Volume changes during head-and-neck radiotherapy and its impact on the parotid dose - A single-institution...

This study aims at assessing the volume changes that occur in the targets (gross tumor volume and planning target volume [PTV]) and the organs at risk in squamous cell carcinoma of the head and neck during radiotherapy and assessing the dose changes that occur as a result of them.

This was a prospective observational study in a tertiary care center after obtaining the appropriate scientific and ethics committee clearance.

Forty-five patients diagnosed with squamous cell carcinoma of the head and neck, who were treated with intensity-modulated radiotherapy in the time period from March 2018 to May 2019, were enrolled in the study. A planning computed tomography (CT) scan (CT_plan) was done for all patients, followed by scans after 15 fractions (CT₁₅) and after 25 fractions (CT₂₅). The volume changes and the subsequent dose changes were assessed and recorded.

Data entry was done in MS Excel spreadsheet. The continuous variables were expressed as mean + standard deviation. The comparison of normally distributed continuous variables was done by paired t-test. Data analysis was done by SPSS (Statistical Package for the Social Sciences) version 16.0. P < 0.05 was considered statistically significant. A multivariate linear regression model was constructed to study the correlation between mean dose to the parotid glands and the other variables. All statistical modeling and analysis were done using SAS (Statistical Analysis Software) version 9.4.

Of the 45 patients, 25 were male and 20 were female. The majority of the patients had malignancies in the oral cavity (16) and hypopharynx (14). Most of them had Stage III/IV (AJCC v 8) disease (41). There were a 36% decrease in the PTV-high risk (PTV-HR) volume and a 6.05% decrease in the PTV-intermediate risk (PTV-IR) volume CT₁₅. In CT₂₅, the volume decrease in the PTV-HR and the PTV-IR was 47% and 9.06%, respectively. The parotid glands also underwent a reduction in their volume which has been quantified as 21.7% and 20.9% in the ipsilateral and contralateral parotids in CT₁₅ and 36% and 33.6% in CT₂₅, respectively. The D2 (dose received by 2% of the volume) and D98 (dose received by 98% of the volume) of the PTV-IR showed changes of +3.5% and -0.2% in CT₁₅ and + 4.6% and -0.31% in CT₂₅, respectively. The homogeneity index and conformity number of the PTV-IR changes by 0.03 and 0.08 in CT₁₅ and by 0.04 and 0.12 in CT₂₅, respectively. The mean dose to the ipsilateral parotid gland increased by 14% in CT₁₅ and 19% in CT₂₅. The mean dose to the contralateral parotid gland increased by 17% in CT₁₅ and 25% in CT₂₅.

The dose to the parotid glands increases as a result of the changes that occur during the course of radiation. The changes are significant after 15 fractions of radiation. A replanning at this juncture might be considered to reduce the dose to the parotid glands.