Changes of tumor and normal structures of the neck during radiation therapy for head and neck cancer requires adaptive strategy.

The treatment period over which radiation therapy is administered extends over several weeks. Since tumor shrinkage in response to radiation therapy and weight loss due to radiation-induced mucositis may impact on the dose distribution in both target and organ at risk in patients with head and neck cancer, the anatomical changes of tumor and neck volumes during this period should be taken into consideration. We investigated the anatomical changes that occurred in the target and normal structure of the neck during radiation therapy for pharyngeal cancer, and evaluated the necessity of an adaptive strategy. Ten patients with pharyngeal cancer who underwent radical chemoradiation therapy using 3-dimensional conformal radiation therapy RT (66-70 Gy in 33-35 fractions) between April 2009 and September 2010 were enrolled in the study. Patients underwent CT scans every week during their course of treatment. We analyzed the CT data in the radiation treatment planning system and measured changes of tumor, organ at risk, and neck volume. Gross tumor volume (GTV) was rapidly reduced by 28% of the original volume on average in the first 3 weeks. The right and left submandibular glands volume decreased to 70% and 63% of their initial volumes on average, respectively. The volume of the neck in the radiation fields decreased to 89% of its initial volume on average by the sixth week mainly caused by body weight loss due to acute radiation morbidity. Considerable anatomical change in the radiation filed that will affect dose distribution of the target and organ at risk was observed during radiation therapy for head and neck cancer.