We abstracted the radiation therapy treatment parameters from each patient¡¯s radiation therapy record. Treatment fields included direct chest wall (37 % of patients), medial and lateral tangentials (45 % ), supraclavicular (SCV, 64 % ), internal mammary (IM, 44 % ), SCV and IM together (16 % ), axillary (52 % ), and breast/chest wall boosts (7 % ). The beam types used were 60Co (45 % of fields), orthovoltage (33 % ), megavoltage photons (11 % ), and electrons (10 % ). The population median prescribed dose to the target volume ranged from 21 Gy to 40 Gy. We reconstructed the doses over the length of the esophagus using abstracted patient data, water phantom measurements, and a computational model of the human body.
Fields that treated the SCV and/or IM lymph nodes were used for 85 % of the patients and delivered the highest doses within 3 regions of the esophagus: cervical (population median 38 Gy), upper thoracic (32 Gy), and middle thoracic (25 Gy). Other fields (direct chest wall, tangential, and axillary) contributed substantially lower doses (approximately 2 Gy). The cervical to middle thoracic esophagus received the highest dose because of its close proximity to the SCV and IM fields and less overlying tissue in that part of the chest. The location of the SCV field border relative to the midline was one of the most important determinants of the dose to the esophagus.
Breast cancer patients in this study received relatively high incidental radiation therapy doses to the esophagus when the SCV and/or IM lymph nodes were treated, whereas direct chest wall, tangentials, and axillary fields contributed lower doses.