Potential role of intensity-modulated photons and protons in the treatment of the breast and regional nodes

Abstract

Purpose: To investigate, using comparative treatment planning, the potential improvements that could result through the use of intensity-modulated photons (intensity-modulated radiation therapy [IMRT]) and protons for the locoregional treatment of complex-target breast cancer. Methods and Materials: Using CT data from a breast cancer patient, treatment plans were computed using “standard” photon/electron, IMRT, and forward-planned proton techniques. A dose of 50 Gy was prescribed to the target volume consisting of the involved breast, internal mammary, supraclavicular, and axillary nodes. The standard plan was designed using 6-MV X-ray beams to the breast, axillary, and supraclavicular areas and a mixture of 6-MV X-rays and 12-MeV electrons for the internal mammary nodes. Two IMRT (IMX1 and IMX2) plans were calculated for nine evenly spaced beams using dose-volume constraints to the organs at risk. For plan IMX1, precedence was given to optimizing the reduction in lung and heart dose while preserving target dose homogeneity. For plan IMX2, an increased precedence was given to the lungs, heart, and contralateral breast to further reduce doses to these organs and to study the effect on target coverage. The proton plan consisted of two oblique, energy-modulated fields. Target dose homogeneity and the doses to neighboring organs were both considered when comparing the different plans. Results: For the standard plan, dose-volume histograms (DVHs) of the target volumes showed severe dose heterogeneity, whereas target coverage for the IMRT and proton plans was comparable. Lung DVHs for the standard and IMRT plans were also comparable, while the proton plan showed the best sparing over all dose levels. Mean doses to the ipsilateral lung for the three plans were found to be 17 Gy, 15 Gy, and 13 Gy for the standard, IMRT, and proton plans, respectively. For the heart, the IMRT plan delivered the highest mean dose (16 Gy), reflecting the extra dose delivered through this organ to spare the lungs. This was reduced somewhat by the standard plan (15 Gy), with the best sparing being provided by the proton plan (6 Gy). When the IMRT plan was reoptimized with an increased precedence to the normal tissues, the mean doses to all neighboring organs at risk could be reduced, but only at the cost of substantial target dose heterogeneity. Conclusions: In comparison with the standard plan, IMRT photons have the potential to greatly improve the target dose homogeneity with only a small increase in the doses delivered to the neighboring critical structures. However, when attempting to further reduce doses to the critical structures, substantial loss of target dose homogeneity was found. In conclusion, only the two-field, energy-modulated proton plan had the potential to preserve target dose homogeneity while simultaneously minimizing the dose delivered to both lungs, heart, and the contralateral breast.