[A Quality Assurance (QA) System with a Web Camera for High-dose-rate Brachytherapy].

PURPOSE: The quality assurance (QA) system that simultaneously quantifies the position and duration of an (192)Ir source (dwell position and time) was developed and the performance of this system was evaluated in high-dose-rate brachytherapy. METHODS: This QA system has two functions to verify and quantify dwell position and time by using a web camera. The web camera records 30 images per second in a range from 1,425 mm to 1,505 mm. A user verifies the source position from the web camera at real time. The source position and duration were quantified with the movie using in-house software which was applied with a template-matching technique. RESULTS: This QA system allowed verification of the absolute position in real time and quantification of dwell position and time simultaneously. It was evident from the verification of the system that the mean of step size errors was 0.31±0.1 mm and that of dwell time errors 0.1±0.0 s. Absolute position errors can be determined with an accuracy of 1.0 mm at all dwell points in three step sizes and dwell time errors with an accuracy of 0.1% in more than 10.0 s of the planned time. CONCLUSION: This system is to provide quick verification and quantification of the dwell position and time with high accuracy at various dwell positions without depending on the step size.

Asymmetric margin setting at the cranial and caudal sides in respiratory gated and non-gated stereotactic body radiotherapy for lung cancer.

OBJECTIVE: To evaluate total errors, including setup errors, and tumour motion changes with a electronic portal imaging device (EPID) cine at the cranial and caudal sides in respiratory gated and non-gated radiotherapy. METHODS: Co-ordinates of the tumour centres (TCs) in the craniocaudal direction were obtained by using four-dimensional CT (4DCT) for each bin and EPID cine frame. During the 100% duty cycle (DC100), 50% duty cycle (DC50) and 30% duty cycle (DC30), both centred on the 50 phase, the co-ordinates of the TCs were compared at the most cranial and caudal positions on both 4DCT and EPID cine. RESULTS: During DC100, total errors were -0.2 ± 2.1 and 1.1 ± 2.6 mm at the cranial and caudal sides, respectively. During DC50, the corresponding values were -0.2 ± 2.1 and 1.7 ± 3.2 mm, respectively; during DC30, they were -0.1 ± 2.1 and 1.8 ± 2.9 mm, respectively. The tumour motion changes at the caudal side were strongly correlated with tumour motion observed on 4DCT during DC100 (R(2) = 0.59). CONCLUSION: Total errors and tumour motion changes on the caudal side were larger than on the cranial side because of the patients' breathing levels. Owing to variations of the TCs at beam-trigger events, the larger margin was required at the caudal side in gated radiotherapy. ADVANCES IN KNOWLEDGE: Variations of the TCs were evaluated at the cranial and caudal sides separately. Providing some margins to compensate for tumour motion changes was a significant requirement at the caudal side in gated and non-gated radiotherapy.