[Verification of Dose Distribution in Cervical Cancer Brachytherapy Using Metal and Plastic Applicators].

PURPOSE: To validate the point-A dose and dose distribution of metal and resin applicators in comparison with those of TG-43U1. METHODS: The metal and resin applicators consisting of tandem and ovoid were modeled by the egs_brachy. The doses to point A and dose distributions considering each applicator were calculated and compared to those of TG-43U1. RESULTS: The dose to point A considering the metal applicator was 3.2% lower than that of TG-43U1, but there was no difference in the dose to point A considering the resin applicator. The dose distribution considering the metal applicator was lower than that of TG-43U1 at all calculation points, but there was no difference in the dose distribution considering the resin applicator at almost all calculation points. CONCLUSION: In this study, the dose distribution considering the metal applicator was lower than that of TG-43U1 at all calculation points, but there was no difference in the dose distribution considering the resin applicator at almost all calculation points. Therefore, TG-43U1 can accurately calculate the dose distribution when changing from the metal applicator to the resin applicator.

Multi-Institutional Study of End-to-End Dose Delivery Quality Assurance Testing for Image-Guided Brachytherapy Using a Gel Dosimeter.

PURPOSE: To quantify dose delivery errors for high-dose-rate image-guided brachytherapy (HDR-IGBT) using an independent end-to-end dose delivery quality assurance test at multiple institutions. The novelty of our study is that this is the first multi-institutional end-to-end dose delivery study in the world. MATERIALS AND METHODS: The postal audit used a polymer gel dosimeter in a cylindrical acrylic container for the afterloading system. Image acquisition using computed tomography, treatment planning, and irradiation were performed at each institution. Dose distribution comparison between the plan and gel measurement was performed. The percentage of pixels satisfying the absolute-dose gamma criterion was reviewed. RESULTS: Thirty-five institutions participated in this study. The dose uncertainty was 3.6% ± 2.3% (mean ± 1.96σ). The geometric uncertainty with a coverage factor of k = 2 was 3.5 mm. The tolerance level was set to the gamma passing rate of 95% with the agreement criterion of 5% (global)/3 mm, which was determined from the uncertainty estimation. The percentage of pixels satisfying the gamma criterion was 90.4% ± 32.2% (mean ± 1.96σ). Sixty-six percent (23/35) of the institutions passed the verification. Of the institutions that failed the verification, 75% (9/12) had incorrect inputs of the offset between the catheter tip and indexer length in treatment planning and 17% (2/12) had incorrect catheter reconstruction in treatment planning. CONCLUSIONS: The methodology should be useful for comprehensively checking the accuracy of HDR-IGBT dose delivery and credentialing clinical studies. The results of our study highlight the high risk of large source positional errors while delivering dose for HDR-IGBT in clinical practices.

Investigation of fiducial marker recognition possibility by water equivalent length in real-time tracking radiotherapy.

Real-time tumor tracking radiotherapy (RTRT) systems typically use fiducial markers implanted near the tumor to track the target using X-ray fluoroscopy. Template pattern matching, used in tracking, is often used to automatically localize the fiducial markers. In radiotherapy of the liver, the thickness of the body that can recognize the fiducial markers must be clinically assessed. The purpose of this study was to quantify the recognition of fiducial markers according to body thickness in stereotactic body radiotherapy of the liver using clinical images obtained using SyncTraX FX4. The recognition scores of fiducial markers were examined in relation to water equivalent length (WEL), tube current, and each flat panel detector. The relationship between the contrast ratio of the fiducial marker and the background and the WEL was also investigated. The average recognition score was found to be less than 20 when the WEL was greater than 25 cm. The probability of successful tracking of image recognition was mostly smaller than 0.8 when the WEL was over 30 cm. The relationship between WEL and tube current did not significantly differ between 100 and 140 mA, but there was a significant difference (p < 0.05) for all other combinations. To ensure tracking of fiducial markers during SBRT, if the WEL representing body thickness is longer than 25 cm, the X-ray fluoroscopy arrangement should be determined based on the WEL.

Quantitative analysis of the intra-beam respiratory motion with baseline drift for respiratory-gating lung stereotactic body radiation therapy.

This study aimed to quantitatively clarify the baseline drift for each respiratory cycle in two respiratory-gating methods using the intra-beam respiratory motion data of lung cancer patients. The residual motion and dose distribution were calculated based on intra-beam respiratory motion data with the baseline drift. To quantify the baseline drift $\Delta$ during irradiation, it was defined as the inclination between the detected expiration point and the expiration point in the next cycle in the anterior-posterior (AP), cranial-caudal (CC) and left-right (LR) directions obtained using an in-house programme. The baseline drift value reached up to 0.74 mm/s in the CC direction as per the respiratory motion data of 10 patients. The homogeneity index (HI) of the phase-gating method tended to increase because the target was irradiated even when the amplitude position of the target differed from period to period. In contrast, the amplitude-gating method enabled irradiation considering the amplitude position of the target because the gating window was set considering the amplitude position of the respiratory motion. The respiratory-gating methods and respiratory phase in respiratory-gating lung stereotactic body radiation therapy (SBRT) must be determined based on the respiratory motion of the patients.