In–vivo endorectal dosimetry of prostate high dose rate brachytherapy and TomoTherapy ® using MOSkinTM detectors

Hypofractionated radiotherapy treatments of the prostate deliver very high doses to the target volume, whilst attempting to spare healthy surrounding tissue. This leads to an extremely high dose gradient at the anterior rectal wall. Small geometrical deviations in target localisation may lead to a high dose delivered to the rectal wall. The aim of this phantom based study was the development and testing of a system for measurement of anterior rectal wall doses during hypofractionated high dose radiotherapy treatments of HDR brachytherapy and TomoTherapy®. A Perspex replica transrectal ultrasound probe was utilised within a surrogate rectum, with clinical treatments delivered to the phantom. MOSkinTM detectors were attached to the probes to measure dose rates to the rectal wall in real–time. A comparison of a hollow and solid Perspex probe determined if the solid probe increased rectal wall dose by introducing extra backscatter material. For brachytherapy, similar results were produced between probes. Dual MOSkinTM detectors, employed to improve the accuracy of measurement over a single detector arrangement, increased agreement between measured and TPS doses, with all dual detector results demonstrating acceptable agreement (within 5%). For TomoTherapy®, the probe was utilised as a Perspex surrogate for the Rectafix®. At the anterior rectal wall using single detectors and accounting for acceptable Distance–to–Agreement measurements, 87.5% of detectors measured dose to within ±5% of the TPS. Dual MOSkinTM detectors further improved results, allowing for acceptable agreement of all detector measurements with the TPS. The dual MOSkinTM system, if employed clinically, could help to detect errors, such as patient shifts or incorrect set–up, in dose delivery for HDR brachytherapy and Helical TomoTherapy® for the treatment of prostate cancer.