Abstract book - ESTRO meets Asia

S13 ESTRO meets Asia 2018

6MV UFB IMRT plans need 15-60% more MU’s required due to generate a better plan as compared to 6MV FB IMRT plan. However 6MV UFB IMRT plan reduce the BOT of 32- 50% in comparison to 6MV FB IMRT, due to removal of flattening filter, dose rate of UFB were 2.3 times higher than FB. Conclusion The 6MV UFB IMRT plans dosimetrically advantage to different tumour sites, did not find any major variation in target coverage. UFB is innovative technology offers shorter treatment duration may allows faster delivery, reduce the patient waiting time, institution may increase the workload to treat number of patients treated on a unit. Lesser low dose volume effect on non tumour tissue will help to reduce the secondary cancer risk. PV-030 Comparison of Hybrid-VMAT technique using two treatment planning systems for esophageal cancer M. Miyazaki 1 , Y. Ueda 1 , S. Ohira 1 , M. Isono 1 , A. Masaoka 1 , S. Inui 1 , T. Teshima 1 1 Osaka International Cancer Institute, Radiation Oncology, Osaka, Japan Purpose or Objective Volumetric arc therapy (VMAT) has been shown to give better conformity and sparing of organ-at-risk (OAR). However, VMAT may also produce a large volume of low dose region in the surrounding normal tissue. In esophageal cancer treatment, such wide distribution of low dose region to the surrounding normal lung parenchyma can be harmful to the patient. To reduce the volume of normal lung that receives low dose, a Hybrid- VMAT (h-VMAT) technique is used. This technique combines static and VMAT beams treated concurrently. The purpose of this study is to compare dosimetric parameters of three h-VMAT technique using two treatment planning systems for esophageal cancer patients. Material and Methods Subjects were 11 cases of esophageal cancer treated with h-VMAT at our institute between 2017 and 2018. The two treatment planning systems were RayStation (RS) version 4.7.4.4 (RaySearch Medical Laboratories AB, Stockholm, Sweden) and Eclipse (EC) version 13.7.29 (Varian, palo Alto, CA). In all cases, high-risk planning target volume (PTVb) was based on the primary tumor and clinical lymph node metastases, while PTVelective (PTVel) included elective dose areas. The Dmean was defined as the prescribed dose. The prescription doses were 40 Gy to PTVel in 20 fractions, followed by 20 Gy in 10 fractions to PTVb. The prescription dose was delivered 50% with VMAT, and 50 % were delivered in antero-posterior (A-P) directions. In clinical treatment planning for esophageal cancer, dose delivery was the A-P directions and using avoidance sectors (240-300° and 60-120°), concurrently (ECcut_plan). For compare with this technique, using two treatment planning systems, dose delivery was the A-P directions and two arcs, concurrently (RS_plan and EC_plan). The h-VMAT planning of RS_plan is recalculating the anisotropic analytical algorithm of Eclipse. PTV coverage, protection on OAR (lung, heart, spinalcord) and dose rate of three planning techniques were compared. Results 8The average volume PTVel and PTVb were 532.5±171.5 cc and 225.7±101.1 cc. All plans had sufficient PTV coverage and no significant differences were observed, except D50 of PTVel and PTVb (RS_plan vs EC_plan : p<0.05, RS vs ECcut_plan : p<0.01). Conformity Index (CI)

of RS_plan was significantly higher for EC_plan and ECcut_plan. Homogeneity Index (HI) was not significantly different for all plans. The V5, V13, V20, mean of Lung- PTVel were lowest in RS_plan (V5 56.4±9.9%, V13 18.8±5.6%, V20 11.9±4.2%, mean 9.6±1.9%). The V40 of heart was lowest in EC_plan (22.9±17.6%). The Dmax of spinal cord was not significantly different for all plans. The average dose rate (MU/min) were lowest for EC_plan (RS_plan 147.8±114.0, EC_plan 136.6±41.2, ECcut=plan 216.7±47.4). The minimum of dose rate (MU/min) were lowest for RS_plan (49.5 MU/min).

Conclusion The h-VMAT planning using RS_plan is an approach for resolving reduction low dose area of lung for the treatment of esophageal cancer. PV-031 Dosimetric error factor in radiosurgery for brain mets using the single isocenter technique Y. Sonoda 1 , M. Akimoto 2 , S. Itasaka 2 , K. Kondou 1 1 Kurashiki Central Hospital, Radiological Technology, Kurashiki, Japan 2 Kurashiki Central Hospital, Radiation Oncology, Kurashiki, Japan Purpose or Objective Brain stereotactic radiosurgery (SRS) using the single isocenter technique (SIDCA) can simultaneously treat multiple targets, but there is concern about dosimetric errors due to off-axis irradiation. In this study, we verified the impact of the isocenter (IC) – planning target volume (PTV) distance and MLC leaf width to dosimetric errors in SIDCA plans were created with Elements (BRAINLAB) for 10 patients with multiple brain metastases (a total number of PTV: 31). Dose calculation algorithm was PBC and the calculation grid size was 1.0 mm. Treatments equipment was True Beam STx (Varian) integrated with multi-leaf collimators which have their leaf width of 2.5 mm at a central region and 5.0 mm at the other regions. A dome- shape in-house developed phantom which consisted of ABS plastic was set to PTV center position and absolute dose was measured in each field and a total fields of the SIDCA plans using CC 01 micro chamber (PTW) inserted to the center of the phantom. Then, the measured absolute dose were compared with the planned dose and the relationship between IC – -PTV distance and the dose difference was verified in total fields results. In addition, the dose difference in each field was verified in two groups: with or without 5.0 mm leaf width MLCs. Results The average ± standard deviation (SD) of the errorsdifference between measured and planned dose in total fields was -3.90 ± 1.94%, and figure shows the relationship of IC – PTV distance and dosimetric errors. Tthe errors weredifference was constant under change of IC – -PTV distance. In each field verification, the average ± SDs of the difference errors were -3.38 ± 2.08% and -3.12 brain SRS with SIDCA. Material and Methods