The doses received by the breast during mantle radiotherapy.

Purpose: To accurately measure the dose received by the breast during mantle radiotherapy.

Methods And Materials: A phantom containing lung-equivalent material was used to measure the doses received by the breast during mantle radiotherapy given by anterior and posterior opposing fields. These were measured using thermoluminescent dosimeters and compared with point dose calculations obtained by computer planning.

Results: Most of the breast lies under the lung shields or inferior to the mantle field, but the upper outer quadrant of the breast remains unshielded. In the unshielded areas of the breast, the average dose measured was nearly 13% higher than the dose prescribed at the central axis. In the shielded parts of the breast, the average measured dose was nearly 10% of the dose prescribed at the central axes, decreasing from 18% superiorly to 4% inferiorly. The posterior field contributed 45% to the dose in the breast, even though doses were prescribed at the midplane. The computer calculations systematically varied from measured doses by up to 35%, becoming less accurate towards the inferior edge of the field.

Conclusions: In a conventional course of mantle radiotherapy (for example, 36 Gy in 20 fractions), most of the breast is shielded but will receive a dose of 3-4 Gy, higher than expected largely due to internally scattered radiation passing through the lungs from the posterior field. Computer dose calculations may poorly reflect actual off-axis doses in large fields with complex shielding, containing inhomogeneous tissue.