Phase I Dose Escalation Study of Stereotactic BOOST for Prostate cancER

Prostate cancer accounts for one third of all new cancer diagnoses in men and approximately 30% of men will have External Beam Radiotherapy (EBRT) as primary local therapy. Local persistence of prostate cancer treated with radiotherapy is under-appreciated. Prostate cancer has a long natural history and the consequences of local persistence may not be realized for many years.

Prostate cancer has a slow growth with a potential doubling time ranging from weeks to months (median 42 days) which has led to the hypothesis that prostate cancer will behave more like a late reacting tissue(1). Brenner et al. (2) used data from prostate Low Dose Rate (LDR) permanent seed implants and EBRT series to derive an alpha/beta of approximately 1.5. Many groups have also calculated the alpha/beta ratio to be in the < 3.0 range(3).

If the alpha/beta ratio for prostate cancer is lower than the surrounding normal tissues (Brenner et al. have estimated the alpha/beta for the rectum to be over 5.0), doses greater than 2 Gy (hypofractionation) will afford an advantage as there will be a greater sensitivity of prostate cancer to radiation, as compared to the bladder or rectum. This benefit has been exploited for many years with High Dose Rate (HDR) brachytherapy using Iridium -192. Typical doses for HDR are 19Gy in two fractions in addition to 46Gy in 23 fractions of EBRT (4). With reference to our own institutional data from HDR brachytherapy we calculated the dose distribution in recently treated patients. Due to the nature of the iridium dose distribution, there are considerable areas of much higher dose delivered; 200% of the dose (38Gy) to 12.7% of the target volume, 150% (13.75Gy) to 32.6% of the target volume and 125% (11.9Gy) to 60.1% of the target. Despite the high doses to the periphery of the prostate it is possible to limit the dose to the rectum and bladder to 1cc < 75% (14.25Gy) and the 1cc urethra to <125% (23.75Gy) (4). HDR brachytherapy series have reported durable long term Biochemical Failure Free Survival (BFFS), which are as good or better than comparable external beam or surgical series(5). However despite the low toxicity and excellent biochemical outcomes, HDR brachytherapy utilization in Australia is low, with under 300 cases in total per year. This is due to the logistical difficulty of HDR brachytherapy, anaesthetic requirements, nursing care and patient discomfort.

There have been five published studies investigating stereotactic boost in addition to standard fractionation EBRT without using brachytherapy, four reports with the Cyberknife platform (6-9) and one with Intensity Modulated Radiotherapy (IMRT) (10). All of these series have early follow up (FU) with reported BFFS between 77% and 100%. The largest experience is from Katz et al. (8) who reported 73 patients with both intermediate (n= 41) and high risk (n=32) disease. They treated patients with the Cyberknife platform to deliver between 18 and 21 Gy in three fractions in addition to 45 Gy in 25 fractions EBRT. With a median follow up of 33 months, BFFS was 89.5% and 77.7% for the intermediate and high risk patients respectively. A 5 mm expansion from the prostate to the Planned Treatment Volume (PTV) was used except posteriorly, where the margin was 3mm. There was 7% Grade 2 acute genitourinary (GU) and gastrointestinal (GI) toxicity, with late Grade 2 estimates at three years of 5.5% (GU) and 8.2% (GI). Three other series using Cyberknife to mimic HDR were reported between 2008 and 2012 (6, 7, 9). These studies used margins of 0-2 mm posteriorly and 3-5 mm in other directions and reported low rates of Grade 2 and Grade 3 toxicity. Two of these reports specifically attempted to reproduce the heterogeneity of HDR with large areas of the PTV receiving >125% (40% to 45%) and >150% (5% to 10%) of the prescribed dose. Miralbell et al. (10) reported a Linac based IMRT boost in 50 patients in 2010. This series used an endorectal balloon but no image guidance, which may be the cause of their unacceptably high five year estimates of late GI Grade 2 toxicity of 26%.

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