With widespread use of prostate-specific antigen (PSA) screening, a majority of newly diagnosed prostate cancers are confined to the organ and are typically treated with radical prostatectomy or radiotherapy.

An emerging radiotherapy option is stereotactic ablative radiosurgery (SABR), also called stereotactic body radiation therapy (SBRT), in which multiple- focused radiation beams deliver a large ablative or destructive dose of radiation to a tumor target.

SABR, which is administered in far fewer but larger doses than conventional radiotherapy, has the potential to benefit a substantial proportion of patients with low-, intermediate-, and potentially even high-risk prostate cancers. While all patients are potentially eligible for radiation treatmenfigut, these shortened courses may be the treatment of choice for those who, due to comorbidities, are not candidates for surgery, or those who have difficulty meeting scheduling demands of traditional, longer-term radiotherapy.

Clinical trials over the past decade have revealed therapeutic advantages of various short-course, or hypofractionated, approaches in prostate cancer, including SABR. Investigations also are underway to address a specific challenge associated with SABR to the prostate: the potential for rectal injury.

Evidence has indicated that dose escalation of conventionally fractionated external-beam radiation improves prostate cancer control and can provide a survival advantage for patients with intermediate- or high-risk disease.

With this approach, 3-dimensional conformal or intensity-modulated radiation therapy (IMRT) approaches can limit toxicity—although many fractions are required, typically 5 days a week for a period of 8 or 9 weeks.

Given prostate cancer’s damage-repair characteristics, which indicate that the disease might be better eradicated with fewer, more powerful radiation doses than with longer, lower-dose courses of treatment,

 modest hypofractionated regimens (with fraction sizes ranging from ~2-3 Gy) have been proposed to improve treatment efficacy as well as patient convenience. Recent clinical trials using regimens that are more hypofractionated than conventional treatment schedules have shown that efficacious therapy can be delivered more efficiently, and with manageable toxicity, using equivalent effective doses with hypofractionation.

 Subsequently, a handful of studies have demonstrated excellent therapeutic outcomes, along with acceptable safety profiles, using even more hypofractionated approaches (6.5-10 Gy per fraction). Among them:

In a phase I/II trial, 40 men with low-risk prostate cancer (Gleason score ≤6, PSA <10 ng/mL and clinical stage ≤T2a) received 5 fractions, 6.7 Gy per fraction, for a total dose of 33.5 Gy targeting the prostate plus a 4 to 5 mm margin. Four-year actuarial freedom from biochemical recurrence was 90% under the Phoenix failure definition (nadir + 2 ng/mL).

A phase II trial involving 67 patients with low-to-intermediate risk (Gleason score 3+3 or 3+4, PSA ≤10 ng/mL, and clinical stage ≤T2b) tested 5 fractions of 7.25 Gy, a total dose of 36.25 Gy, delivered to the prostate with a margin of 3 to 5 mm. PSA relapse-free survival was 94% at 4 years.

In a trial testing SABR in 304 low-, intermediateand high-risk patients with prostate cancer, the majority of whom were low risk, 50 patients were administered 5 fractions of 7 Gy (total dose 35 Gy); the rest received 5 fractions of 7.25 Gy (36.25 Gy total). With median follow-up of 30 months for the lower-dose patients and 17 months for the higher-dose group, actuarial 5-year biochemical recurrence-free survival was 97% for the patients with low-risk cancers, almost 91% for intermediate- risk patients, and 74% for high-risk patients.

 In a phase I/II study led by the UT Southwestern Medical Center at Dallas and 4 other sites,

investigators have tested SABR in low- and intermediate-risk patients using substantially higher doses, starting in 15 patients at 45 Gy in 5 fractions—similar to those used in hypofractionated high-dose-rate brachytherapy.

The 5-fraction total doses were escalated to 47.5 Gy and finally to 50 Gy. No grade 3-5 toxicities occurred within 90 days after treatment. Overall, grade ≥2 GI toxicities occurred in 18% of patients, and grade ≥3 in 2%, while grade ≥2 GU toxicities were experienced by 31% of patients, with grade ≥3 occurring in 4%.

		In the subsequent phase II expansion, 46 additional patients were enrolled and treated at the 50-Gy dose level. At median follow-up of 54 months, actuarial freedom from biochemical failure was 98.6% at 5 years.

 However, 6 patients developed high-grade rectal toxicity, primarily ulceration to the anterior rectal wall, that ultimately required 5 patients to undergo a diverting colostomy—highlighting a crucial limitation to dose escalation in prostate cancer radiotherapy.

		Aiming to reduce the radiation dose to the rectal wall while retaining the therapeutic benefit of highdose treatment, the UT Southwestern team has been investigating the use of a minimally invasive, biodegradable spacer to increase the distance between the prostate and the rectum in patients with low- and intermediate-risk prostate cancer treated with SABR in a prospective clinical trial (NCT02353832).

		The spacer is an FDA-approved polyethylene glycol (PEG)-based gel that can be placed via transperineal injection at the same time radio-opaque markers are implanted to help target the prostate during radiation therapy (Figure). The spacer dissolves after 12 weeks.

		This approach already has proved successful in conventional and IMRT, with no evidence of ulceration, stricture, or necrosis to rectal tissue after a year.

 The trial accrual is nearly complete, and results should be available within a year. Additionally, a trial investigating treatment of both the prostate and pelvis with SABR in 5 fractions in conjunction with androgen deprivation therapy for patients with high-risk prostate cancer has opened and is currently accruing at UT Southwestern (NCT02353819).

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Stereotactic ablative radiosurgery, which is administered in far fewer but larger doses than conventional radiotherapy, has the potential to benefit a substantial proportion of patients with low-, intermediate-, and potentially even high-risk prostate cancers.

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