PURPOSE: To assess clinical factors and volumetric parameters associated with clinically significant symptomatic radiation pneumonitis (RP), which requires steroid medication after radiotherapy (RT). METHODS: Medical records of 204 irradiated breast cancer patients were reviewed. Percent lung volume (PLV) receiving more than 20 Gy was measured from CT-based treatment plan and was correlated with the central lung distance (CLD) of local and regional fields. PLV was also evaluated as a predictive factor of symptomatic RP, along with other previously reported clinical factors. RESULTS: Average (+/-standard deviation) actual irradiated lung volume and PLV for breast/chest wall irradiation were 169 (+/-50.6) cm3 and 14.9 (+/-3.8)%, respectively. Addition of regional irradiation resulted in increase of 183 (+/-80.2) cm3 in actual irradiated lung volume and 16.5 (+/-6.2)% in PLV. The correlation between CLD of the local fields and PLV was significant, with 1 cm of CLD corresponding to approximately 6% of PLV. CLD of the regional field was also significantly associated with PLV: a CLD of 3 cm corresponds to a PLV of approximately 13%; a CLD of 4 cm, approximately 17%; and a CLD of 5 cm, approximately 21%. RP developed in 11 patients (5.4%). There was an increased incidence of RP among patients who underwent local RT vs local and regional RT (2.4% vs 12.1%, p=0.0192). In terms of PLV, total PLV > or =23% was associated with the development of RP (p=0.0467). Previously reported clinical factors failed to show statistically significant association. CONCLUSION: Correlation between CLD and PLV for local and regional fields was significant on volumetric analysis. Although symptomatic RP requiring steroid medication was a rare complication, regional irradiation increased the incidence of RP, and such relationship can be expressed with a volumetric parameter of PLV.