A Novel Approach to Support Quality Assurance (QA) of Intensity Modulated Neutron Therapy (IMNT)

Abstract

Neutron therapy is a form of high linear energy transfer (LET) radiation treatment shown to be beneficial for the treatment of locally advanced head and neck cancers (HNC) resistant to low LET x-ray and proton therapy treatments. The Clinical Neutron Therapy System (CNTS) at our institution has been in clinical operation for over 35 years, and over 3,400 patients have been treated using 3D conformal neutron radiation therapy. In October of 2022, the clinical commissioning of IMNT was completed and the first-ever patient was treated. A novel patient specific quality assurance (PSQA) program has been developed to support IMNT. We present an analysis of our early experiences and PSQA findings for the first 16+ patients treated with IMNT. Our pre-treatment IMNT PSQA program includes (1) ionization chamber measurements, (2) a log-file analysis of treatment delivery, and (3) a γ-analysis of the expected and measured doses (fluences) from a novel positron emission portal imaging system based on 12C(n,2n)11C reactions. Patient setup is confirmed on a daily basis with kV portal imaging. We use a modulation factor (MF), defined as the total number of monitor units (MU)/prescribed dose (cGy) per fraction, to identify and help anticipate IMNT plans that may fail our PSQA program. We have found that IMNT treatments for large, irregularly shaped treatment volumes (e.g., chest wall or head and neck cancers that require treatment of nodes in the lower neck) are more likely to fail PSQA when the MF exceeds 3 MU/cGy. For smaller, more spherical treatment volumes (e.g., base of tongue cancers), plans with a MF up to 3.5 MU/cGy pass our PSQA criteria. Re-optimized plans subsequently passed PSQA with insignificant or no change in tumor coverage and organ-at-risk (OAR) dose. Re-optimized plans also reduced the total number of MU and number of segments, which has the added benefit of decreasing the overall treatment time and patient time on the table. About 10% of early IMNT patients have required plan re-optimization. Our pre-treatment clinical PSQA program and workflow provides useful information to guide IMNT treatment planning and delivery, and helps ensure the safe and accurate delivery of IMNT. Our early experiences suggest IMNT plans with smaller MF values are more likely to pass PSQA than plans with larger values of the MF.