3027 - Cardiac and Breast Sparing Pediatric Whole Lung Irradiation Using Proton Beam Therapy a Cohort Comparison vs. IMRT and 3DCRT
Presenter(s)
B. J. Jacobs1, M. Dhaliwal2, K. Leach3, A. Billoo1, R. O'Neal1, I. Macewan1,3, and B. Chou3,4; 1Department of Radiation Medicine and Applied Sciences, University of California - San Diego, La Jolla, CA, 2Vanderbilt University, Nashville, TN, 3California Protons Cancer Therapy Center, San Diego, CA, 4Department of Radiation Medicine and Applied Sciences, University of California San Diego, La Jolla, CA
Purpose/Objective(s): Whole lung irradiation (WLI) is routinely used to treat children with lung metastases from certain solid tumors, who may also receive cardiotoxic chemotherapy. Dose to the heart and its substructures is a known predictor of long-term morbidity and mortality, and dose to mammary tissue increases lifetime risk of breast cancer. We hypothesized pencil beam proton therapy (IMPT) may lower the integral dose to these high-risk organs at risk (OARs). We sought to compare the dosimetry of proton versus photon WLI on the cardiac and mammary structures that correlate with late morbidity. We also defined a novel central cardiac nexus (CN) substructure informed by the post hoc analysis of RTOG 0617 by McWilliam et al., which signals survival detriment with increasing dose.
Materials/Methods: Ten pediatric patients with rhabdomyosarcoma, Ewing sarcoma, or Wilms tumor treated with WLI using protons (n=5), 3DCRT (n=2), or IMRT (n=3) were retrospectively analyzed from our institution. Dosimetry to the newly defined CN, heart, aortic valve, and breast buds were evaluated. The novel CN substructure—encompassed the left main coronary artery root, proximal left anterior descending artery, and AV nodal region—was retrospectively contoured on original CT simulation. Dosimetric comparisons between IMPT and photon techniques were performed using an independent two-sample two-tailed t-test. Percentage of the prescription dose was used to normalize comparisons, accounting for differences in prescribed dose between patients.
Results: IMPT maintained similar PTV coverage compared with photons (PTV V95%: 98.2% vs 99.7%, p = 0.1) while significantly reducing cardiac dose across all evaluated substructures (as a percentage of prescription dose). IMPT decreased relative mean cardiac nexus dose by 60% of prescription dose (abs 11.2 vs 2.9 Gy, p = 0.0046), relative mean heart dose by 41% of prescription dose (abs 10.8 vs 5.3 Gy, p = 0.018), and relative aortic valve dose by 59% (abs 8.5 vs 0.41 Gy, p = 0.015). Mean breast bud dose evaluated in 4 female pediatric patients was reduced with IMPT (10.5 Gy vs 3.1 Gy, NS).
Conclusion: IMPT for low-dose WLI in pediatric patients significantly reduced radiation dose to the heart and all evaluated cardiac substructures, with the greatest reductions observed in the aortic valve and the cardiac nexus (CN) substructure. Additionally, absolute mean breast bud dose was lower with IMPT. IMPT appears to better adhere to ALARA principles for these critical structures, which have been associated with late cardiac morbidity and an increased risk of secondary malignancy. Although limited by a small sample size, this study provides important preliminary data to support larger cohort investigations aimed at validating these early findings.