Is a reduction in radiation lung volume and dose necessary with paclitaxel chemotherapy for node-positive breast cancer?

PURPOSE: To evaluate and quantify the effect of irradiated lung volume, radiation dose, and paclitaxel chemotherapy on the development of radiation pneumonitis (RP) in breast cancer patients with positive lymph nodes.

METHODS AND MATERIALS: We previously reported the incidence of RP among 41 patients with breast cancer treated with radiotherapy (RT) and adjuvant paclitaxel-containing chemotherapy. We recorded the central lung distance, a measure of the extent of lung included in the RT volume, in these patients. We used this measure and the historical and observed rates of RP in our series to model the lung tolerance to RT in patients receiving chemotherapy (CHT) both with and without paclitaxel. To evaluate the risk factors for the development of RP, we performed a case-control study comparing paclitaxel-treated patients who developed RP with those who did not, and a second case-control study comparing patients receiving paclitaxel in addition to standard CHT/RT (n = 41) and controls receiving standard CHT/RT alone (n = 192).

RESULTS: The actuarial rate of RP in the paclitaxel-treated group was 15.4% compared with 0.9% among breast cancer patients treated with RT and non-paclitaxel-containing CHT. Our mathematical model found that the effective lung tolerance for patients treated with paclitaxel was reduced by approximately 24%. No statistically significant difference was found with regard to the dose delivered to specific radiation fields, dose per fraction, central lung distance, or percentage of lung irradiated in the case-control study of paclitaxel-treated patients who developed RP compared with those who did not. In the comparison of 41 patients receiving RT and CHT with paclitaxel and 192 matched controls receiving RT and CHT without paclitaxel, the only significant differences identified were the more frequent use of a supraclavicular radiation field and a decrease in the RT lung dose among the paclitaxel-treated patients. This finding indicates that the major factor associated with development of RP was paclitaxel treatment.

CONCLUSIONS: The use of paclitaxel chemotherapy and RT in the primary treatment of node-positive breast cancer is likely to increase the incidence of RP. In patients treated with paclitaxel, reducing the percentage of lung irradiated by 24% should reduce the risk of RP to 1%, according to our calculations of lung tolerance. Future clinical trials using combination CHT that includes paclitaxel and RT should carefully evaluate the incidence and severity of RP and should also accurately monitor the extent of lung included within the RT volume to develop safe guidelines for the delivery of what is becoming standard therapy for node-positive breast cancer.