Optimisation techniques in vaginal cuff brachytherapy

Area of Science:

• Radiation Oncology
• Medical Physics
• Gynecologic Oncology

Background:

• Vaginal cuff brachytherapy is a crucial treatment for gynecologic cancers.
• Ensuring homogeneous dose distribution in the target volume and minimizing dose to organs at risk are critical for treatment efficacy and patient safety.
• Standardization of dosimetry protocols and optimization techniques is essential for consistent and effective brachytherapy.

Purpose of the Study:

• To evaluate an in-house dosimetry protocol for achieving homogeneous dose distribution in high dose rate (HDR) iridium-192 (Ir-192) vaginal cuff brachytherapy.
• To determine the frequency of optimization required to meet dose constraints for organs at risk.
• To investigate the relationship between ovoid diameter and dose distribution to the target and organs at risk.

Main Methods:

• Treatment planning for 109 fractions in 33 patients undergoing HDR Ir-192 brachytherapy using Fletcher ovoids.
• Dose prescription and normalization to catheter-oriented lateral dose points (dps).
• Calculation of doses to vaginal apex point (Vk), alternative vaginal apex point (Vk'), International Commission on Radiation Units and Measurements (ICRU) rectal point (Rg), and bladder point (Bl).
• Time-position optimizations performed considering dps, Vk, and Rg doses, aiming for Vk >95% and Rg <85% of prescribed dose.

Main Results:

• The in-house protocol resulted in a mean target dose of 99±7.4% of the prescription dose, indicating good homogeneity.
• Optimization was required in a high percentage of fractions (83%, 92/109).
• Ovoid diameter significantly impacted Rg, Vk, Vk', and dps, with larger ovoids (2.5 cm) leading to higher doses to these points compared to smaller ovoids (2.0 cm and 1.5 cm).

Conclusions:

• The in-house dosimetry protocol and optimization method can achieve homogeneous dose distribution within the target volume in vaginal cuff brachytherapy.
• Frequent time-position optimization is necessary to adhere to dose constraints for organs at risk, particularly the rectum.
• Ovoid size is a significant factor influencing dose distribution and requires careful consideration during treatment planning.