CASPIR trial: Using prostatic calculi as an alternative to fiducial markers for IGRT in for localized prostate cancer.

60 Background: Image guided Radiotherapy (IGRT) for prostate cancer (PCA) frequently employs surgically implanted fiducial markers (FMs). However, it is estimated that up to 35% of prostate radiotherapy patients have prostatic calculi (PC) visible on treatment cone beam CTs (CBCT). The purpose of this clinical trial is to directly compare FMs with PCs as an aid to prostate IGRT. Methods: We report data from a single institution prospective clinical trial investigating the feasibility of using prostate calcifications as natural FMs for IGRT. Patients planned for standard prostate radical EBRT +/- brachytherapy were eligible. Prior to CT planning, 3 gold fiducial markers are inserted into the prostate by the trans-perineal route under TRUS guidance. PCs visible within the PTV were contoured. Participants were aligned to FMs for EBRT using daily CBCT image guidance on a Varian TrueBeam linac. Off-line, a single experienced user analyses CBCTs using Image Registration in Eclipse (version 13.6). Random and systematic set-up errors are determined based on FMs, PCs (where present), prostate gland (PG) and bony landmarks (BL) and CTV-PTV margins derived for each data set. Results: 30 participants with PC have been recruited. Data from the first 19 patients (365 fractions), resulted in 2555 individual image registrations (7665 individual data points). The PTV margins required based on each reference structure are summarised in Table 1. The maximum difference between the CTV-PTV (PC) margin and CTV-PTV (FM) margin is 2.2mm in the X or L/R dimension. Margins required for FM, PG and PC in the Y and Z dimensions are comparable, with a maximum difference of 0.5mm between CTV-PTV (PC) and CTV-PTV (PG) . Conclusions: Preliminary results from this study demonstrate some evidence to support the use of PCs as an alternative to FMs for prostate IGRT. Future analysis will include location of PC according to the PIRADs schema and patient feedback in relation to FM implantation. Clinical trial information: NA. [Table: see text]