Treatment of synchronous rectal and prostate cancer utilising intensity-modulated radiotherapy (IMRT) with a simultaneous integrated boost (SIB): Assessment of acute toxicity and response

Abstract

Introduction: Prostate and colorectal cancers are the two most prevalent cancers in men and diagnosis of synchronous tumours is increasingly common. Dependent on a number of factors including patient preference, tumour stage and localization, different management strategies are utilized for both tumours. External beam radiotherapy (EBRT) is frequently required in their management and IMRT enables simultaneous delivery of different dose levels to separate structures. We report our experience of delivering 2-phase EBRT utilising IMRT to this group of patients. Method(s): 6 patients with synchronous prostate and rectal cancer were treated with IMRT between 2014-2017 at Oxford University Hospitals Foundation NHS Trust. Patient and tumour demographics, treatment volumes, dose/fractionations, and concurrent chemotherapy were collected. An assessment of planning parameters was made. Prospective toxicities were recorded weekly. Post-treatment scans were reported by MRI tumour regression grade (mrTRG) for the rectal primary and serial post-treatment prostate specific antigen (PSA) measurements were recorded. Result(s): Patient and tumour demographics available in Table 1. All patients received androgen deprivation therapy and 5/6 patients received concurrent chemotherapy (Capecitabine 900mg/m2 BD on treatment days). One patient, with high risk features, was treated adjuvantly following transanal endoscopic resection. Rectal and prostate clinical target volumes (CTV) were contoured as per local guidelines. Phase I involved pelvic doses for rectal disease of 45-50.4Gy in 1.8Gy fractions with a simultaneous integrated boost to the prostate +/- seminal vesicles receiving 2Gy fractions. Phase II delivered a further 2Gy/# to prostate +/- seminal vesicles to a total dose of 74Gy. Dose to elective and involved nodal volumes varied and were dependent on whether the nodes were thought to be related to the rectal or prostate primary. Radiotherapy was delivered using either a 5-field IMRT plan (n=1) or RapidArc (n=5) using a 2-phase technique. Dose prescription was defined according to ICRU83. Optimal organs at risk objectives, as per PIVOTAL and CHHIP trials, included bladder (V50% <65Gy, V70Gy <35%), rectum (V60%<50%, V50% <60Gy), femoral heads (V50%<50Gy) and bowel (V45Gy <158cc, V65Gy <0cc). PTV coverage was achieved in all plans, although prostate defined rectal planning objectives were not met (V60% <50Gy). One plan exceeded optimal bowel objectives. Mean rectal dose across plans was 57.5Gy, bladder 42Gy, bowel 27.6Gy and right and left femoral heads 33.7Gy and 34.9Gy respectively. All patients completed radiotherapy. There were no hospital admissions. Acute toxicity was available for all patients and there were no grade 3/4 toxicities. Grade 1 toxicities were fatigue (n=5), pelvic pain (n=4), diarrhoea (n=3), constipation (n=1) anorexia (n=1), and palmoplantar erythema (n=1). All patients demonstrated posttreatment mrTRG responses of grade 3 or higher with recorded PSA biochemical remission. 2 patients proceeded to resection, where pathological complete responses were found, 3 patients continue on surveillance and liver metastases were noted on post-treatment imaging in one patient. Conclusion(s): 2-phase EBRT using IMRT with a SIB has been delivered safely, with minimal acute toxicity to patients with synchronous tumours of the rectum and prostate. Appropriate planning objectives need to be defined. Long-term follow-up is required to assess for on-going response and possible late toxicity. (Table Presented).