Rationale for and preliminary results of proton beam therapy for mediastinal lymphoma

Purpose: To evaluate the potential of three-dimensional proton beam therapy (3D-PBT) for reducing doses to normal structures in patients with mediastinal lymphomas compared with conventional photon radiation therapy (RT). Methods and Materials: We treated 10 consecutive patients with mediastinal masses from lymphomas with 3D-PBT between July 2007 and February 2009 to 30.6-50.4 cobalt-Gray equivalents (CGE). Of those patients, 7 had primary refractory or recurrent disease, and 8 had Hodgkin lymphoma. Dosimetric endpoints were compared with those from conventional RT plans. Results: PBT delivered lower mean doses to the lung (6.2 vs. 9.5 Gy), esophagus (9.5 vs. 22.3 Gy), and heart (8.8 vs. 17.7 Gy) but not the breasts (5.9 vs. 6.1 Gy) than did conventional RT. Percentages of lung, esophagus, heart, and coronary artery (particularly the left anterior descending artery) volumes receiving radiation were consistently lower in the 3D-PBT plans over a wide range of radiation doses. Of the 7 patients who had residual disease on positron emission tomography before PBT, 6 (86%) showed a complete metabolic response. Conclusions: In patients with mediastinal lymphomas, 3D-PBT produced significantly lower doses to the lung, esophagus, heart, and coronary arteries than did the current conventional RT. These lower doses would be expected to reduce the risk of late toxicities in these major organs.