Development of adaptive radiotherapy (ART) protocol based on multiple plan optimisation for head and neck cancer

Abstract

The radiation response of head and neck squamous cell carcinoma (HNSCC) and the consequent volumetric changes for tumor volume and surrounding organs at risk are considerably affecting the quality of the initial radiotherapy treatment plan. This study aims to quantify the volumetric and morphological changes for the tumor volume, organ at risk, and surrounding tissue and its effect on the dose distribution for the irradiated volume. A total of 36 patients who underwent head and neck squamous cell carcinomas (HNSCC) treatment were enrolled in this study. Initial CT-simulation was performed using a Siemens SOMATOM DEFINITION, ERLANGEN, and GERMANY computed tomography scanner. Monaco treatment planning system version 5.11 was used for IMRT treatment planning (tumor and organs at-risk delineation and plan design). All patients underwent rescanning and multiple plan optimisation, after the initial one. The rescanning was done at radiotherapy treatment fraction numbers 7, 14, and 21. Tumor and organ-at-risk volumes were determined and compared to the initial volumes. The dosimetric effects of the volumetric and morphological changes were evaluated. The best timing for applying ART was determined based on effective radius (ER) reduction, where a 3 mm ER decrease at any time was considered significant. Adaptive radiotherapy is a continuously changing treatment plan delivered to patients based on observed anatomical changes caused by weight loss, tumor shrinkage, or internal organ motion. The results showed significant volumetric and morphological changes. Tumor volume reductions between the initial CT and the fourth CT at fraction number 21 were 34.2%, 30%, and 37% for PTV, CTV, and GTV, respectively, P-value of 0.001. Volume reductions of parotid glands and sub-mandibular glands at fraction 21 were 48% and 46% (p-value 0.001) for the right and left parotid glands, respectively, and 43% (pvalue 0.001) for each sub-mandibular gland. Regarding morphological changes, the average weight loss at fraction number 21 was 6 kg. The statistical analysis performed on the dosimetric alterations due to volume reductions showed a significant improvement in the dose distribution of tumor volumes and a decrease in the mean and maximum dose of organs at risk after applying ART. In light of this study, the best timing for applying ART is at fraction number 14 for patients with small PTV volumes less than 100 cc. For medium and large PTV volumes from 100 to 500 cc, the best timing for ART is after fractions number 7 and 14. Using the ER method instead of percentage volume can clearly demonstrate the distance between reference isodose volume and PTV, which is the best timing for ART. Adaptive radiotherapy planning is important to eliminate dosimetric alteration due to volumetric changes. The initial radiotherapy plan is not adequate for the best outcome results in head and neck cancer patients. Based on tumor volume changes during radiotherapy, the ER method can be used as a quantitative measurement to determine the best timing to apply ART.