Purpose: Patients with lower mediastinal lymphoma (LML) benefit dosimetrically from proton therapy (PT) compared with intensity modulated radiation therapy (IMRT). The added dosimetric benefit of deep-inspiration breath-hold (DIBH) is unknown; therefore, we evaluated IMRT versus PT and free-breathing (FB) versus DIBH among patients with LML. Methods and Materials: Twenty-one patients with LML underwent 4-dimensional computed tomography and 3 sequential DIBH scans at simulation. Involved-site radiation therapy target volumes and organ-at-risk contours were developed for both DIBH and FB scans. FB-IMRT, DIBH-IMRT, FB-PT, and DIBH-PT plans were generated for each patient for comparison. Results: The median difference in lung volume between the DIBH and FB scans was 1275 mL; the average difference in clinical target volume was 5.7 mL. DIBH-IMRT produced a lower mean lung dose (10.8 vs 11.9 Gy; P < .001) than FB-IMRT, with no difference in mean heart dose (MHD; 16.1 vs 15.0 Gy; P = .992). Both PT plans produced a significantly lower mean dose to the lung, heart, left ventricle, esophagus, and nontarget body than DIBH-IMRT. DIBH-PT reduced the median MHD by 4.2 Gy (P < .0001); left ventricle dose by 5.1 Gy (P < .0001); and lung V5 by 26% (P < .0001) versus DIBH-IMRT. The 2 PT plans were comparable, with DIBH-PT reducing mean lung dose (7.0 vs 7.7 Gy; P = .063) and with no difference in MHD (10.3 vs 9.5 Gy; P = .992). Conclusions: Among patients with LML, DIBH (IMRT or PT) improved lung dosimetry over FB but had little influence on MHD. PT (DIBH and FB) significantly reduced lung, heart, esophagus, and nontarget body dose compared with DIBH IMRT, potentially reducing the risk of late complications.