Background and purpose: Local relapse after radiotherapy for prostate cancer mostly originates at the original tumor location. Dose escalation reduces local relapse rates. It may be of benefit to focus the highest dose to the intraprostatic lesion (GTVMRI) using intensity-modulated radiotherapy (IMRT). Therefore, the visualization of the GTVMRI and its inclusion into computer optimization is mandatory.
Materials and methods: Fifteen patients with prostatic adenocarcinoma were referred for IMRT. All these patients had a palpable lesion on digital rectal examination (DRE) and/or a PSA >10.0 ng/ml. A T2-weighted MR examination of the prostate was performed in order to detect a GTV(MRI) and correlate the location of the GTV(MRI) with the site of the tumour-containing cylinder (biopsy). Two IMRT plans were compared: a plan without the inclusion of the GTV(MRI) (IMRT-CONV) versus a plan including the GTV(MRI) into the plan optimization (IMRT-GTV(MRI)). For comparison, both physical and biological endpoints of the GTV(MRI), CTV, PTV and rectum were taken into account. After the finalization of the planning study, the IMRT-GTV(MRI) plans were clinically delivered using step-and-shoot IMRT. Acute gastro-intestinal (GI) and genito-urinary (GU) toxicity were recorded.
Results: In all cases, the location of the GTV(MRI) corresponded with the site of the tumor containing biopsy cylinder. The mean and median distance of the GTV(MRI) to the anterior rectal wall was 3 and 2mm, respectively (range: 0-12 mm). For the GTV(MRI), its inclusion in the optimization led to a significant increase of all physical endpoints (P<0.01), without compromising the dose to the CTV, PTV and rectum. Mean GTV(MRI) dose was 78.3 Gy (IMRT-GTV(MRI)) versus 76.9 Gy (IMRT-CONV) (P<0.00001). All IMRT treatments were successfully delivered within 6 min. We did not observe grade 3 acute GI toxicity. One patient developed grade 3 GU toxicity (nocturia), that disappeared after administration of medication. Grade 2 GI and GU toxicity was observed in, respectively, four and six patients.
Conclusion: Using T2-weighted MR, the visualization of an intraprostatic lesion is feasible. The inclusion of the GTV(MRI) into planning optimization leads to a modest increase in dose, without compromising the dose to the CTV, PTV and organs at risk. The clinical delivery of these plans runs without problems. Acute toxicity is mild.