St George Cancer Care Centre has moved to real time ultrasound based HDR brachytherapy for prostate brachytherapy boosts, performing the implantation, planning and radiation delivery in the one theatre session. In vivo measurements, using a wireless dosimetry system (MOSkin), of the rectal wall dose were performed in real-time to evaluate the efficacy of moving from two fractions (2 x 9 Gy separated by 1 week) to single fraction (15 Gy). Patient data was collected for a year’s worth of patients for both two fraction and single fraction treatments.
Materials and Methods
For each fraction of a real time TRUS based HDR brachytherapy of the prostate, the TRUS probe was fitted with 4 MOSkin detectors, placed 1.5 cm apart, along the right hand edge of the probe, 90° to the longitudinal transducer. The probe and MOSkin detectors were then placed inside an ultrasound balloon before insertion within the patient. After the TRUS guided HDR catheters have been inserted and the treatment plan constructed and optimised, the TRUS probe is rotated 90°, placing the MOSkin detectors against the anterior rectal wall, and stepped to place the most superior detector in line with the prostate base. The doses delivered to the rectal wall at the 4 desired locations are monitored in real time. Readout and analysis of the MOSkin measurements is performed every three seconds for the duration of the radiation delivery, with the total dose received compared to planned dose points. The accepted dose threshold to the rectal wall was restrained to 70% of the prescription dose (630 cGy for two fraction treatments, 1050 cGy for single fraction treatments). 14 Patients received 2 fractions (with 19 separate fractions recorded) throughout the first year of the trial and 19 patients received single fraction treatments.
For the two fraction treatments, the maximum recorded dose was 577 cGy for an expected dose of 516 cGy. This is an increase of 12%, but still below the 70% rectal dose threshold (64%). The average discrepancy of measurement of all points was -0.26%, with a standard deviation of 5.7%. This indicates the planning and delivery are adequate for the implementation of single fraction treatment. The single fraction dosimetric results were shown to replicate the trends of the two fraction measurements.
In-vivo rectal dose measurements have demonstrated the efficacy of single fraction real time brachytherapy boost for prostate cancer treatment.
Dr Dean Cutajar –Dr Dean Cutajar is a lecturer in Physics with the University of Wollongong, and a researcher with St George Cancer Care Centre, specialising in brachytherapy dosimetry.