Radiation Pneumonitis: A Possible Lymphocyte-mediated Hypersensitivity Reaction

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Abstract
To determine if unilateral thoracic irradiation results in a lymphoid alveolitis in both irradiated and unirradiated lung fields. A prospective, nonrandomized study. Women receiving postoperative radiotherapy for carcinoma of the breast were evaluated both before and 4 to 6 weeks after radiotherapy. Findings after radiotherapy in 15 asymptomatic patients were compared with findings in a group of patients with clinical radiation pneumonitis. History, physical examination, chest radiograph, quantitative gallium lung scanning, respiratory function tests, bronchoalveolar lavage, and lavage lymphocyte subset analysis. After irradiation, lavage lymphocytes increased significantly (34.5% versus 46.8%; P = 0.01) in the 17 patients studied prospectively. There was an associated reduction in vital capacity (102.5% versus 95.5%; P = 0.04). Comparison of results in patients before treatment, after treatment without clinical pneumonitis, and after treatment with clinical pneumonitis showed a dramatic increase in total lymphocytes after irradiation (6.3 versus 9.4 versus 35.2 million, respectively; P = 0.005), particularly in those with clinical pneumonitis. Only in those with clinical pneumonitis was this accompanied by an increase in the gallium index (3.7 versus 3.4 versus 9.0, respectively; P < 0.001). Vital capacity was also progressively reduced (102.5% versus 96.9% versus 76.7%, respectively; P = 0.04), as was diffusing capacity (98.6% versus 91.4% versus 72.6%, respectively; P = 0.003). No statistical differences existed between irradiated and unirradiated sides of the chest in either lavage or gallium lung scan studies. In most patients, a lymphocytic alveolitis develops in both lung fields after strictly unilateral thoracic irradiation; this is more pronounced in patients developing clinical pneumonitis. These findings suggest that radiotherapy may cause a generalized lymphocyte-mediated hypersensitivity reaction.

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