Dose distributions produced by a shielded vaginal cylinder using a high-activity iridium-192 source

Area of Science:

• Medical Physics
• Radiation Oncology
• Brachytherapy

Background:

• Current algorithms for 192Ir dose calculation often neglect scatter dose components.
• This omission can lead to overestimation of tumor dose when scatter is reduced by shielding.
• Vaginal cylinder brachytherapy utilizes sources like 192Ir for gynecological cancer treatment.

Purpose of the Study:

• To investigate the impact of shielded vaginal cylinders on dose distribution.
• To quantify dose reductions caused by tungsten shields of varying angles.
• To assess the accuracy of current dose calculation algorithms in shielded brachytherapy scenarios.

Main Methods:

• Relative dose measurements were performed in a water phantom using a thimble ionization chamber.
• A 2.5 cm diameter vaginal cylinder with and without 0.8 cm thick tungsten shields (90°, 180°, 270°) was used.
• Dose measurements were taken at various distances and angles relative to the 192Ir source.

Main Results:

• Dose reduction on the unshielded side increased with distance and shield angle, reaching 15% at 10 cm for the 270° shield.
• On the shielded side, surface dose was reduced by ~13%, with maximum reductions of 16-23% occurring 2-4 cm from the cylinder.
• Dose reductions did not exceed 2% within 1 cm of the cylinder surface for any shield configuration.

Conclusions:

• Tungsten shielding significantly alters dose distribution around a vaginal cylinder, particularly at greater distances.
• Standard dose calculation algorithms may overestimate tumor dose due to neglecting scatter reduction by shields.
• Accurate dose calculation accounting for scatter is crucial for effective brachytherapy treatment planning with shielded applicators.