Prostate Cancer Brachytherapy Protocol
For prostate cancer brachytherapy, use CT-based postimplant dosimetry performed at 4 weeks for Iodine-125 (or 2-3 weeks for Palladium-103), with mandatory reporting of D90 (target ≥prescribed dose), V100, V150, V200 for the prostate, and maximum doses to the urethra and rectum, using Task Group 43 dosimetric parameters for dose calculation. 1
Imaging Protocol
Pre-implantation Planning
- Transrectal ultrasound (TRUS) is the standard imaging modality for predictive dosimetry and implantation guidance 1
- Dosimetric planning must be performed either prior to implantation or during the procedure 1
- While freehand versus US/CT-guided template systems show no definitive superiority, both are acceptable options 1
Postimplantation Imaging
- CT scanning is mandatory for postimplantation dosimetry with calculation of dose-volume histograms (DVHs) 1
- Timing is critical: 4 weeks post-implant for Iodine-125, and 2-3 weeks for Palladium-103 1
- MRI-based planning is emerging as feasible and can provide superior soft tissue definition, though not yet standard 2, 3
Target and Organ-at-Risk Contouring
Target Volume Definition
- Contour the entire prostate volume as visualized on CT
- For intermediate-risk disease, consider including proximal seminal vesicles as clinically appropriate 2
- No additional margin beyond the prostate gland itself is used for permanent seed implants (unlike external beam where PTV margins are added)
Organs-at-Risk (OARs)
The following structures must be contoured and dose-constrained:
- Urethra: Critical structure requiring careful dose monitoring
- Rectum: Specifically the anterior rectal wall 1
- Bladder: Particularly the bladder neck region
- For HDR brachytherapy, also contour neurovascular bundles and penile bulb 2
Dose Calculation
Calculation Method
- Use Task Group 43 (TG-43) dosimetric parameters for dose calculation—this is the standard 1
- Calculate dose-volume histograms for all contoured structures
Dose Grid
While not explicitly specified in the guidelines, standard practice uses:
- Grid size of 2-3mm for adequate resolution
- Calculation extending beyond the prostate to capture rectal and bladder doses
Dosimetric Parameters
Target Coverage (Prostate)
A good-quality implant must achieve 1:
- D90 ≥ prescribed dose (this is the primary quality metric)
- D100: Minimal peripheral dose covering 100% of prostate volume
- D80: Dose covering 80% of prostate volume
Target Volume Coverage
Report the following percentages of prostate volume receiving specified doses:
- V100: Percentage receiving 100% of prescribed dose (aim ≥90%)
- V90: Percentage receiving 90% of prescribed dose (aim ≥95%)
- V80: Percentage receiving 80% of prescribed dose
- V150: Percentage receiving 150% of prescribed dose (aim ≤30%)
- V200: Percentage receiving 200% of prescribed dose (aim ≤15%)
Organ-at-Risk Constraints
- Urethra: Length receiving >200% of prescribed dose should be minimized; use modified peripheral implantation technique to avoid urethral overdose 1
- Rectum: Report D0.1cc or D2cc and doses to anterior rectal wall 1
- Bladder: Report doses received
Prescribed Doses
Monotherapy (Low-Risk Disease)
Combination with External Beam (40-50 Gy)
EQD2 Conversion
The guidelines from 2003 do not explicitly address EQD2 conversion for permanent seed implants, as this concept is more relevant for comparing different fractionation schemes. However:
- For comparing brachytherapy with external beam, use standard linear-quadratic model
- Assume α/β ratio of 1.5-3 Gy for prostate cancer (low α/β reflects slow-growing tumor)
- For organs at risk: α/β = 3 Gy for late effects in rectum/bladder
Plan Verification and Quality Assurance
Mandatory Documentation
The implant report must specify 1:
- Volume implanted (total prostate volume from postimplant dosimetry)
- Number of seeds implanted
- Number of needles used
- Total activity delivered
- Prescribed dose
- Delay between brachytherapy and postimplantation dosimetry
Quality Metrics
- Postimplant dosimetry is mandatory, not optional 1
- Brachytherapy should be performed at centers following strict quality-assurance standards 5
- Training in brachytherapy techniques is essential and should be evaluated 1
Patient Selection Considerations
Ideal Candidates (Monotherapy)
Contraindications
Absolute:
- Previous large transurethral resection of prostate (TURP) 1
Relative:
- Previous limited TURP
- Prostate volume >50-60 cm³
- Median lobe hypertrophy
- Moderate to severe baseline urinary symptoms 1, 4
Implantation Technique
- Modified peripheral implantation technique is recommended to minimize urethral overdose (>200% of prescribed dose) 1
- Evaluate urinary function with questionnaires prior to implantation 1
Patient Counseling Requirements
Patients must receive information about 1:
- Radio-protective measures for children and pregnant women
- Need to use condoms
- Need to filter all urine (to capture any dislodged seeds)
- Requirement to inform physicians before any pelvic intervention
Common Pitfalls to Avoid
- Performing postimplant dosimetry too early: Wait the full 4 weeks for I-125 to allow for seed migration and edema resolution
- Ignoring D90 as primary quality metric: V100 alone is insufficient; D90 must meet prescribed dose
- Inadequate urethral dose monitoring: The modified peripheral technique specifically addresses this risk
- Treating patients with prior TURP: This significantly increases complication risk
- Performing response biopsies too early: Wait 18-24 months post-brachytherapy before biopsy 1
The more recent ASCO/CCO 2017 guideline 5 confirms these technical standards remain appropriate while updating clinical indications, noting that for intermediate-risk disease, brachytherapy boost combined with external beam should be offered, and for high-risk disease receiving EBRT and ADT, brachytherapy boost (LDR or HDR) should be offered to eligible patients.