Image‐based high dose rate brachytherapy for prostate cancer

Abstract

The passage from conventional radiographic film‐based to image‐based high dose rate (HDR) brachytherapy for prostate cancer has significantly improved our ability to define the targets and organs at risk (OAR). Three‐dimensional (3D) anatomical information are obtained from Computed Tomography (CT) or Magnetic Resonance Imaging (MRI), and cancer areas are validated using functional imaging with magnetic resonance spectroscopy imaging (MRSI). Together with inverse planning optimization, image‐based HDR brachytherapy can deliver a highly conformal dose distribution to the target while sparing OAR. However, uncertainties inherent to the use of the new imaging modalities may impact on the dose distribution. In this work, we have selected the major uncertainties and studied their impact within the context of the clinical procedures of ultrasound guided HDR prostate brachytherapy delivered in two fractions. The feasibility of using functional imaging to guide the delivery of higher dose to dominant intraprostatic lesions (DIL) within the prostate was also investigated. The average cranio‐caudal displacement of catheters between fractions was 2.7 mm and 5.4 mm for bony anatomy and gold seed marker measurement, respectively. Either increase or reduction of prostate volume was observed with an average of 7.8% and a maximum of 17% between fractions, resulting in minimal dose changes. The dose uncertainty due to the planning CT slice thickness showed a relative error of 1% on average for current 3 mm planning CT, independent of the transversal region of the prostate. A retrospective study taking advantage of MRI/MRSI for the dose escalation of the DIL demonstrated that high dose areas could be redistributed to boost the DIL by 120% without any additional dose delivered to OAR compared to a reference plan. On average, the rigid endorectal probe was shown to rotate the prostate anteriorly by to expand it in the antero–posterior direction by 1.2 mm, and compress it in lateral direction with 1.5 mm. Finally, MRI scans with and without the probe could be registered with a 2 mm precision using translation only. The use of 3D anatomical and functional information combined with inverse planned HDR brachytherapy is a precise procedure for the treatment of localized prostate cancer with potential for dose escalation.