy in 5 fractions with GTV-PTV margins of 3mm, 5mm and 7mm. Plans were normalized for 95% coverage by prescription isodose and Dmax 125-140%. Dose was then escalated or de-escalated in 2.5 Gy increments until the OAR dose limits were exceeded. The highest dose level meeting OAR criteria was compared between techniques for each GTV-PTV margin, with assessment of boost dose and total cumulative dose including the phase one EBRT (EQD2-10). Results With 20 Gy in 5 fractions and 5mm margin, mean GTV dose with Cyberknife was 23.0 Gy (total 72 Gy) and VMAT-SBRT 24.3 Gy (74.1 Gy). Conformity index was 1.1 vs 1.2 and dose drop off Cyberknife 5.0, VMAT 4.5. Using isotoxic planning to OAR tolerances, CK 3mm was 23.8 Gy (total 73.3 Gy), VMAT 3mm using equivalent prescription 25.7 Gy (76.5 Gy) and highest deliverable with VMAT 3mm 28.7 Gy (81.6 Gy). With 5mm margin, CK 21.6 Gy (69.8 Gy), VMAT 5mm equivalent dose 22.6 Gy (71.3 Gy) and highest deliverable 26.1 Gy (77.2 Gy), while a 7mm margin with VMAT is 24.0 Gy (73.6 Gy). Conclusion SBRT can significantly increase total dose to GTV compared to conventional radiotherapy techniques. With an isotoxic approach VMAT-SBRT can deliver higher doses to GTV than Cyberknife, even when a larger GTV-PTV margin is used to allow for the lack of real time tracking.