Dosimetry comparison between volumetric modulated arc therapy with RapidArc and fixed field dynamic IMRT for local-regionally advanced nasopharyngeal carcinoma

A dosimetric study was performed to evaluate the performance of volumetric modulated arc radiotherapy with RapidArc on locally advanced nasopharyngeal carcinoma (NPC) The CT scan data sets of 20 patients of locally advanced NPC were selected randomly. The plans were managed using volumetric modulated arc with RapidArc and fixed nine-field coplanar dynamic intensity-modulated radiotherapy (IMRT) for these patients. The dosimetry of the planning target volumes (PTV), the organs at risk (OARs) and the healthy tissue were evaluated. The dose prescription was set to 70 Gy to the primary tumor and 60 Gy to the clinical target volumes (CTV) in 33 fractions. Each fraction applied daily, five fractions per week. The monitor unit (MU) values and the delivery time were scored to evaluate the expected treatment efficiency. Both techniques had reached clinical treatment's requirement. The mean dose (D-mean), maximum dose (D-max) and minimum dose (D-min) in RapidArc and fixed field IMRT for PTV were 68.4 +/- 0.6 Gy, 74.8 +/- 0.9 Gy and 56.8 +/- 1.1 Gy; and 67.6 +/- 0.6 Gy, 73.8 +/- 0.4 Gy and 57.5 +/- 0.6 Gy (P < 0.05), respectively. Homogeneity index was 78.85 +/- 1.29 in RapidArc and 80.34 +/- 0.54 (P < 0.05) in IMRT. The conformity index (CI: 95%) was 0.78 +/- 0.01 for both techniques (P > 0.05). Compared to IMRT, RapidArc allowed a reduction of D-mean to the brain stem, mandible and optic nerves of 14.1% (P < 0.05), 5.6% (P < 0.05) and 12.2% (P < 0.05), respectively. For the healthy tissue and the whole absorbed dose, D-mean of RapidArc was reduced by 3.6% (P < 0.05), and 3.7% (P < 0.05), respectively. The D-mean to the parotids, the spinal cord and the lens had no statistical difference among them. The mean MU values of RapidArc and IMRT were 550 and 1,379. The mean treatment time of RapidArc and IMRT was 165 s and 447 s. Compared to IMRT, the delivery time and the MU values of RapidArc were reduced by 63% and 60%, respectively. For locally advanced NPC, both RapidArc and IMRT reached the clinic requirement. The target volume coverage was similar for the different techniques. The RapidArc technique showed some improvements in OARs and other tissue sparing while using reduced MUs and delivery time.