Residency · Residency · Radiation Oncology
Prostate Cancer: Moderate Hypofractionation and Ultra-Hypofractionation (SBRT)
Introduction
Prostate cancer exhibits a unique radiobiology compared to other solid tumors, primarily due to its characteristically low alpha/beta ratio, estimated between 1.5 and 3.0 Gy. This low alpha/beta ratio indicates that prostate cancer cells are more sensitive to larger doses per fraction than the surrounding normal tissues, which typically have higher alpha/beta ratios. This radiobiologic feature has driven the development and clinical adoption of hypofractionated radiation regimens. These regimens deliver fewer but larger fractions, aiming to maintain or improve tumor control while reducing the overall treatment burden for patients.
Radiobiologic Rationale
Alpha/Beta Ratio
The alpha/beta ratio for prostate cancer is approximately 1.5 Gy, with a range from 1.0 to 3.0 Gy. In contrast, late-reacting normal tissues such as the rectum and bladder have alpha/beta ratios around 3 Gy. This difference implies that delivering larger fraction sizes is therapeutically advantageous because the biologically effective dose (BED) increases more for the tumor than for the late-reacting normal tissues when hypofractionation is used. Consequently, hypofractionation can enhance tumor control without proportionally increasing late toxicity.
Practical Benefits
Hypofractionated regimens also offer practical benefits. By reducing the total treatment time, they improve patient convenience and access to care. Shorter treatment courses may lead to healthcare cost savings and can increase departmental throughput, allowing more patients to be treated efficiently.
Moderate Hypofractionation
Key Trials
Several pivotal trials have established the efficacy and safety of moderate hypofractionation in prostate cancer treatment. The CHHiP trial compared 60 Gy delivered in 20 fractions (3 Gy per fraction) over four weeks to the conventional regimen of 74 Gy in 37 fractions. The trial demonstrated non-inferior biochemical and clinical failure-free survival at five years, with comparable late toxicity profiles between the two arms.
Similarly, the PROFIT trial evaluated 60 Gy in 20 fractions versus 78 Gy in 39 fractions in patients with intermediate-risk prostate cancer. It found non-inferior biochemical failure rates and similar late gastrointestinal (GI) and genitourinary (GU) toxicity between the regimens.
The RTOG 0415 trial, which is equivalent to the HYPO-RT-PC trial in the US, enrolled low-risk prostate cancer patients and compared 70 Gy in 28 fractions (2.5 Gy per fraction) to 73.8 Gy in 41 fractions. This study also showed non-inferior disease-free survival, although there was a slightly higher incidence of acute GI and GU toxicity with hypofractionation.
ASTRO/ASCO/AUA Guideline Recommendation
Based on these data, the ASTRO/ASCO/AUA guidelines recommend moderate hypofractionation as a standard-of-care option for all risk groups of prostate cancer patients. This recommendation applies regardless of whether androgen deprivation therapy (ADT) is used. Importantly, image guidance (IGRT) is strongly recommended when delivering hypofractionated radiation to ensure accurate targeting and minimize toxicity.
Typical Prescription
A typical moderate hypofractionation prescription involves delivering 60 Gy in 20 fractions to the prostate, with or without inclusion of the seminal vesicles. Treatment is administered over approximately four weeks with daily image guidance using fiducial markers or intraprostatic transponders to ensure precise delivery.
Ultra-Hypofractionation (SBRT)
Landmark Trials
Ultra-hypofractionation, commonly delivered as stereotactic body radiotherapy (SBRT), has been investigated in several landmark trials. The Scandinavian HYPO-RT-PC trial compared 42.7 Gy in 7 fractions (6.1 Gy per fraction) to the conventional 78 Gy in 39 fractions in patients with intermediate- and high-risk prostate cancer. The trial demonstrated non-inferior failure-free survival at five years and comparable late toxicity, marking the first phase III evidence supporting ultra-hypofractionation.
The PACE-B trial evaluated 36.25 Gy in 5 fractions against either 78 Gy in 39 fractions or 62 Gy in 20 fractions in low- and intermediate-risk patients. The results showed comparable acute toxicity between the arms, with long-term efficacy data still maturing.
