Relation of radiotherapy dose and sternocleidomastoid muscle (SM) mass or late response in patients with nasopharyngeal carcinoma

Objective To investigate the relationship of radiation dose with the volume and late toxicity of the sternocleidomastoid muscle (SM) in patients with nasopharyngeal carcinoma. Methods SM was divided into upper part and lower part based on the lower edge of cricoid cartilage. Patients were divided into three groups according to the prescribed dose for clinical target volume at the lower neck (CTV2)(0, 54, 60 Gy). The dosimetric parameters included Dmean, V66, and V60 for the upper, lower, and whole SM. SM was delineated and the volume was calculated on computed tomography images in the treatment planning system before and at 6, 12, and 18 months after treatment. The anteroposterior and transversal diameters of SM at C3-C4, C4-C5, C5-C6, and C6-C7 levels were measured and recorded. Late toxicity of neck skin and SM was evaluated according to the Common Terminology Criteria for Adverse Events V4.0 criteria. Between-group comparison was made by t-test or Kruskal-Wallis non-parametric test. Between-group comparison of the sample rate was made by one-way analysis of variance. The correlation analysis was made by Spearman correlation. Results There were significant difference in SM volume between the three time points after treatment (P=0.000). At 12 or 18 months after treatment, the volume of SM wasignificantly reduced (P=0.000, 0.000); the reduction in SM volume was significantly correlated with V66 of the SM and the upper SM (P=0.015, 0.020). At 18 months after treatment, SM fibrosis was significantly correlated with V60 of the upper SM (P=0.030); the fibrosis of neck skin was significantly correlated with the Dmean and V60 of the upper SM (P=0.029, 0.005). Conclusions In order to prevent the incidence of the fibrosis of neck skin and SM, the dose homogeneity should be as high as possible, while the number of hot spots should be as small as possible. Key words: Nasopharyngeal neoplasms/radiotherapy; Radiation dose; Sternocleidomastoid muscle; Late toxicity