ESTRO 2021 Abstract Book

S1667

ESTRO 2021

Purpose or Objective To assess the dosimetric accuracy of SBRT plans calculated directly on CBCT images. Materials and Methods

Prostate SBRT is performed using an online adaptive method implemented in our department. By using the CBCT scan of each session, the “plan of the day” is computed in real time while the patient is lying on the treatment couch. The AAA algorithm of the Eclipse TPS was used to compute adaptive IMRT plans with 6 MV beams from a Varian Clinac 2100 CD. A specific CBCT calibration curve (electron density vs. CT number) for CBCT images was used in Eclipse. Monte Carlo (MC) codes are the gold standard for computing radiation doses. The accuracy of the dose distribution planned on CBCT images was investigated using the PRIMO MC software v 0.3.64.1800 (www.primoproject.net). PRIMO used the phase-space files (v. 2, Feb. 27, 2013) provided by Varian for 6 MV photon beams for a Truebeam linac on the website www.myvarian.com/montecarlo. According to the PO-1393 ESTRO 2020, this PSF files can be used for a Varian Clinac 2100 CD as well. Twenty seven CBCT-based adaptive plans were retrospectively recalculated using the PRIMO MC software. In order to perform an independent verification of the CBCT calibration, a generic mass density vs CT number was used in PRIMO. The 3D gamma analysis tool of the PRIMO software was used to compare the Eclipse and MC dose calculations. The global gamma passing rates (GPRs) for 3%/2 mm were computed for the patient body (with a dose threshold of 10% of the maximum dose), PTV, rectum, bladder and femoral heads. Results • The average 3D GPRs were (mean ± SD): 99.9% ± 0.4%, 98.9% ± 2.8%, 99.8% ± 0.7%, 99.8% ± 0.7% and 100.0% ± 0.2%, for the patient body, PTV, rectum, bladder and femoral heads, respectively. • The statistical uncertainty (k= 2) of the MC dose distributions was 1.3% ± 0.2%. Conclusion Excellent accuracy of the CBCT-based SBRT plans computed with the Eclipse TPS was found. CBCT images are adequate to be used for online adaptive SBRT dosimetry. PO-1958 Parotid gland dose reduction in the hippocampus avoidance whole-brain radiotherapy H. Shimizu 1 , K. Sasaki 2 , T. Aoyama 1 , H. Tachibana 1 , Y. Koide 1 , T. Iwata 1 , T. Kitagawa 1 , T. Kodaira 1 1 Aichi Cancer Center Hospital, Radiation Oncology, Nagoya, Japan; 2 Gunma Prefectural College of Health Sciences, Graduate School of Radiological Technology, Maebashi, Japan Purpose or Objective As a selectable treatment for patients with multiple brain metastases, whole-brain radiotherapy (WBRT) is an important technique. There are two notable risk organs in the WBRT. One of them is the hippocampus, which is known to cause cognitive deterioration by irradiation. The other risk organ is the parotid gland. A recent cohort study revealed that xerostomia in the WBRT case was statistically significantly associated with the parotid gland dose. This study aimed to reduce the parotid gland dose in the hippocampus avoidance WBRT (HA-WBRT) using helical tomotherapy. Materials and Methods Ten patients who had previously undergone WBRT were selected randomly. The WB was delineated as a clinical target volume (CTV). A planning target volume (PTV) was created by adding 5-mm isotropic margins for the CTV. The hippocampus, bilateral parotid glands, and lens were delineated as high-risk organs. The hippocampus was delineated with reference to the T1-weighted magnetic resonance image. The planning organ at risk volume (PRV) of the hippocampus was defined by adding a 5-mm isotropic margin to the hippocampus. The PTV eval was defined as the volume subtracted the PRV of the hippocampus from the PTV. During the treatment planning, jaw mode was applied in two techniques ( 1.0 cm fixed jaw and 2.5 cm dynamic jaw ) per patient. To reduce the bilateral parotid glands dose, they were set as directional blocks (DBs), which is a function of the specific treatment planning system for the dose reduction of risk organs. To compare the plan quality, the standard HA-WBRT plan which was not added the parotid gland onto the DB mode was also designed. The prescribed dose was set as 30 Gy to the 95% volume of PTV eval , and it was delivered in 10 fractions. Results Compared to the standard HA-WBRT plan, the mean parotid gland doses for the 1.0 cm fixed jaw and the 2.5 cm dynamic jaw could be reduced from 8.7 ± 1.6 Gy to 2.5 ± 0.3 Gy and from 11.6 ± 2.1 Gy to 2.7 ± 0.3 Gy without extending the delivery time by adding the parotid gland on the DB mode. The difference of dosimetric parameters for the PTV eval and risk organs between the plans by the 1.0 cm fixed jaw and the 2.5 cm dynamic jaw was small. In addition, the delivery times in the 1.0 cm fixed jaw and the 2.5 cm dynamic jaw were approximately 14 min and 6 min, respectively. Conclusion In conclusion, by applying the DB mode of the helical tomotherapy to the parotid gland, the parotid gland dose could be greatly reduced without extending the delivery time. In addition, the plan by using the 2.5 cm dynamic jaw was more practical because that could significantly shorten the delivery time (about 60% shorter) than the 1.0 cm fixed jaw. PO-1959 Intensity modulated proton therapy plans for left-sided breast cancer patients with pectus excavatum A. Gullhaug 1 , D. Høynes 2 , V. Vingelsgård 2 , D. Tor-Ove 2 , J. Senum 2 , J.G. Svestad 3 1 oslo Metropolitan University, Department of life sciences and health, OSLO, Norway; 2 Oslo Metropolitan University, Department of life sciences and health, Oslo, Norway; 3 Oslo University Hospital, Department of Medicals Physics, Oslo, Norway Purpose or Objective Pectus excavatum (PE) is an anatomical deformity of the thoracic wall that may complicate radiation therapy (RT) for patients with left-sided breast cancer (BC). When using photon treatment techniques, reaching adequate target coverage without unacceptable high doses to nearby organs at risk (OAR) is not always achievable. Therefore, investigating the advantages of alternative treatment techniques is of interest. With proton treatment, the dose is deposited in a more defined area. The purpose of this study was to compare three-dimensional conformal RT (3D CRT), hybrid-Volumetric Modulated Arc Therapy (VMAT) and Intensity Modulated Proton Therapy (IMPT) treatment plans focusing on target volume dose coverage and dose to heart and left lung.