Initial three- and half-year machine performance assessment of a superficial radiation therapy unit

PURPOSE: This work reports on results of periodic machine quality assurance (QA) tests for our superficial radiation therapy (SRT) unit (SRT-100 Vision) performed for the first three- and half- year.

METHODS: Results of machine warm-up, dosimetry, mechanical, safety and imaging QA tests for our SRT-100 Vision unit were reviewed and analyzed. Dosimetry tests included output constancy, backup timer, timer accuracy, beam quality (half value layer [HVL]), applicator factors (AFs), absolute output calibration, output constancy with varying x-ray tube head rotation, output reproducibility, output linearity, timer and end-effect error, percent depth dose (PDD) verification and congruence between radiation field and applicator size. Mechanical tests encompassed unit stability as well as applicator integrity and indicators. Safety tests consisted of visual and audio monitors, beam-on indicator, interlocks, and energy/filter indicators. Imaging tests for ultrasound covered functional check, spatial integrity, and image quality check. The QA tests were performed for three kilovoltage (kV) x-ray energies (50, 70, and 100 kV p) and six applicators (1.5, 2.0, 2.5, 3.0, 4.0, and 5.0 cm in diameter).

RESULTS: Daily mandatory machine warm-up was successfully completed on each day of treatment. The results of all dosimetry tests for three energies were within the recommended tolerance. Daily, and monthly outputs and annual absolute outputs were < ± 3.0%, < ± 2.0%, and < ± 1.3%, respectively. Backup timer displayed timer set ± 0.0%. Timer accuracy was < ± 1.0 sec. HVLs and AFs were < ± 1.1% and < ± 0.3%, respectively. Outputs for different head angles were < ± 1.0% and output reproducibility was within 0.1%. Output linearity was < ± 0.6% and end-effect errors were < ± 0.0006 min. PDDs were < ± 2.2%. Congruence was < ± 0.5 mm. Monthly mechanical tests confirmed mechanical robustness of our unit. Daily safety features were verified to be functional each day. Monthly imaging tests for ultrasound showed good functionality, image quality, and spatial integrity meeting the set tolerance.

CONCLUSIONS: The QA results demonstrated high performance and stability of our SRT-100 Vision unit over the first three and a half years of operation.