Volumetric modulated arc therapy in differentiated thyroid cancer: a treatment planning comparison with intensity-modulated radiotherapy

Introduction Adjuvant radiotherapy is indicated in differentiated thyroid cancer patients with post-op gross residual disease or microscopic disease with high-risk features. Intensity-modulated radiotherapy (IMRT) is now the standard radiotherapy technique. This is a dosimetric study to compare the static-field IMRT and volumetric modulated arc therapy (VMAT) for patients with differentiated thyroid cancer. Methods Consecutive 15 patients with differentiated thyroid cancer needed post-operative radiotherapy were recruited. A pair IMRT and VMAT plan was generated for each patient. Comparison of dosimetric parameters was performed between IMRT and VMAT. The monitor units (MU) and delivery time by each radiation technique were also compared. Results All pairs of treatment plans in IMRT and VMAT fulfilled the acceptance requirement in terms of target coverage and critical organ sparing. The conformity index and homogeneity index between IMRT and VMAT for PTV66 and PTV60 were not statistically significant. For the organs at risk, the mean dose to parotids was significantly increased by 4.8 Gy (14.4 %) in VMAT when compared to that in IMRT (IMRT vs. VMAT, 33.35 +/- 6.86 Gy vs. 38.15 +/- 7.82 Gy, p = 0.01). The dose parameters of the spinal cord, brain stem, esophagus, and lungs were not statistically significant between the two techniques. Compared to IMRT, the mean MU and delivery time of VMAT were reduced by 71.6 % (p = 0.0004) and 63.1 % (p = 0.004), respectively. Conclusion For adjuvant radiotherapy for differentiated thyroid cancer, IMRT resulted in better parotid sparing at the expense of longer treatment time. Otherwise, the tumor coverage is essentially the same between these two techniques.