Treatment replanning in head and neck cancer radiotherapy: an investigation based on clinical dosimetry

Radiotherapy is a medical specialty based on controlled use of ionizing radiation for therapeutic purposes. Patients submitted to external beam radiation therapy of head and neck may suffer anatomical changes during the course of treatment. Frequent causes of such variations are the reduction of tumor volume, the progression of the disease during the course of radiotherapy, the loss of weight, and other causes. The conventional treatments are made of 33 to 35 fractions. Thus, the interval of time between the treatment planning computed tomography (CT) and the conclusion of the treatment surpasses two months. Monitoring significant anatomical variations and making adjustments to the plans of radiotherapy during the course of radiotherapy could be a relevant strategy to achieve the objectives proposed for the treatments. The objective of this study was to analyze how the variations in volume of interest, both target structures and healthy tissues could affect the dose distribution in the patient in radiation therapy treatment of head and neck cancer. This analysis will be made through investigation of metrics dose-volume, volumetric variations of target structures, as well as the parotid glands, patient thickness in the cervical region, image fusion, and isodose surfaces plotted on the CTs. The timing to Replan head and neck cancer appears to be around the 15th fraction of treatment, however a study with a larger number of patients is necessary. In case of adaptive radiotherapy, reduction of mean dose in the ipsilateral parotid was observed in 100% of patients, while in the contralateral parotid reduction was observed in 80% of patients. The decision to replan head and neck cancer has benefit in the spinal cord and brainstem since this study showed reduction in the maximum dose in 70% of patients in case of treatment replan. Decrease of dose including 95% of volume and minimum dose in the PTV (mean percentage decrease observed was 7.71%) was observed in case of no treatment replan. Increase in the maximum dose in the PTV was observed, for which the mean percentage increase observed was 2.42%. Adaptive radiotherapy improves the coverage of target volumes as well as the doses in OARs.