Magnetic Resonance Image Guided Adaptive Radiation Therapy: Clinical Applications in the Pediatric and Adolescent and Young Adult Population

Abstract

<h3>Purpose/Objective(s)</h3> To demonstrate the feasibility of online adaptive magnetic resonance (MR) image guided radiotherapy in the pediatric, adolescent and young adult (AYA) population. <h3>Materials/Methods</h3> All pediatric (age < 18) and AYA patients (age < 30), treated on an MR linear accelerator (MRL) from 2019 to 2021 were enrolled onto a prospective registry. The system is used with a related treatment planning system which employs a Monte Carlo dose calculation algorithm to account for effects of the magnetic field on the dose distribution. Step-and-Shoot intensity-modulated radiation treatment (IMRT) plans were generated for all Unity patients with a dose grid of 3 mm and a statistical uncertainty of ≤ 1% per plan. CT and MR images were obtained for each patient for initial radiation treatment planning. Daily treatment setup consisted of a volumetric MRI scan followed by rigid registration used to evaluate the agreement between the reference plan contours and daily shifts in internal anatomy. If necessary, based on evaluation of the daily anatomy, online recontouring and plan re-optimization was performed. <h3>Results</h3> A total of 14 pediatric and AYA patients have been treated. The median patient age is 13 years (range: 14 mo -27 yrs). Four patients were ≤ 8 years old. Reasons for treatment on the MRL were improved visualization of the primary tumor, re-irradiation in a critical area, and need for daily adaptive replanning to minimize dose to critical structures The clinical applications of MR-guided radiotherapy included Ewing sarcoma (primary and metastatic, n = 3), recurrent diffuse intrinsic pontine glioma (DIPG, n = 2), nasopharyngeal carcinoma (n = 1), clival chordoma (n = 1), primitive neuroectodermal tumor of the pancreas (n = 1), recurrent gluteo-sacral germ cell tumor (n = 1), Wilms tumor with unresectable IVC thrombus (n = 1), C-spine ependymoma (n = 1), and posterior fossa ependymoma (n = 1). Only one child (age 14 mo. with Wilms tumor required anesthesia. One AYA patient with Ewing sarcoma of C1 could not complete the treatment course due to pain related to longer treatment times and was transferred to a conventional linear accelerator. No patient experienced treatment interruptions or unexpected toxicity. Reasons for plan adaptation included changes in stomach and small bowel anatomy, target shape change on a near-daily basis, critical location adjacent to the brainstem, and manipulation of hot spots near the spinal cord. <h3>Conclusion</h3> Radiotherapy on the MRL was well-tolerated by pediatric and AYA patients. There was no increased use of anesthesia outside of our usual practice. Significant dosimetric advantages were seen for patients with tumors in critical locations such as adjacent to or involving the brainstem, spinal cord or near abdominal organs that change shape and location daily.