Is linac-based volumetric modulated arc therapy better than helical tomotherapy in the radiotherapy of nasopharyngeal carcinoma?

Abstract

Volumetric arc therapy (VMAT) and helical tomotherapy (HT) are currently the two state-of-the-art rotational intensity-modulated radiotherapy (IMRT). When compared with conventional IMRT, linac-based VMAT can shorten the treatment time while HT has been claimed to produce a better dosimetric outcome. The aim of our study was to evaluate whether the more “efficient” VMAT can produce a comparable or better dosimetric outcome when compared with HT for the treatment of nasopharyngeal carcinoma (NPC) patients. Sixteen primary NPC patients treated by HT were recruited. In addition to the original HT plans, two-arc VMAT plans using the same patient data and dose requirements were generated for each patient. The dosimetric evaluation of the plans included the comparison of conformity index (CI), homogeneity index (HI), dose to 2 and 98 % volume (D 2 and D 98) of the planning target volume (PTV) of the NP, and left and right lymphatic regions. For the organs at risk (OARs), maximum dose or D 2 and mean dose were used for serial and parallel organs, respectively. Student’s paired t test or Wilcoxon signed-rank test was used to check for significant difference. The dose to the NP target was similar between the two techniques. For the nodal targets, the VMAT plans produced higher mean D 2 and lower HI values. For the OARs, the spinal cord and spinal cord doses were higher in the VMAT plans than in the HT plans, whereas the opposite happened in the optic chiasm and pituitary gland. The doses to the brain stem, lens, and optic nerve did not show a significant difference. Both systems delivered satisfactory dose distributions and kept the dose to OARs below their tolerance. However, if the parotid gland toxicity is a concern in the treatment, HT would be more favorable because it is relatively more effective in sparing the parotid gland. Otherwise, the linac-based double-arc VMAT would be recommended because of its shorter treatment time.