Citation: Chinniah, S.; Deisher, A.J.; Herman, M.G.; Johnson, J.E.; Mahajan, A.; Foote, R.L. Rotating Gantries Provide Individualized Beam Arrangements for Charged Particle Therapy. Cancers 2023, 15, 2044. https://doi.org/10.3390/ cancers15072044 Academic Editors: Charles B. Simone and David A. Gewirtz Received: 31 January 2023 Revised: 12 March 2023 Accepted: 25 March 2023 Published: 29 March 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). cancers Article Rotating Gantries Provide Individualized Beam Arrangements for Charged Particle Therapy Siven Chinniah 1 , Amanda J. Deisher 2 , Michael G. Herman 2 , Jedediah E. Johnson 2 , Anita Mahajan 3 and Robert L. Foote 3, * 1 Mayo Clinic Alix School of Medicine, Jacksonville, FL 32224, USA 2 Department of Radiation Oncology, Division of Medical Physics, Rochester, MN 55905, USA 3 Department of Radiation Oncology, Mayo Clinic, Rochester, MN 55905, USA * Correspondence: foote.robert@mayo.edu; Tel.: +1-507-284-3551 Simple Summary: Carbon ion radiotherapy (CIRT) facilities are proliferating throughout Asia and Europe. They are costly to build, operate and maintain. Rotating gantries can deliver individualized treatments to patients with CIRT-eligible malignancies due to the increased number of choices of beam angles for treatment delivery. They help to minimize compromises in the treatment planning process, optimizing local tumor control and reducing the risk for acute and late adverse events, but they are expensive. Our aim is to retrospectively report the number and range of beam angles utilized to deliver proton therapy with a rotating gantry to a wide variety of patients with CIRT-eligible malignancies and to determine the proportion of tumor sites treated with unimodal, bimodal and multimodal groupings of beam angles. We found that only esophagus and pancreas cancers were treated with unimodal or bimodal beam groupings. Rotating gantries provide individualized beam arrangements for most CIRT-eligible patients. Abstract: Purpose: This study evaluates beam angles used to generate highly individualized proton therapy treatment plans for patients eligible for carbon ion radiotherapy (CIRT). Methods and Mate- rials: We retrospectively evaluated patients treated with pencil beam scanning intensity modulated proton therapy from 2015 to 2020 who had indications for CIRT. Patients were treated with a 190 ◦ rotating gantry with a robotic patient positioning system. Treatment plans were individualized to provide maximal prescription dose delivery to the tumor target volume while sparing organs at risk. The utilized beam angles were grouped, and anatomic sites with at least 10 different beam angles were sorted into histograms. Results: A total of 467 patients with 484 plans and 1196 unique beam angles were evaluated and characterized by anatomic treatment site and the number of beam angles utilized. The most common beam angles used were 0 ◦ and 180 ◦ . A wide range of beam angles were used in treating almost all anatomic sites. Only esophageal cancers had a predominantly unimodal grouping of beam angles. Pancreas cancers showed a modest grouping of beam angles. Conclusions: The wide distribution of beam angles used to treat CIRT-eligible patients suggests that a rotating gantry is optimal to provide highly individualized beam arrangements. Keywords: charged particle therapy; rotating gantries; carbon ion radiotherapy 1. Introduction The use of charged particle therapy in cancer treatment has been growing in the United States and worldwide, with 38 centers in the United States and more than 110 worldwide and an additional 65 more under construction or in the planning phase globally [1–3]. The expansion of proton therapy (currently 99 centers) and carbon ion radiotherapy (CIRT, currently 13 centers) has required a significant financial investment, with the cost of a single-room proton therapy center estimated at about $20–40 million USD and multi- room or multi-ion centers costing on the order of $200 million USD [4]. One of the major Cancers 2023, 15, 2044. https://doi.org/10.3390/cancers15072044 https://www.mdpi.com/journal/cancers