Radiosurgery Society guide to localized prostate SBRT
TL;DRCase-based RSS guide to intact-prostate SBRT (≤5 fx): large gland, high baseline IPSS and prior TURP are not contraindications if OAR constraints are met.
PACE-BMIRAGE
The useful RT detail is the prescription spread: 100% isodose with V105% < 20-30% vs 75-85% isodose with Dmax 130-140%, which makes cross-center dose comparisons unreliable. On gland size, PACE-B had only 5% of pts ≥80 mL, so offering SBRT to large glands rests on retrospective data plus spacer and bladder filling.
In a pt with a large prostate, high baseline IPSS or prior TURP, this guide supports offering SBRT with counselling on a harder transient GU flare when OAR constraints are achievable; it does not support neoadjuvant ADT for cytoreduction alone.
Prescription conventions vary widely (100% isodose with V105% < 20-30% vs 75-85% isodose with Dmax 130-140%), so dose comparisons across series need care. Large glands are supported mainly by retrospective data, since PACE-B had only 5% ≥80 mL; spacer, bladder filling and MR guidance (MIRAGE) are the stated mitigations.
8 details 2 trials watching
Case-based practical guide endorsed by the Radiosurgery Society, built on two modified real-world cases. Expert synthesis of existing evidence, not a trial or systematic review.
Both cases 40 Gy in 5 fractions to prostate + proximal SV; Case B added an MR-guided SIB of 42 Gy to the dominant intraprostatic lesion. Case A prescribed to the 100% isodose covering PTV, limiting V105% (42 Gy) < 20-30%, with hydrogel spacer and QOD delivery.
Large prostate is not a contraindication if OAR constraints are met; spacer and bladder filling help, and neoadjuvant ADT purely for cytoreduction is discouraged. Pts with baseline IPSS ≥15 had low G3 GU toxicity with prophylactic α1-antagonists but should be counselled on a harder transient flare.
PACE-B is the reference trial but enrolled few large glands (5% ≥80 mL) and allowed prior TURP. MIRAGE (NCT04384770) is cited for MR guidance reducing toxicity.
Case A's own neoadjuvant ADT shrank the gland from 112 to 61 mL, so it does not test SBRT on an untreated large gland. Whether large-gland GU risk is specific to SBRT fractionation remains unresolved pending PACE-B subgroup analyses.
| Approach | Prescription | Hot spot handling |
|---|---|---|
| 100% isodose | Covers CTV or PTV, goal 95-99% receiving Rx | Case A limited V105% (42 Gy) < 20-30% |
| Robotic-style | 75-85% isodose covering PTV | Dmax 130-140% of Rx; inhomogeneity deliberately allowed |
- Is large-gland GU toxicity specific to SBRT fractionation? n=2205 · primary completion 2025-12 · randomizes prostate SBRT vs conventional fractionation
- Does MR-guided SBRT reduce GU toxicity in large glands? recruiting Image-Guidance and Online Adaptation With Stereotactic Body Radiation Therapy for the Treatment of Localized Prostate Cancer, MANTICORE Trial Phase NAn=186 · primary completion 2031-12 · CT- vs MRI-guided SBRT arms, side-effect focus
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The longer read
This piece is a practical, opinion-based companion to the trials that moved prostate SBRT into standard practice. It is not new evidence. Its value is in naming the decisions where the randomized data run thin and saying, as a society, where the authors would stand. The main example is gland size. PACE-B is the trial most readers lean on, but the guide points out that only 5% of its pts had volumes ≥80 mL, so a radiation oncologist offering SBRT to a 100 mL gland is relying on retrospective series. Those series disagree: one found volume ≥50 mL predicted late and any grade ≥2 GU toxicity, while others reported acceptable toxicity at 50-100 mL with no loss of biochemical control. The guide reads the balance as permissive, conditional on meeting constraints. That is a reasonable judgment, but it is a judgment.
Case A complicates that message. The authors say neoadjuvant ADT should not be used only to shrink the gland, yet the illustrative pt received leuprolide, which took the gland from 112 to 61 mL before SBRT. Unfavorable intermediate-risk disease justifies ADT on oncologic grounds, so the choice can be defended. Still, the case does not show SBRT tolerated by an untreated large gland. A reader looking for reassurance on that exact scenario will not find it here.
The prescription discussion is probably the most transferable part. Prescribing 40 Gy to the 100% isodose with V105% held under 20-30% is a very different dose distribution from prescribing to the 75-85% isodose with Dmax at 130-140%. Single-institution toxicity and control figures are hard to compare without knowing which convention a center uses, and that applies to any dose-escalation claim, including the 42 Gy SIB shown in Case B.
On urinary function, the guide rests its position on retrospective series with generous α1-antagonist prophylaxis in pts with IPSS ≥15. The open question it names is the right one: is the large-gland or poor-baseline GU risk specific to five-fraction delivery, or common to prostate RT of any schedule? The authors point to future PACE-B subgroup analyses for the answer. MIRAGE is cited as randomized support for MR guidance reducing toxicity, which makes MR-guided SBRT a lever for the high-risk-of-toxicity pt rather than only a convenience.
The guide could be wrong if large-gland toxicity turns out to be fractionation-dependent. In that case moderate hypofractionation would be the safer default for these pts, and the permissive stance here would need to narrow. The excerpt was truncated before the high-risk, ADT and IBD sections, so this read covers only the large-gland, baseline urinary and TURP discussion.