Organ at Risk Dose Constraints for Prostate SBRT
For prostate SBRT, ensure organ at risk constraints are not exceeded when delivering ultrahypofractionated regimens of 36.25 Gy in 5 fractions or 42.7 Gy in 7 fractions, with particular attention to rectal wall, bladder wall, and urethral sparing. 1
Recommended SBRT Dose Regimens
The most current European guidelines recommend two specific ultrahypofractionated SBRT schedules for prostate cancer:
- 36.25 Gy (40 Gy to the prostate) in 5 fractions delivered on alternate days 1
- 42.7 Gy in 7 fractions delivered on alternate days 1
These regimens should only be offered at facilities with appropriate technology, physics, and clinical expertise. 2
Critical OAR Dose Constraints
Rectum/Rectal Wall
The rectal wall requires the most stringent dose constraints in prostate SBRT. The following parameters should be respected:
- V36 Gy ≤ 1 cc (though this may be challenging to achieve in all cases) 3
- D30% < 66.0 Gy (cumulative equivalent dose) 4
- D60% < 38.0 Gy (cumulative equivalent dose) 4
- V122.1 Gy < 5% (biological equivalent dose) 4
Research demonstrates that optimized planning can reduce rectal wall median dose and D30%-D60% by approximately 50% compared to standard optimization, while maintaining target coverage. 5
Bladder/Bladder Wall
Bladder wall constraints for SBRT include:
- D30% < 57.9 Gy (cumulative equivalent dose) 4
- Mean bladder wall dose should be minimized, with achievable values around 14 Gy for 5-fraction SBRT 6
Urethra
Urethral sparing is increasingly recognized as critical for reducing urinary toxicity. 3, 5
- The urethral planning risk volume (PRV) should be defined as a 2 mm expansion of the urethra 3
- When dose-escalating to 5 × 9 Gy to the prostate, the urethral PRV should be limited to 5 × 7.25 Gy (36.25 Gy total) 3
- V34.4 Gy should approach 99.8% for the urethral PRV 3
- D5% should not exceed 38.7 Gy for the urethral PRV 3
Femoral Heads
- Mean femoral head dose should be < 6.8 Gy for 5-fraction SBRT 6
- Maximum dose constraints should respect standard tolerance limits
Planning Optimization Strategy
A standardized stepwise optimization approach significantly improves OAR sparing without compromising target coverage. 5, 7
Sequential Optimization Algorithm:
Initial optimization: Achieve PTV coverage (D98% ≥ 36.2 Gy, D2% ≤ 46.9 Gy for 5 × 7.25 Gy regimen) 3
Urethral constraint application: Limit urethral PRV dose while maintaining PTV-PRV coverage (D95% ≥ 40.6 Gy) 3
Sequential rectal wall optimization: Apply stepwise dose constraints to reduce D30%-D60% by approximately 50% 5
Bladder wall optimization: Minimize low-to-intermediate dose spillage 5
Final verification: Ensure homogeneity index and Dice similarity coefficient remain stable 5
Anatomic Considerations for Constraint Feasibility
The achievability of OAR constraints depends on the 3D proximity of organs to the target. 7
- The expansion-intersection volume (EIV) quantifies organ proximity: intersection volume between target and OAR expanded by 5 mm 7
- Linear correlation exists between EIV and V75% for bladder and rectum 7
- Patients with larger EIV values may require modified constraints or alternative treatment approaches 7
Use of Endorectal Balloon
Endorectal balloon use does not significantly alter achievable rectal wall dose constraints when proper optimization is performed. 5
- No statistical differences in rectal wall dosimetry between plans with and without ERB when using standardized optimization 5
- ERB may provide geometric stability benefits but is not mandatory for achieving acceptable OAR doses 5
Absolute Contraindications Requiring Alternative Treatment
The following conditions preclude safe SBRT delivery:
- Prior pelvic irradiation (cumulative dose toxicity risk) 2
- Active inflammatory rectal disease (active proctitis) 2
- Permanent indwelling Foley catheter (incompatible with precise targeting) 2
Common Pitfalls to Avoid
- Do not proceed with SBRT if OAR constraints cannot be met without compromising target coverage—consider conventional fractionation or alternative modalities 1
- Do not use standard conventional fractionation constraints without converting to appropriate biological equivalent doses for hypofractionation 4
- Do not neglect urethral contouring and constraint application—this is essential for minimizing urinary toxicity 3
- Do not assume all patients are suitable for SBRT—careful patient selection based on anatomy and baseline function is mandatory 2