137: First Clinical Results of an Adaptive Off-Line Image Guidance Strategy Using Cone-Beam CT Scans for Prostate Cancer

Abstract

Correction strategies usually tend to focus on the prostate motion only and neglect the effect of a changed anatomy on Intensity Modulated Radiotherapy Treatment (IMRT) planning. Linear accelerators equipped with kV cone-beam CT (CBCT) imaging provide soft tissue information of both prostate and normal tissues. We developed an adaptive radiotherapy scheme for prostate cancer based on CBCT images. The aim was to improve knowledge of the average prostate position and average rectum shape to safely reduce the PTV margin. We evaluated the first clinical results of patients routinely treated according to this scheme. Nineteen intermediate risk prostate cancer patients have been treated according to the protocol. The treatment started with a 5-field IMRT plan based on a conventional CT scan, the PTV was the prostate (+/- seminal vesicles) with a 3D expansion of 10 mm. In the first six days of treatment CBCT scans were acquired on Elekta Synergy systems just prior to treatment. These scans were matched on the planning CT scan using the pelvic bones. Automatic grey-value matching was then used to match the prostates of the CBCT-scans to the prostate of the planning CT scan. A radiation oncologist visually evaluated the matches; a minimum of four approved matches was needed to continue with the protocol. The mean of the obtained translations and rotations was used to move the prostate of the planning CT scan to its average position. Subsequently, the rectal wall was delineated in the CBCT-scans, and coordinates of corresponding points of the rectums were averaged to obtain the average rectum. Based on average prostate and rectum a new IMRT treatment plan was made with a reduced PTV margin of 7mm. Weekly CBCT-scans were made to verify that the new PTV encompassed the prostate. Sixteen patients were successfully treated with our adaptive treatment scheme. For 3 patients the adaptive scheme was aborted: for two patients due to poor image quality of the CBCT scan, the third patient received a urinary catheter because of urinary retention leading to a systematic shift in prostate position. For 85% of the CBCT-scans a successful grey-value match was obtained, the other scans were discarded. On all except one of the verification scans (marginal miss of the seminal vesicle) the prostate was inside the PTV. With the adaptive treatment scheme the new PTV volume was on average 30% smaller than for the conventional treatment (p<0.0001). The mean dose received by the rectum was on average reduced by 7.6%. The percentage of rectal wall receiving 40 Gy or more was reduced by 11 % (p<0.001) and 65 Gy or more was reduced by 23 % (p<0.0001) leading to an estimated reduction in risk of late rectal bleeding by 24 % (p<0.0001) or a 24 % reduction in anal incontinence (p<0.0001). This is the first routine clinical application of soft tissue image guidance for the prostate using kV CBCT. Contrary to adaptive schemes that use implanted markers, our method is non-invasive and improves localization of prostate, seminal vesicles and rectum. The proposed strategy safely reduces the PTV by 30 % leading to a significant reduction in the risk of late rectal complications.