Common SBRT Prescriptions
The most widely used SBRT regimen is 36.25 Gy delivered in 5 fractions (7.25 Gy per fraction). Another common prescription is 40 Gy in 5 fractions (8 Gy per fraction). These treatments are typically delivered every other day or twice weekly over one to two weeks.
| Regimen | Total Dose / Fractions | Dose/Fx | Duration | Key Trial | BED (α/β=1.5 Gy) |
|---|---|---|---|---|---|
| Conventional | 78 Gy / 39 fx | 2.0 Gy | 8 weeks | — | 182 Gy |
| Moderate hypofractionation | 60 Gy / 20 fx | 3.0 Gy | 4 weeks | CHHiP, PROFIT | 180 Gy |
| Moderate hypofractionation | 70 Gy / 28 fx | 2.5 Gy | 5.5 weeks | RTOG 0415 | 187 Gy |
| Ultra-hypofractionation | 42.7 Gy / 7 fx | 6.1 Gy | 2.5 weeks | HYPO-RT-PC | 216 Gy |
| SBRT | 36.25 Gy / 5 fx | 7.25 Gy | 1.5–2 weeks | PACE-B | 212 Gy |
| SBRT | 40 Gy / 5 fx | 8.0 Gy | 1.5–2 weeks | — | 253 Gy |
Technical Requirements
Safe and effective delivery of prostate SBRT requires precise technical measures. Intraprostatic fiducial markers or electromagnetic transponders, such as the Calypso system, are essential for real-time tracking and intrafraction motion management. Planning target volume (PTV) margins are tight, generally 3-5 mm, with smaller margins of 2-3 mm posteriorly to protect the rectum. The use of a rectal spacer, such as SpaceOAR hydrogel, is strongly recommended to reduce rectal dose and toxicity.
Dose Constraints (5-Fraction SBRT)
Dose constraints for 5-fraction SBRT are carefully defined to minimize toxicity. For the rectum, volumes receiving 36 Gy should be less than 1 cc, volumes receiving 32 Gy less than 3 cc, and volumes receiving 29 Gy less than 5 cc. For the bladder, volumes receiving 37 Gy should be under 5 cc, and volumes receiving 18.1 Gy should be less than 50%. The urethra maximum dose should not exceed 42 Gy (105% of the prescription dose), and the mean dose to the penile bulb should remain below 25 Gy.
SBRT with ADT in Higher-Risk Disease
The HYPO-RT-PC trial included patients with intermediate- and high-risk prostate cancer, supporting the use of SBRT in these groups. SBRT combined with ADT is increasingly employed for unfavorable intermediate- and high-risk disease. Prospective registry data indicate biochemical control rates exceeding 90% at five years. Ongoing investigations are exploring pelvic nodal SBRT or moderate hypofractionation to pelvic nodes combined with an SBRT boost to the prostate.
Rectal Spacer
The SpaceOAR hydrogel is a biocompatible material that creates a 1 to 1.5 cm separation between the prostate and the anterior rectal wall. This separation reduces the rectal volume receiving high doses, such as V70, by approximately 75% during conventional fractionation. The benefit is particularly significant in SBRT, where single-fraction rectal doses are high. The hydrogel is generally well tolerated and is placed transperineally under ultrasound guidance.
Key Clinical Pearls
The low alpha/beta ratio of prostate cancer, approximately 1.5 Gy, underpins the radiobiologic rationale for hypofractionation, making larger fraction sizes therapeutically advantageous. Moderate hypofractionation, typically 60 Gy in 20 fractions, is now considered a standard-of-care option supported by multiple phase III trials such as CHHiP and PROFIT. SBRT, most commonly delivered as 36.25 Gy in 5 fractions, has demonstrated non-inferior outcomes in the phase III HYPO-RT-PC trial and is appropriate for low- and intermediate-risk disease. Critical technical components for safe prostate SBRT delivery include the use of rectal spacer hydrogel and real-time intrafraction motion management to minimize toxicity and ensure precise treatment.
References
- Dearnaley D, Syndikus I, Mossop H, et al. Conventional versus hypofractionated high-dose intensity-modulated radiotherapy for prostate cancer: 5-year outcomes of the randomised, non-inferiority, phase 3 CHHiP trial. Lancet Oncol. 2016;17(8):1047-1060.
- Catton CN, Lukka H, Gu CS, et al. Randomized Trial of a Hypofractionated Radiation Regimen for the Treatment of Localized Prostate Cancer. J Clin Oncol. 2017;35(17):1884-1890.
- Widmark A, Gunnlaugsson A, Beckman L, et al. Ultra-hypofractionated versus conventionally fractionated radiotherapy for prostate cancer: 5-year outcomes of the HYPO-RT-PC randomised, non-inferiority, phase 3 trial. Lancet. 2019;394(10196):385-395.
- Brand DH, Tree AC, Ostler P, et al. Intensity-modulated fractionated radiotherapy versus stereotactic body radiotherapy for prostate cancer (PACE-B): acute toxicity findings from an international, randomised, open-label, phase 3, non-inferiority trial. Lancet Oncol. 2019;20(11):1531-1543.