Background and Purpose: Intensity modulated radiation therapy (IMRT) is an established technique for the treatment of prostate cancer. Volumetric modulated arc therapy (VMAT) has recently been introduced clinically. Nucletron BV (Veenendal, Netherlands) offers both optimisation algorithms on the same platform in Oncentra® MasterPlan 3.3.
Methods: In this planning study five patients with localised prostate cancer are enrolled to compare these two algorithms. The planning target volume (PTV) and the rectum volume, urinary bladder and femoral heads are delineated as organs at risk (OAR). Collimator and table angles are kept fixed during the treatment. The planning is accomplished using the machine data of a SynergyS linear accelarator of the Elekta Company. For the IMRT seven equispaced beams are chosen, starting at a gantry angle of 0°, followed by 51°, 103°, 154°, 206°, 257° and 309°. The VMAT calculations are made based on a dual arc technique rotating from 182° to 178° and back. The dose-volume objectives are kept identical: Planning Target Volume minimum dose 68 Gy, maximum dose 72 Gy, both weighted 3000, urinary bladder 40 Gy to 50% of the volume, weight 1000, rectum 40 Gy to 30% of the volume, weight 1000, and both femoral heads set to a maximum dose of 30 Gy, weight 300. Average doses for the organs at risk and target dose homogeneity H are evaluated.The homogeneity is hereby defined as (D5-D95)/DAV. Furthermore the number of monitor units are compared. For one representative case the treatment times are measured.
Results: The results of the dose distribution are rather similar: the average homogeneity for the VMAT plans is 0.088, in IMRT 0.094, both exceed the dose-volume objectives for the rectum (46,1 and 44,7 Gy) and the bladder (43,3 and 44.1 Gy). The objectives for the femoral heads are kept rather well although the weight is low (34,1 and 32,1 Gy).
The number of monitor units is clearly higher for the VMAT plans than for the IMRT (1016 and 698 MU). But as the measurement of the treatment times in one representative case (1088 and 775 MU) shows, there is nevertheless an advantage in treatment time for the VMAT case (8,09 min versus 10,14 min).
Conclusion: The results of the dose distribution are similar enough that VMAT is an interesting alternative for the treatment of prostate cancer. The treatment time, which is the crucial factor regarding intrafractional organ movements is advantageous for the VMAT technique.