Room: Exhibit Hall
Purpose: To investigate the impact of patient immobilization devices on calculated entrance skin doses of the supraclavicular field for three-field prone breast radiotherapy.
Methods: Two patients who received three-field prone breast radiotherapy had their treatment plan calculated using six scenarios with 265 and 200 cGy per fraction for breast and supraclavicular targets. The immobilization devices considered were the prone breast board, MOLDCARE cushions and couch support structure. The breast board and cushion contours were united with the body contour in order for the planning system to account for their presence in the path of the treatment fields. The six scenarios evaluated were: (1) Body only, (2) Body + Couch, (3) Body + Breast Board, (4) Body + Breast Board +Couch, (5) Body + Breast Board + Cushion, and (6) Body + Breast Board + Cushion + Couch. A field aperture incident on the patient was contoured to create a skin irradiated contour used for skin dose comparisons with in-vivo film measurements using dose histograms.
Results: Without any inclusion of immobilization, the mean entrance skin doses were 120.5Â±50.9 cGy for patient A and 112.3Â±40.8 cGy for patient B. With the inclusion of all immobilization and couch structures, the mean entrance skin dose was 233.3Â±26.1 cGy for patient A and 224.9Â±15 cGy for patient B. Various combinations of immobilization and couch with the body contour produced means between these values for each patient. The film measurements were 213Â±23 cGy for patient A and 192Â±30.5 cGy for patient B.
Conclusion: Our study showed that patient immobilization and support structure had a non-negligible effect on entrance skin dose calculations and that it would be prudent to include patient immobilization in the treatment planning stage of the three-field prone breast radiotherapy to provide accurate skin dose estimation.