Three-dimensional dose distribution of tangential breast irradiation: results of a multicentre phantom dosimetry study

Background and purpose: One aspect of good radiotherapeutic practice is to achieve dose homogeneity. Dose inhomogeneities occur with breast tangent irradiation, particularly in women with large breasts. Materials and methods: Ten Australian radiation oncology centres agreed to participate in this multicentre phantom dosimetry study. An Alderson radiation therapy anthropomorphic phantom with attachable breasts of two different cup sires (B and DD) was used. The entire phantom was capable of having thermoluminescent dosimeters (TLD) material inserted at various locations. Nine TLD positions were distributed throughout the left breast phantom including the superior and inferior planes. The ten centres were asked to simulate, plan and treat (with a prescription of 100 cGy) the breast phantoms according to their standard practice. Point doses from resultant computer plans were calculated for each TLD position. Measured and calculated (planning computer) doses were compared. Results: The dose planning predictability between departments did not appear to be significantly different for both the small and large breast phantoms. The median dose deviation (calculated dose minus measured dose) for all centres ranged from 2.3 to 5.3 cGy on the central axis and from 2.1 to 7.5 cGy for the off-axis planes. The highest absolute dose was measured in the inferior plane of the large breast (128.7 cGy). The greatest dose inhomogeneity occur-red in the small breast phantom volume (median range 93.7-105 cGy) compared with the large breast phantom volume (median range, 100.1-107.7 cGy). There was considerable variation in the use (or not) of wedges to obtain optimized dosimetry. No department used 3D compensators. Conclusion: The results highlight areas of potential improvement in the delivery of breast tangent radiotherapy. Despite reasonable dose predictability, the greatest dose deviation and highest measured doses occurred in the inferior aspects of both the small and large breast phantoms. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.