Jun Tomihara1,2, Jun Takatsu3, Satoru Sugimoto3, Naoto Shikama3, Keisuke Sasai3. 1. Department of Radiation Oncology, Graduate School of Medicine, Juntendo University, Tokyo, Japan. 2. Department of Radiology, Juntendo University Hospital, Tokyo, Japan. 3. Department of Radiation Oncology, Juntendo University, Tokyo, Japan.
Abstract
PURPOSE: We investigated the immobilization accuracy of a new type of thermoplastic mask-the Double Shell Positioning System (DSPS)-in terms of geometry and dose delivery. METHODS: Thirty-one consecutive patients with 1-5 brain metastases treated with stereotactic radiotherapy (SRT) were selected and divided into two groups. Patients were divided into two groups. One group of patients was immobilized by the DSPS (n = 9). Another group of patients was immobilized by a combination of the DSPS and a mouthpiece (n = 22). Patient repositioning was performed with cone beam computed tomography (CBCT) and six-degree of freedom couch. Additionally, CBCT images were acquired before and after treatment. Registration errors were analyzed with off-line review. The inter- and intrafractional setup errors, and planning target volume (PTV) margin were also calculated. Delivered doses were calculated by shifting the isocenter according to inter- and intrafractional setup errors. Dose differences of GTV D99% were compared between planned and delivered doses against the modified PTV margin of 1 mm. RESULTS: Interfractional setup errors associated with the mouthpiece group were significantly smaller than the translation errors in another group (p = 0.03). Intrafractional setup errors for the two groups were almost the same in all directions. PTV margins were 0.89 mm, 0.75 mm, and 0.90 mm for the DSPS combined with the mouthpiece in lateral, vertical, and longitudinal directions, respectively. Similarly, PTV margins were 1.20 mm, 0.72 mm, and 1.37 mm for the DSPS in the lateral, vertical, and longitudinal directions, respectively. Dose differences between planned and delivered doses were small enough to be within 1% for both groups. CONCLUSIONS: The geometric and dosimetric assessments revealed that the DSPS provides sufficient immobilization accuracy. Higher accuracy can be expected when the immobilization is combined with the use of a mouthpiece.
PURPOSE: We investigated the immobilization accuracy of a new type of thermoplastic mask-the Double Shell Positioning System (DSPS)-in terms of geometry and dose delivery. METHODS: Thirty-one consecutive patients with 1-5 brain metastases treated with stereotactic radiotherapy (SRT) were selected and divided into two groups. Patients were divided into two groups. One group of patients was immobilized by the DSPS (n = 9). Another group of patients was immobilized by a combination of the DSPS and a mouthpiece (n = 22). Patient repositioning was performed with cone beam computed tomography (CBCT) and six-degree of freedom couch. Additionally, CBCT images were acquired before and after treatment. Registration errors were analyzed with off-line review. The inter- and intrafractional setup errors, and planning target volume (PTV) margin were also calculated. Delivered doses were calculated by shifting the isocenter according to inter- and intrafractional setup errors. Dose differences of GTV D99% were compared between planned and delivered doses against the modified PTV margin of 1 mm. RESULTS: Interfractional setup errors associated with the mouthpiece group were significantly smaller than the translation errors in another group (p = 0.03). Intrafractional setup errors for the two groups were almost the same in all directions. PTV margins were 0.89 mm, 0.75 mm, and 0.90 mm for the DSPS combined with the mouthpiece in lateral, vertical, and longitudinal directions, respectively. Similarly, PTV margins were 1.20 mm, 0.72 mm, and 1.37 mm for the DSPS in the lateral, vertical, and longitudinal directions, respectively. Dose differences between planned and delivered doses were small enough to be within 1% for both groups. CONCLUSIONS: The geometric and dosimetric assessments revealed that the DSPS provides sufficient immobilization accuracy. Higher accuracy can be expected when the immobilization is combined with the use of a mouthpiece.
Authors: Daniel Reitz; Johannes Muecke; Vanessa da Silva Mendes; Guillaume Landry; Michael Reiner; Maximilian Niyazi; Claus Belka; Philipp Freislederer; Stefanie Corradini Journal: Phys Imaging Radiat Oncol Date: 2022-07-25