Isodose Curves in Brachytherapy
Isodose curves are lines connecting points of equal radiation dose in brachytherapy treatment plans, serving as the fundamental tool for visualizing dose distribution around radioactive sources and ensuring adequate target coverage while respecting normal tissue constraints. 1
Definition and Clinical Purpose
Isodose curves represent spatial dose distributions in brachytherapy planning and are essential for:
- Visualizing three-dimensional dose coverage of the target volume on CT or MRI imaging 2
- Identifying underdosed regions within the tumor and overdosed areas in adjacent normal tissues 2
- Quantifying dose-volume relationships through dose-volume histograms (DVHs) that calculate what percentage of a structure receives specific dose levels 3
The curves are typically displayed as percentages of the prescribed dose (e.g., 100%, 150%, 200% isodose lines) overlaid on cross-sectional imaging. 3
Key Dosimetric Parameters Derived from Isodose Analysis
Target Coverage Metrics
For prostate brachytherapy with permanent implants, post-implantation dosimetry must report: 3
- D100, D90, D80: The isodose covering 100% (minimal peripheral dose), 90%, and 80% of the prostatic volume, respectively 3
- Quality benchmark: A good-quality implant requires at least D90 equal to the prescribed dose 3
- V200, V150, V100, V90, V80: The percentage of prostatic volume receiving 200%, 150%, 100%, 90%, and 80% of the prescribed dose 3
For cervical cancer brachytherapy, the traditional point A dosing system remains standard, with doses calculated at: 3, 1
- Point A (tumor surrogate, typically 2 cm superior and 2 cm lateral to external cervical os) 3, 1
- Point B (pelvic sidewall reference) 3, 1
- Bladder and rectal reference points for normal tissue assessment 3, 1
Dose Constraints for Organs at Risk
Critical normal tissue constraints to prevent toxicity: 3
- Urethral dose: The modified peripheral implantation technique is recommended to minimize urethral overdose (>200% of prescribed dose) 3
- Rectal and bladder doses: Must be documented at standardized reference points 3
- Small bowel: Cumulative EQD2 for D2cc should not exceed approximately 75-80 Gy 4
Recommended Dose Prescriptions
Prostate Cancer (Permanent Implants)
Minimum peripheral doses for monotherapy: 3
- Iodine-125: 144 Gy as monotherapy; 100-110 Gy when combined with external beam (40-50 Gy) 3
- Palladium-103: 115 Gy as monotherapy; 80-90 Gy when combined with external beam 3
Post-implantation dosimetry timing: 3
Cervical Cancer (Intracavitary Brachytherapy)
Total point A dose requirements (combining external beam + brachytherapy): 1, 5
Common HDR fractionation: Five insertions with tandem and colpostats, each delivering 6 Gy to point A (total 30 Gy in 5 fractions), biologically equivalent to 40 Gy LDR 3, 1, 5
Modern Image-Guided Approaches
Three-dimensional image-guided brachytherapy optimizes dose distribution by: 3, 1
- Using CT or MRI to delineate actual tumor volumes and organs at risk 1, 6
- Calculating DVH parameters for high-risk clinical target volume (HR-CTV) and intermediate-risk CTV (IR-CTV) 6
- Employing inverse planning optimization to achieve total dose constraints while respecting organ-at-risk limits 6
Critical caveat: When implementing 3D image-guided techniques, traditional point A dosing must not be abandoned to avoid tumor underdosage relative to proven historical benchmarks. 3, 1 The weight of clinical experience and tumor control results remains based on the point A system. 3
Quality Assurance Requirements
Mandatory dosimetric documentation includes: 3
- Volume implanted and total activity delivered 3
- Number of seeds/sources and needles used 3
- Prescribed dose and dose-volume histograms 3
- Doses to organs at risk (urethra, rectum, bladder) 3
CT-based treatment planning with conformal blocking and rigorous dosimetric verification is the standard of care. 1, 4 Transrectal ultrasound should be used for predictive dosimetry and implantation guidance. 3
Common Pitfalls to Avoid
- Relying solely on visual verification: Even when surgical clips appear within implant boundaries on radiographs, actual target coverage may be suboptimal—only 68% median coverage of target volume in one breast brachytherapy series 7
- Two-dimensional planning limitations: Radiographic films alone cannot define three-dimensional dose distributions or identify underdosed regions within the target 2
- Substituting external beam for brachytherapy: Conformal techniques like IMRT or SBRT cannot replace brachytherapy for central disease in intact cervical cancer 1
- Exceeding treatment duration: The entire radiation course must be completed within 8 weeks; each day beyond this decreases pelvic control by 0.5-1% 1, 4