Is Stereotactic Body Radiation Therapy (SBRT) delivering 36 Gy in 6 fractions to the prostate and nodes a viable treatment option for a patient with localized or locally advanced prostate cancer?

SBRT 36 Gy in 6 Fractions to Prostate and Nodes: Evidence Assessment

The 36 Gy in 6 fractions SBRT regimen is supported for salvage treatment of intraprostatic recurrence after prior radiotherapy, but there is insufficient evidence to recommend this specific dose-fractionation schedule for treating both the prostate and pelvic lymph nodes in the definitive setting for newly diagnosed prostate cancer. 1

Evidence for 36 Gy in 6 Fractions

Salvage Setting (Post-Radiation Recurrence)

The 36 Gy in 6 fractions regimen has established evidence specifically in the salvage context:

• This dose schedule stems from the prospective phase 2 GETUG-AFU 31 study and is recognized as a commonly used external-beam regimen for reirradiation of locally recurrent prostate cancer after prior definitive radiotherapy. 1

• The regimen is appropriate for whole-gland salvage treatment of intraprostatic radiorecurrence with acceptable safety profiles demonstrated in prospective series. 1

Definitive Treatment Setting (Newly Diagnosed Disease)

For upfront treatment of newly diagnosed prostate cancer, the evidence does NOT support 36 Gy in 6 fractions:

• The established SBRT regimens for definitive treatment are 36.25 Gy in 5 fractions or 42.7 Gy in 7 fractions, delivered on alternate days—NOT 36 Gy in 6 fractions. 2, 3

• These validated regimens (36.25 Gy/5fx or 42.7 Gy/7fx) have demonstrated 5-year biochemical relapse-free survival rates of 95% for low-risk, 84% for intermediate-risk, and 81% for high-risk patients in pooled analyses. 1, 3

Critical Issue: Nodal Irradiation with SBRT

There is no high-quality evidence supporting SBRT to pelvic lymph nodes at any dose-fractionation schedule in the definitive setting. The literature addresses this gap:

• SBRT for prostate cancer has been studied exclusively for prostate-confined disease in prospective trials, with no validation for elective nodal irradiation using stereotactic techniques. 4

• When pelvic nodal coverage is indicated (unfavorable intermediate-risk or high-risk disease), conventional approaches use intensity-modulated radiotherapy (IMRT) to deliver 45-50 Gy to the pelvis combined with either prostate boost or brachytherapy—not SBRT to nodes. 1

• The combination of EBRT to nodes plus brachytherapy boost has Level I evidence for improved outcomes in high-risk disease, whereas SBRT to nodes remains investigational. 1

Technical and Safety Considerations

Facility Requirements

If considering any prostate SBRT regimen:

• SBRT should only be performed at centers with appropriate technology, physics expertise, and clinical experience with image-guided delivery systems. 2, 3

• Strict quality assurance standards and precise targeting capabilities are mandatory. 3

Contraindications to Prostate SBRT

Absolute contraindications that would preclude this approach entirely:

• Prior pelvic irradiation (cumulative radiation toxicity risk). 2
• Active inflammatory disease of the rectum/active proctitis. 2
• Permanent indwelling Foley catheter. 2

Relative contraindications requiring careful consideration:

• Very low bladder capacity, chronic moderate-severe diarrhea, bladder outlet obstruction requiring suprapubic catheter, or inactive ulcerative colitis. 2

Organ-at-Risk Constraints

For the validated 36.25 Gy in 5 fractions regimen (closest to your proposed schedule):

• Rectal wall, bladder wall, and urethral sparing constraints must be strictly respected—if these cannot be met without compromising target coverage, conventional fractionation should be used instead. 2

• Urethral planning risk volume (2 mm expansion) should be limited to 5 × 7.25 Gy (36.25 Gy total). 2

Risk-Stratified Treatment Recommendations

Low-Risk Disease (PSA <10, Gleason ≤6, ≤T2a)

• SBRT monotherapy to prostate only using validated regimens (36.25 Gy/5fx or 42.7 Gy/7fx) is appropriate. 3
• No nodal irradiation indicated. 3

Intermediate-Risk Disease

• Favorable intermediate-risk: SBRT monotherapy to prostate only may be offered. 3
• Unfavorable intermediate-risk: SBRT should be combined with 6 months ADT; if nodal coverage needed, use conventional IMRT to pelvis (45 Gy) with prostate boost rather than SBRT to nodes. 3, 1

High-Risk Disease (PSA >20, Gleason 8-10, ≥T3)

• SBRT alone is NOT appropriate—Level I evidence demonstrates RT combined with at least 24 months ADT significantly improves overall survival compared to RT alone. 3
• Nodal irradiation requires conventional fractionation (45-50 Gy IMRT to pelvis) combined with dose-escalated prostate treatment. 1

Common Pitfalls to Avoid

1. Do not extrapolate salvage reirradiation data (36 Gy/6fx) to the definitive treatment setting—these are distinct clinical scenarios with different evidence bases. 1

2. Do not attempt SBRT to pelvic lymph nodes outside of clinical trials—this lacks validation and appropriate dose-fractionation schedules for nodal targets have not been established. 1

3. Do not proceed if organ-at-risk constraints cannot be met—this indicates the patient is not a suitable SBRT candidate and should receive conventional fractionation. 2

4. Do not use SBRT monotherapy for high-risk disease—this contradicts Level I evidence showing survival benefit from combined RT plus long-term ADT. 3