TRUEBEAM Stereotactic Body Radiotherapy for Localized Prostate Cancer

1.0 BACKGROUND 1.1 Prostatic adenocarcinoma is one of the most common forms of malignancy in men. Every year over 200000 patients are diagnosed with prostate cancer in the United States. Treatment options for these patients include active surveillance, radical prostectomy, external beam radiation therapy, permanent source interstitial brachytherapy and high dose rate (HDR) brachytherapy.

1.2 Each of these treatment options vary in regards to the logistics, anticipated outcomes, and potential side effects of therapy.

1.3 High-dose rate (HDR) brachytherapy has been used in the treatment of prostate cancer since the 1980's with good results. Catheters are placed temporarily in the prostate, and then loaded with a high-dose Iridium-192 source, delivering a few fractions of very high-dose RT. Brachytherapy allows the delivery of conformal, high-dose radiotherapy to the prostate, with a rapid dose fall-off outside of the region. It also takes advantage of low alpha/beta ratio of prostate cancer by using a hypofractionated approach.

1.4 The TrueBeam is a noninvasive radiosurgical system, capable of treating any part of the body from multiple targeting angles, creating a highly conformal three-dimensional radiosurgical treatment volume, guided by orthogonal X-ray-based targeting feedback, and delivering radiation by a highly collimated, robotically controlled linear accelerator. The TrueBeam system targets implanted fiducial markers with sub-millimeter set-up accuracy.

1.5 From a dosimetry standpoint, TrueBeam Stereotactic radiosurgery is capable of producing a dose distribution comparable to that created by prostate HDR brachytherapy treatment, without the need for invasive transperineal catheters, anesthesia, or inpatient admission. It would therefore be possible to deliver the HDR boost portion of a patient's treatment in a non-invasive fashion. As such, the TrueBeam prostate dose fractionation schedule prescribed in this study is based upon prior published prostate HDR brachytherapy experience both as a monotherapy and as a boost to external beam radiation therapy in patients with higher risk disease. The therapeutic volume in this study will also be made to resemble prostate HDR brachytherapy therapeutic volume, with similar dose limitation objectives to the adjacent tissues, including the rectum, bladder and urethra. It is theorized that such an approach should result in similar cancer control rates while lowering overall morbidity and improving the patient's comfort and convenience.

1.6 The feasibility of stereotactic body radiation therapy for treating localized prostate cancer was first described by King at Stanford University. Their phase I protocol delivered 36.25Gy in 5 fractions of 7.25Gy. In a recent report of acute and 18-month late toxicity in 26 "low-risk" patients, no patient experienced grade 3 or 4 acute or late toxicity, and only one patient experienced a grade 2 late morbidity (urethral stricture). Toxicity was less than that reported in MD Anderson's external beam dose escalation trial. Mean PSA 18 months after treatment was 0.22ng/ml. Naples Community Hospital reported a series of more than 70 low and intermediate risk patients treated with the SBRT. The prostate received 35 Gy in 5 fractions of 7 Gy each; acute toxicity was minimal. San Diego Cyberknife, which used a virtual HDR technique, reported a series of more than 124 low and intermediate risk patients treated. The prostate received 38Gy in 4 fractions of 9.5Gy each; acute toxicity was minimal.

1.7 Another potential benefit of stereotactic body radiosurgery relative to HDR brachytherapy is possibly better preservation of potency, even if the radiation distribution is essentially identical between these modalities. This is so because needle trauma has been identified as a potentially significant contributory factor to erectile dysfunction with brachytherapy, including HDR-based monotherapy technique, presumably due to direct physical injury to the neurovascular bundle and/or bulb of the penis, particularly when greater than 13 needle insertions are performed. By comparison, stereotactic body radiosurgery is noninvasive, and so removes this particular erectile dysfunction risk factor.