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Is intraoperative nomogram-based overplanning of prostate implants necessary?
Warren D D'Souza1, Hoon K Lee, Matthew B Palmer
1Division of Radiation Oncology, The University of Texas M. D. Anderson Cancer Center, Houston, TX, USA. wdsou001@umaryland.edu
Summary
Intraoperative planning for prostate brachytherapy using a nomogram adequately predicts necessary activity for optimal dosimetry. This method ensures quality implants and keeps critical structure doses within tolerance, despite dynamic dose distribution changes.
Area of Science:
- Medical Physics
- Radiation Oncology
- Urologic Oncology
Background:
- Intraoperative planning for prostate implants using nomograms is an established practice.
- The adequacy of nomograms in predicting optimal dosimetry requires further investigation.
Purpose of the Study:
- To evaluate the accuracy of a nomogram in predicting the total activity required for optimal dosimetry in prostate brachytherapy.
- To assess the impact of nomogram-predicted activity on treatment plan quality and critical structure doses.
Main Methods:
- Eighty CT-based postimplant treatment plans were analyzed for patients with early-stage prostate cancer treated with I-125 permanent implants.
- Dosimetric parameters (V100, D90, etc.) for prostate, urethra, and rectum were measured at CT1 (day of implant) and CT2 (1 month post-implant).
- Pearson correlation and linear regression were used to assess the relationship between dosimetric parameters and the difference between implanted and nomogram-predicted activity (dA).
Main Results:
- Prostate V100 and D90 at CT1 correlated significantly with dA, indicating nomogram accuracy at this time point.
- Higher prostate dose volumes (V150, V200) correlated with dA.
- Rectal V100 and V70 correlated significantly with dA, suggesting potential for increased rectal dose with increased activity.
- No significant correlation was found between urethral doses and dA.
Conclusions:
- CT1 dosimetry underestimates, while CT2 dosimetry overestimates the delivered dose.
- Intraoperative planning using nomogram-predicted activity achieves target coverage consistent with quality implant guidelines.
- Critical structure doses, particularly for the rectum, should be monitored, though urethral doses remain within tolerance.