What is the indication for prostate Stereotactic Body Radiation Therapy (SBRT) with 36.25 Gy in 5 fractions in a patient with localized prostate cancer and unfavorable intermediate-risk disease?

Prostate SBRT 36.25 Gy in 5 Fractions: Indications

Prostate SBRT at 36.25 Gy in 5 fractions (ultrahypofractionation) is indicated for patients with low-risk or favorable intermediate-risk localized prostate cancer who decline active surveillance, but should only be offered to intermediate-risk patients within a clinical trial or multi-institutional registry due to limited comparative evidence. 1

Risk-Appropriate Patient Selection

Low-Risk Disease

• SBRT ultrahypofractionation may be conditionally offered as an alternative to conventional fractionation for low-risk prostate cancer patients (T1-T2a, Gleason ≤6, PSA <10 ng/mL) who decline active surveillance. 1
• This recommendation is based on moderate-quality evidence, as active surveillance remains the preferred approach for very low and most low-risk patients with life expectancy >10 years. 2

Favorable Intermediate-Risk Disease

• For favorable intermediate-risk patients (single intermediate-risk factor: T2b-c, Gleason 7 with primary pattern 3, or PSA 10-20 ng/mL), ultrahypofractionation may be offered but patients should be strongly encouraged to enroll in a clinical trial or multi-institutional registry. 1
• The evidence quality is low due to limited prospective comparative studies in this population. 1

Unfavorable Intermediate-Risk Disease: NOT INDICATED

• Ultrahypofractionation should NOT be offered for unfavorable intermediate-risk disease (primary Gleason pattern 4, ≥50% positive biopsy cores, or multiple intermediate-risk factors) outside of clinical trials. 1, 3
• These patients have significantly worse outcomes (5-year PSA relapse-free survival 38-58% vs 85% for favorable risk) and require dose-escalated radiation with 4-6 months of ADT. 2, 4

Critical Technical Requirements

Mandatory Image Guidance

• Image-guided radiation therapy (IGRT) with daily prostate localization using CT, ultrasound, implanted fiducials, electromagnetic targeting, or endorectal balloon is strongly required for safe delivery. 1
• Without proper IGRT, the risk of geographic miss and increased toxicity is unacceptable. 1

Required Treatment Planning

• Intensity-modulated radiation therapy (IMRT) or more advanced techniques must be used; non-modulated 3D conformal techniques are contraindicated due to excessive toxicity risk. 1
• At least 2 dose-volume constraint points for rectum and bladder (one high-dose, one mid-dose) must be applied and strictly adhered to. 1
• Deviating from published reference study dose constraints significantly increases acute and late toxicity risk. 1

Contraindications and Exclusions

High-Risk Disease: CONTRAINDICATED

• Ultrahypofractionation should NOT be offered outside clinical trials for high-risk disease (T3a, Gleason 8-10, or PSA >20 ng/mL). 1
• High-risk patients require dose-escalated radiation (≥78 Gy conventional or equivalent) combined with 2-3 years of ADT for optimal survival outcomes. 2

Unfavorable Anatomic Features

• Large prostate size may preclude safe ultrahypofractionation due to dosimetric constraints. 1
• Patients with significant obstructive urinary symptoms should be counseled that SBRT can exacerbate these symptoms. 2

Comparative Context

Standard Alternatives Remain Preferred

• Moderate hypofractionation (60-70 Gy in 20-28 fractions) is recommended across all risk categories with high-quality evidence showing equivalent cancer control and toxicity to conventional fractionation. 1
• For unfavorable intermediate-risk patients specifically, dose-escalated EBRT (≥78 Gy) or combined EBRT-brachytherapy with ADT provides superior outcomes compared to ultrahypofractionation. 2, 4

Evidence Quality Hierarchy

The strength of evidence decreases from moderate hypofractionation (high-quality RCTs) to ultrahypofractionation (limited prospective data), particularly for intermediate-risk disease. 1 This explains why ultrahypofractionation remains investigational for most intermediate-risk patients despite theoretical radiobiologic advantages.

Common Pitfalls to Avoid

• Do not use ultrahypofractionation for patients with unfavorable intermediate-risk features (primary Gleason 4, ≥50% core involvement, multiple risk factors) outside trials. 1, 3
• Do not deliver consecutive daily treatments for 5-fraction regimens without adequate inter-fraction repair time. 1
• Do not exceed validated dose constraints for organs at risk, as this dramatically increases toxicity. 1
• Do not omit ADT in unfavorable intermediate-risk patients receiving any radiation modality, as this compromises survival outcomes. 2