Evaluation of rectal shielding in a Henschke system applicator.

Purpose: To assess the effectiveness of avoid shielding in the Henschke intracavitary gynaecologic 3-channel applicator.

Material And Methods: An acrylic phantom was used with our locally modified 3-channel Henschke applicator so that standard treatments with caesium-137 sources could be simulated. Thermoluminescent dosimeters were used to measure point A and rectal doses with and without ovoid shielding to assess the benefits of this shielding. Unshielded measurements were also compared to our planning computer calculations to assess its accuracy. The thermoluminescent dosimeters were calibrated against a caesium teletherapy unit. An estimate of shielding effect produced by the ovoid shielding when using iridium-192 wire was also determined.

Results: Doses received at points in the plane containing the rectal point as defined by the ICRU in its report 38 [4] show a reduction ranging from 5% to 15% over the area measured due to the ovoid shielding. As expected this benefit is more pronounced when using iridium-192 sources. Given the steep dose gradients, good agreement is found between computed and measured values of point A doses.

Conclusion: While ovoid shielding will never provide a large reduction in rectal dose using caesium-137 sources, our results indicate that it can be a worthwhile option when using the Henschke applicator.