ESTRO 37 Abstract book

S1048

ESTRO 37

three actual OAR/CTVs representing the actual position of the structures. Using Monte-Carlo simulation with 100 random uncertainties for each type of plan and uncertainty, we estimated the probability of constraint violation. Results All OAR- and PRV-plans met the dose constraints. Adequate PTV coverage was achieved in all plans (average, OAR-plan: 94.6%; PRV-plan: 94.8%). Coverage of actual CTVs was adequate in OAR plans and PRV plans (97.8±2.1% vs. 97.3±1.6%). Conformity index (0.978 vs. 0.972, p<.001) and homogeneity index (0.150 vs. 0.200, p<.001) were better in OAR-plan; however, the maximal dose to OAR was greater in OAR-plan (SC: 10.89Gy vs. 9.32Gy, p=.001; CE: 13.53Gy vs. 12.32Gy, p=.04). Dose constraints were violated for at least one of the three actual OARs in 5 out of 22 OAR-plans while all actual OARs met the constraints in all PRV-plans. In Monte-Carlo simulation, violation probability of SC and CE was >1% in 73% (8/11) and 29% (4/14) of OAR-plans, respectively; however, all PRV-plans resulted in violation probability ≤1% with any type of uncertainty. Conclusion Spinal SBRT without PRV margin may risk violating constraints for neural OARs due to geometric uncertainties. Introduction of 1mm PRV margin seems to guarantee safe treatment as well as adequate CTV coverage. Therefore, 1mm PRV margin should be routinely applied in spinal SBRT planning. EP-1926 Planning approaches and impact of breathing motion for proton radiotherapy in the mediastinum C. Cases 1 , A.M. Flejmer 2 , A. Dasu 3 1 Hospital de la Santa Creu i Sant Pau, Radiofisica i Radioprotecció, Barcelona, Spain 2 Linköping University, Department of Clinical and Experimental Medicine, Linköping, Sweden 3 The Skandion Clinic, Medical Physics, Uppsala, Sweden Purpose or Objective Proton Pencil Beam Scanning (PBS) has the potential to improve mediastinum treatments by reducing doses to organs at risk (OAR) compared to photons. This study investigates different planning approaches with PBS to maximize target coverage, robustness and OAR sparing accounting for breathing motion. Material and Methods The study was based on CT datasets of twelve thoracic patients acquired in free-breathing (FB), breath-hold-at- inhalation (BHI) and breath-hold-at-exhalation (BHE). CTV and OAR were delineated by an experienced radiation oncologist on each dataset. The prescribed dose was 57.5 Gy (RBE) in 25 fractions of 2.3 Gy (RBE), assuming a Relative Biological Effectiveness (RBE) for protons of 1.1. Two planning approaches were used: a PTV approach with the PTV generated by expanding the CTV 7 mm in Lateral (L) and Craniocaudal (CC) directions and 1 cm in Superio-Inferior (SI) direction and a robust CTV optimization considering 4.5% range uncertainty and 4/6 mm position uncertainty in L, CC/ SI directions respectively, as suggested in the literature. Several field arrangements were compared: 1 Field (1F) technique, with the field at a gantry angle of 0° was compared with 2 Field (2F) techniques (using both single field uniform dose - SFUD and intensity modulated proton therapy - IMPT) with symmetrical gantry positions between 30° and 330° (depending on patient anatomy). PBS plans were designed for the FB dataset and then recalculated to BHI and BHE to assess robustness against breathing motion. Comparisons were carried out for the following DVH parameters: • CTV coverage: D 98% , D 2% , D mean • CTV robustness: D 98%,w , D 2%,w , where w indicates worst-case-scenario.

Figure 1. Dose distribution in the transversal plane at the isocenter

Table 1. Evaluated dosimetric parameters

Conclusion A new method of irradiation was developed using the VMAT technique for the anal cancer patients, which reduces the dose delivered to the bladder to an average of 25 Gy. The average radiotherapy fraction time taken to deliver a VMATnew treatment plan is 5 minutes. Using the developed VMAT new method and the IMRT technique for the treatment of anal canal cancer, it is possible to achieve a 3Gy reduction in dose, delivered to the bladder in comparision to the standard rotational irradiation of VMAT and 13Gy compared to using 3D CRT. With equal doses to the bladder for VMAT new and IMRT, the average time of the radiotherapy session using the developed VMAT new method is 3 times less. Using the developed VMAT new method allows to increase the homogeneity index of the radiation plan by 16% in comparison with the standard VMAT technique (2 arcs), 20% with IMRT and 33% with 3D CRT. EP-1925 Dosimetric implication of planning organ at risk volumes in stereotactic body radiotherapy of spine S.H. Jeon 1 , J.H. Kim 1 , S. Son 1 , S.Y. Park 1 , J.I. Kim 1 , J.M. Park 1 1 Seoul National University Hospital, Radiation Oncology, SEOUL, Korea Republic of Purpose or Objective To investigate the impact of planning organ at risk volumes (PRV) of neural organ at risk (OAR), i.e. spinal cord (SC) and cauda equina (CE), on the safety and efficacy in stereotactic body radiotherapy (SBRT) of spine. Material and Methods Pairs of SBRT plans were generated using either no PRV (OAR-plans) or 1mm PRV margin (PRV-plans) for 22 spinal lesions. An 1 mm planning target volume (PTV) margin was added to clinical target volume (CTV). All plans were generated using a volumetric-modulated arc therapy technique. A single dose of 18Gy was prescribed to cover ≤95% of PTV and dose constraints from RTOG 0631 trial were used. Three types of uncertainties, i.e. pre- treatment residual error, post-treatment residual error, and intrafraction motion, were acquired by matching of pre-/post-treatment cone-beam computed tomography to simulation computed tomography, and used to generate