A Comparative Dosimetric Analysis by Delivery of Adaptive Radiation Therapy in Locally-Advanced Head-and-Neck Cancer

Abstract

Most patients receiving fractionated radiation therapy (RT) for head and neck cancer (HNC) have marked anatomical changes during their course of treatment, including shrinkage of the primary tumor or nodal masses, resolving edema, and changes in overall body habitus / weight loss. This study is aimed to investigate the dosimetric impact of these changes occurring during the intensity modulated radiation therapy (IMRT) for locally advanced head and neck cancer (LAHNC), to compare the planned and delivered dose to target volume (TV) and organ at risk (OAR) during IMRT and to compare the dose distribution with and without replanning. Twenty patients with LAHNC treated with concurrent chemoradiation therapy were enrolled prospectively in this study. Planning CT scans were acquired both pretreatment and after 40 Gy/20 number of RT. Original plan (OPLAN30) and hybrid plan (HPLAN30) for phase 2 was generated. Assessment of dosimetric changes was performed by comparing the HPLAN30 with the OPLAN30. Adaptive replanning (REPLAN30) based on the second CT scan was done and dose distribution for TV and OAR was compared with and without replanning. Planned doses (D98%, D95%, D50% and D2%) to GTV, CTV and PTV for second phase of RT were not significantly different from the actually delivered doses. However an increase in V110% was observed in the hybrid plan in comparison to the original treatment plan. With replanning, dose distribution to TV was made more homogenous as compared to hybrid plan (p = 0.00). Replanning significantly reduced V>110% and V<93% as compared to hybrid plan (p = 0.003 and p = 0.002, respectively). There was a significant increase in delivered doses (Dmax, D2% and D1%) to spinal cord as compared to planned dose (p = 0.00, 0.001 and 0.001, respectively). Adaptive replanning resulted in decrease in Dmax, D2% and D1% of spinal cord (p = Significant). The mean dose, D33%, D50% and D66% to ipsilateral (I/L) parotid was significantly higher in the HPLAN30 in comparison to the OPLAN30 (p < 0.05). On comparison between the HPLAN30 and REPLAN30, dose to I/L parotid was significantly reduced with replanning. Replanning with repeat CT simulation during the course of IMRT after 40 Gy significantly improves the target volume coverage and reduces the dose to OARs.