Location, Location, Location: A Treatment Planning Study Comparing OAR Sparing between IMRT and PBS Plans Based on Disease Location Using the Ann Arbor Staging System for Hodgkin and Aggressive Non-Hodgkin Lymphoma

Abstract

<h3>Purpose/Objective(s)</h3> Radiotherapy (RT) is often used as a component pf combined-modality therapy for patients with Hodgkin (HL) and aggressive non-Hodgkin(aNHL) lymphoma. RT options include photon intensity-modulated (IMRT) or proton (PT) RT. While the latter offers improved organ-at-risk (OAR) sparing, PT is a limited resource. As such, to guide provider and patient decision-making, it is important to identify cases in which PT has the greatest impact. Accordingly, we sought to assess the difference in OAR sparing between photon IMRT and pencil-beam-scanning (PBS) PT based on disease location using the Ann Arbor staging system. <h3>Materials/Methods</h3> Patients with HL or aNHL treated using either photon or proton RT at a single institution between 2007 and 2020 were considered for analysis. Of these, a preliminary subset of 43 patients, all with mediastinal disease, was selected for a comparative dosimetric analysis. Two plans—IMRT and PBS PT—were developed for each patient using consistent planning techniques. OAR differences were assessed based on disease location using the Ann Arbor staging system. Lower mediastinal disease referred to disease below the aortic valve. <h3>Results</h3> The initial dosimetric analysis focused on 6 disease locations and 5 OARs, including the heart, two cardiac substructures, lungs, and breast (female patients only). Table 1 shows PBS plans relative to IMRT, providing the percent decrease in mean dose to the 5 OARs when employing PBS. <h3>Conclusion</h3> The use of PBS resulted in a substantial decrease in the mean dose to OARs, the magnitude of which depended on the disease location. Sparing in the left hilar and right hilar locations did not vary substantially from the general cohort, while patients with left axillary disease did receive a meaningful increase in sparing to the left ventricle when compared to the general cohort. Sparing was less prominent in patients with lower mediastinal disease. However, the absolute dose to the OARs was higher for these patients and sparing may still be clinically valuable. This preliminary analysis represents a portion of the total cohort available to the study. Further work will be performed to fully establish patterns and evaluate statistical significance.