Effect of prostatic edema on CT-based postimplant dosimetry

Area of Science:

• Radiation Oncology
• Medical Physics
• Urologic Oncology

Background:

• Prostate brachytherapy involves implanting radioactive seeds for cancer treatment.
• Edema, or swelling, can occur after the procedure, affecting treatment accuracy.
• Sequential CT scans are crucial for monitoring these changes.

Purpose of the Study:

• To quantify prostate edema post-brachytherapy.
• To assess the impact of edema on CT-based dosimetry using various indices.
• To evaluate dose-volume histograms, conformity, and homogeneity over time.

Main Methods:

• 25 prostate cancer patients underwent CT scans pre-implant and on Days 1, 7, and 28 post-implant.
• Prostate, rectum, and bladder volumes were contoured.
• Dose distributions were calculated using TG43 formalism and brachytherapy planning software.
• Dose-volume histograms and quality indices (e.g., V(100), D(90), CI) were analyzed.

Main Results:

• Prostate volume increased by 30% by Day 1 post-implant, then decreased by 20% by Day 28.
• Prescription dose coverage (V(100)) improved from 77% on Day 1 to 85% on Day 28.
• The dose covering 90% of the prostate (D(90)) increased from 84% to 93% over the same period.

Conclusions:

• Decreasing prostate edema from Day 1 to Day 28 significantly improves dose coverage.
• Postimplant dosimetry accuracy is dependent on the timing of CT scans due to volume changes.
• Day 28 volumes remained ~10% larger than pre-implant volumes, highlighting persistent edema.