Description
Recent studies have suggested an extremely low _ / _ ratio of ~1.5 - 3 Gy for prostate cancer exists. This ratio implies that prostate cancer is more radioresistant than other types of cancers. Radiobiology tells us that the prostate responds well to high dose per fraction and high-dose-rate radiation treatment, namely hypofractionation. A continuing Phase I trial at Palomar Medical Center in Escondido, CA is in progress utilizing a hypofractionated stereotactic body radiation therapy (SBRT) prostate protocol since late 2008. Currently 12 patients diagnosed with low-risk prostate cancer have been treated under this protocol. Five 7 Gy fractions were delivered every other day yielding a total dose of 35 Gy. Treatment was delivered with the Novalis TX system which is comprised of a Varian Trilogy linear accelerator and Brainlab Exactrac 3.0 dynamic patient positioning system. Stereotactic localization and targeting were accomplished through a combination of infrared surface body markers as well as implantation of four gold fiducial radiographic markers within the prostate which were subsequently localized by stereoscopic x-ray system during treatment. A comprehensive evaluation of tumor control, toxicity and sequelae was performed through regularly scheduled prostate specific antigen (PSA) tests and the Expanded Prostate Cancer Index Composite (EPIC) quality of life survey which measured genitourinary (GU) and gastrointestinal (GI) toxicity as well as sexual performance sequelae. The EPIC survey was administered four times within 1 year following treatment. Acute and late sequelae were comprised of 9 and 16 QOL reports, respectively. Only one patient has experienced a biochemical failure at 2 years post-treatment by the ASTRO definition. No patients have had a PSA failure with respect to the Phoenix definition. The median PSA nadir is 1.65 ng/mL (range: 0.0-4.5 ng/mL). No EPIC grade 4 toxicities have been reported for GI or GU function. Only one EPIC grade 4 toxicity was reported immediately following treatment in sexual function; however, it was felt that treatment had little influence on this report since the same patient recorded a grade 4 sexual function prior to treatment. Currently, the Epic surveys have shown this treatment to be quite tolerable with nearly all of the acute and late scores falling within the 0-2 grades. The early results of this study reveal tumor control is being achieved with a hypofractionated treatment regimen while keeping acute and late toxicity to a minimum. While further study is necessary, a modified phase II study seems to be warranted.
