Purpose: To develop an intensity modulated radiotherapy (IMRT) technique for postmastectomy RT that improves target coverage while sparing all appropriate normal tissues.
Materials and methods: IMRT plans were generated using an in-house optimization system. Priority was given to matching the heart doses achieved with partially wide tangent fields (PWTFs) while maintaining 50 Gy +/- 5% to the chest wall, internal mammary nodes, and supraclavicular nodes. Other normal tissue doses were then minimized. Metrics for plan comparisons included minimal, maximal, and mean doses and normal tissue complication probability.
Results: IMRT resulted in more uniform chest wall coverage than did PWTFs. The average chest wall minimal dose was 43.7 +/- 1.1 Gy for IMRT and 31.2 +/- 16.5 Gy for PWTFs (p = 0.04). The average internal mammary node minimal dose was 42.8 +/- 2.1 Gy for IMRT and 21.8 +/- 13.2 Gy for PWTFs (p = 0.001). IMRT matched the <1% heart normal tissue complication probability achieved using PWTFs. The average contralateral breast mean dose was 2.8 +/- 1.7 Gy for IMRT, but a greater breast volume was exposed compared with PWTFs. The mean ipsilateral lung normal tissue complication probability was lower for IMRT (0.0) than for PWTFs (0.07 +/- 0.07; p = 0.02). The mean contralateral lung dose was greater for IMRT (5.8 +/- 1.8 Gy) than for PWTFs (1.6 +/- 0.1 Gy; p = <0.0001).
Conclusion: A new IMRT technique achieves full target coverage while maintaining similar doses to heart and ipsilateral lung as conventional techniques. However, contralateral lung and breast volumes exposed to low doses were increased with IMRT and will need to be reduced in future studies.