# Solution for the External Contour Changes in Cone Beam Computed Tomography-Guided On-demand Online Adaptive Radiotherapy for a Patient With Very Advanced Head and Neck Cancer: A Technical Case Report

## TL;DR

A patient with advanced head and neck cancer was successfully treated using adaptive radiotherapy guided by cone beam CT, offering insights into managing complex tumor contours.

## Contribution

A novel solution for handling external contour changes in adaptive radiotherapy for a large head and neck tumor is presented.

## Key findings

- The treatment successfully managed tumor regression with minimal side effects.
- A solution was implemented to include the full target volume in adaptive radiotherapy optimization.
- The case provides clinical insights for broader application of on-demand adaptive radiotherapy.

## Abstract

This article presents a case of a patient with advanced head and neck cancer, characterized by a large and protruding tumor. The patient was treated with an innovative on-demand online adaptive radiotherapy (ART) technology, guided by cone beam computed tomography (CBCT), on the Ethos adaptive radiotherapy platform (version 1.0, Varian Medical Systems, Palo Alto, CA). A solution was provided for this special case to address the issue where part of the target volume could not participate in the optimization due to exceeding the external contour boundary during online adaptive radiotherapy. The treatment outcome was satisfactory in terms of tumor regression, while only grade 1 radiodermatitis and grade 2 oral mucositis at the end of radiotherapy. This article discusses the clinical diagnosis, treatment process, and follow-up of this case, aiming to provide clinical references for a broader application of this technology.