Oral Scientific SessionsRainbows and Butterflies: Individualized IMRT Planning for Mediastinal Lymphoma

At our institution, mediastinal lymphomas are treated with intensity modulated radiation therapy (IMRT) using a “butterfly” technique, with 3 anterior and 2 posterior beams. We hypothesized that a “rainbow” arrangement, with 5 anterior beams, is superior for patients with anterior mediastinal disease. Rainbow and butterfly IMRT plans were generated for 20 patients with anterior mediastinal lymphoma. The beam arrangement consisted of 1 anterior-posterior (AP) beam and 4 anterior obliques for the rainbow technique (0o, 20-30o, 40-60o, 300-320o, and 335-345o), and 1 AP, 2 anterior obliques, and 2 posterior obliques for the butterfly technique (0o, 25-40o, 155-170o, 190-200o, and 330-345o). The prescription dose was 30.6 Gy in 17 fractions. Dosimetric data were compared using the two-tailed paired t-test. The preferred plan, based on target coverage and normal structure avoidance, was selected for each patient based on scores assigned by PlanIQ™ software. These scores range from 0% to 100%, with higher numbers indicating a greater likelihood of meeting pre-specified dosimetric goals. The plan with the higher PlanIQ™ score was selected for each patient. In addition to anterior mediastinal disease, patients had involvement of the neck (n = 16), hilum (n = 11), axilla (n = 2), and posterior mediastinum (n = 6). For this cohort, the rainbow plans received higher PlanIQ™ scores overall (mean: 82.2% vs 80.1%, P = 0.03). The rainbow plan received a higher score for 13 patients (65%). The 7 patients for whom the butterfly plan was superior had a higher rate of posterior mediastinal involvement (57% vs. 15%; P = 0.05) PTV coverage was equivalent with the 2 techniques (PTV V95%: 95% vs. 96%; P = 0.3). Structures that received lower dose with the rainbow technique included: heart (mean of mean heart dose: 9.9 Gy vs 10.4 Gy, P = 0.0007), left ventricle (mean D10%: 8.9 Gy vs 10 Gy, P = 0.04), left main coronary artery (mean of maximum dose [Dmax]: 30 Gy vs 32 Gy, P = 0.04), left anterior descending artery (mean Dmax: 25 Gy vs 28 Gy, P = 0.03), and spinal cord (mean Dmax: 29 Gy vs 32 Gy, P = 0.002). The lung V5 was lower with the butterfly technique (mean V5: 42% vs 44%, P = 0.02). There was no difference in mean lung dose, lung V20, mean esophagus dose, or breast V5. Overall, the rainbow arrangement was preferred for most patients; however, the butterfly plan was favored for patients with posterior mediastinal disease. Selection of the optimal beam arrangement is encouraged based upon the anatomy and prioritization of organs at risk for each individual patient.