Aim

To study the influence of segment width on plan quality for volumetric modulated arc based stereotactic body radiotherapy.

Background

The redundancy of modulation for regularly shaped small volume tumors results in creation of many small segments and an increase of monitor units, with a consequent prolongation of treatment and uncertainty in treatment delivery.

Materials and methods

Six cases each in lung, abdomen and liver were taken for the study. For each case, three VMAT SBRT plans were generated with different penalties on minimum segment width of 0.5, 1.0 and 1.5[[ce:hsp sp="0.25"/]]cm. A comparison was made on the metrics of dose volume histogram, dosimetric indices, monitor units (MUs) and delivery accuracy.

Results

The mean reduction of total MUs when compared with 0.5[[ce:hsp sp="0.25"/]]cm plan was observed as 12.7[[ce:hsp sp="0.25"/]]±[[ce:hsp sp="0.25"/]]6.0% and 17.5[[ce:hsp sp="0.25"/]]±[[ce:hsp sp="0.25"/]]7.2% for 1.0[[ce:hsp sp="0.25"/]]cm and 1.5[[ce:hsp sp="0.25"/]]cm of minimum segment width, respectively. The p value showed a significant degradation in dosimetric indices for 1.5[[ce:hsp sp="0.25"/]]cm plans when compared with 0.5[[ce:hsp sp="0.25"/]]cm and 1.0[[ce:hsp sp="0.25"/]]cm plans. The average deviation of measured dose with TPS calculated was 3.0[[ce:hsp sp="0.25"/]]±[[ce:hsp sp="0.25"/]]1.1%, 2.1[[ce:hsp sp="0.25"/]]±[[ce:hsp sp="0.25"/]]0.84% and 1.8[[ce:hsp sp="0.25"/]]±[[ce:hsp sp="0.25"/]]0.9% for 0.5, 1.0 and 1.5[[ce:hsp sp="0.25"/]]cm, respectively. The calculated gamma index with pass criteria of 2% dose difference and 2[[ce:hsp sp="0.25"/]]mm distance to agreement was 95.9[[ce:hsp sp="0.25"/]]±[[ce:hsp sp="0.25"/]]2.8%, 96.5[[ce:hsp sp="0.25"/]]±[[ce:hsp sp="0.25"/]]2.6% and 97.8[[ce:hsp sp="0.25"/]]±[[ce:hsp sp="0.25"/]]1.6% as calculated for 0.5, 1.0 and 1.5[[ce:hsp sp="0.25"/]]cm of penalties, respectively. In view of the trade off between delivery efficiency and plan quality, 1[[ce:hsp sp="0.25"/]]cm minimum segment width plans showed an improvement.

Conclusions

VMAT SBRT plans with increased optimal value of minimum segment width showed better plan quality and delivery efficiency for stereotactic body radiotherapy.