A comparison of several modulated radiotherapy techniques for head and neck cancer and dosimetric validation of VMAT

Volumetric modulated arc therapy (VMAT) has the potential to shorten treatment times for fluence modulated radiotherapy. We compared dose distributions of VMAT, step-and-shoot IMRT and serial tomotherapy for typical head and neck (H&N) planning target volumes (PTV) with sparing of one parotid, a complex paradigm and a situation often encountered in H&N radiotherapy. Finally, we validated the dosimetric accuracy of VMAT delivery.

Based on CT datasets of 10 patients treated for H&N cancer (PTV1:60 Gy/PTV2:56 Gy) with IMRT (7/9 fields), serial tomotherapy (MIMiC) and VMAT were compared with regard to plan quality and treatment efficiency. Plan quality was assessed by calculating homogeneity/conformity index (HI/CI), mean dose to parotid and brain stem and the maximum dose to the spinal cord. For plan efficiency evaluation, total treatment time (TTT) and number of monitor units (MU) were considered. A dosimetric evaluation of VMAT was performed using radiosensitive film, ion chamber and 2D-array.

For MIMiC/IMRT(7F)/IMRT(9F)/VMAT, mean CI was 1.98/2.23/2.23/1.82, HI(PTV1) was 1.12/1.20/1.20/1.11 and HI(PTV2) was 1.11/1.15/1.13/1.12. Mean doses to the shielded parotid were 19.5 Gy/14.1 Gy/13.9 Gy/14.9 Gy and the spinal cord received maximum doses of 43.6 Gy/40.8 Gy/41.6 Gy/42.6 Gy. The mean MU's were 2551/945/925/521 and the mean TTT was 12.8 min/7.6 min/8.5 min/4.32 min. The ion chamber measurements showed an absolute deviation of 0.08 ± 1.10% and 98.45 ± 3.25% pixels passed γ-analyses for 3%/3mm and 99.95 ± 0.09% for 5%/5mm for films. 2D-array measurements reported an agreement for 3%/3mm of 95.65 ± 2.47%-98.33 ± 0.65% and for 5%/5mm 99.79 ± 0.24%-99.92 ± 0.09% depending on the measurement protocol.

All treatment paradigms produced plans of excellent quality and dosimetric accuracy with IMRT providing best OAR sparing and VMAT being the most efficient treatment option in our comparison of treatment plans with high complexity.