Image-guided radiotherapy for prostate cancer using transperineal ultrasound

During a course of external beam radiotherapy the prostate gland moves both between radiotherapy fractions (interfractional motion) and during treatment (intrafractional motion). Some of this motion is related to changes in rectal and bladder filling, and movements are more marked in the antero-posterior direction as a result of rectal filling variability, compared to laterally and superoinferiorly. Image guided radiotherapy (IGRT) is defined as: “Any imaging at the pre-treatment and treatment delivery stage that leads to an action that can improve or verify the accuracy of radiotherapy.” The importance of IGRT is to reduce the chance of geographical miss and treatment failure, and to improve rates of bladder and bowel toxicity. For years bony anatomy was used as a surrogate for prostate position but now there are various IGRT modes in order to visualise the prostate gland itself. These include the insertion of fiducial markers into the prostate, an additional procedure for patients with attendant risks of infection, bleeding and marker migration. Prostate volume changes due to hormones or radiotherapy may alter the position of markers. Cone beam computed tomography (CBCT) scans are also used, which are dependent on gaining a satisfactory soft tissue image and subject to interobserver variability. Both the use of fiducial markers and CBCT scans require daily radiation exposure. Systems using magnetic resonance imaging (MRI), which does not require radiation exposure, are in development. However, MRI is not possible in patients with pacemakers, certain metallic implants and in patients with claustrophobia. Clarity (Elekta AB, Stockholm, Sweden) transperineal ultrasound (TPUS) was introduced at the Bristol Cancer Institute (BCI) in February 2014 for interfractional IGRT using threedimensional soft tissue matching (figure 1). Since January 2015 we have treated 240 patients with radiotherapy to the prostate and seminal vesicles using TPUS IGRT. This equates to more than 8,500 x-ray volume imaging (XVI) doses not being delivered as a result. We are the first centre in the UK to use this system clinically for prostate cancer IGRT.