Anatomical and Dose Changes of Gross Tumour Volume and Parotid Glands for Head and Neck Cancer Patients during Intensity-modulated Radiotherapy: Effect on the Probability of Xerostomia Incidence

Aims: To quantify the changes in dose as well as in the prediction of parotid gland toxicity due to anatomical changes during therapy of head and neck cancer patients. Materials and methods: Fifteen patients with advanced locoregional head and neck cancer, with no evidence of distant metastasis, were enrolled in a prospective study. All patients were treated with intensity-modulated radiotherapy. Multiple computed tomography scans were repeated at the end of each treatment week. The original treatment plans were copied to the per-treatment scans to create hybrid plans. The normal tissue complication probability (NTCP) was calculated assuming the end point to be grade ≥3 xerostomia according to the Radiation Therapy Oncology Group late toxicity scale. Results: The gross tumour volume dose coverage was slightly affected by the anatomical changes, whereas the mean dose (D _mean) to the parotids changed from 26.1 ± 6.0 to 27.4 ± 7.4 Gy, with a mean increase of 0.22 Gy/treatment week. Consequently, the mean NTCP increased from 0.15 ± 0.06 to 0.18 ± 0.10, primarily due to a few patients exhibiting a marked increase. The absolute gross tumour volume shrinkage and the percentage parotids shrinkage were the best independent predictors for the NTCP variations. Conclusions: On average, the increase in the parotids D _mean as well as in NTCP during treatment is limited, and the observed variations were strongly patient-dependent.