Evaluation of the RTI Dose Probe for Daily Superficial X-Ray Radiotherapy (SXRT) Output Constancy QA
Abstract
Purpose
To evaluate the suitability of the RTI Dose Probe for monitoring daily output constancy of SXRT machines.
Methods
The RTI Dose Probe, originally designed for radiology applications, was evaluated for daily radiotherapy quality assurance (QA) on an Xstrahl 150 SXRT unit. The detector was housed in customised RW3 solid water, with an additional 2mm RW3 build-up layer to provide mechanical protection. A 10cm diameter open-ended cone at a 20cm source-to-surface distance (SSD) was positioned on top of the build-up. Commissioning tests were designed in recognition that the detector’s intended role was to monitor output constancy rather than absolute dosimetry. Detector response was evaluated across clinically relevant beam qualities with half-value layers (HVLs) ranging from 1 to 5 mm Al. Performance metrics included output linearity, short-and medium-term reproducibility, and sensitivity to output variations. Controlled, known output changes were deliberately introduced to assess the detector’s sensitivity. Detector readings were compared with reference dosimetry measurements to establish baseline output values. Measurement uncertainty and detector stability were used to define appropriate daily QA tolerances and action levels.
Results
Repeated measurements demonstrated output linearity and reproducibility better than 0.1% and 0.2%, respectively. The detector also demonstrated reliable detection of introduced output errors of 0.5% and 1%.
Conclusion
Although the RTI Dose Probe was originally developed for radiology QA and has not been used for radiotherapy, the lack of a robust, dedicated dosimetry system for daily output constancy in superficial radiotherapy motivated its evaluation for SXRT applications. The results indicate that the RTI Dose Probe is suitable for daily SXRT output constancy QA. This methodology may also be applicable to orthovoltage radiotherapy units.