Effect of the simulated half leaf width of a multileaf collimator on volumetric modulated arc therapy plan quality in...

Whole-brain radiotherapy (WBRT) is commonly used in patients with multiple brain metastases. Compared with conventional WBRT, hippocampal avoidance WBRT (HA-WBRT) more favorably preserves cognitive function and the quality of life. The hippocampal volume is considerably small (approximately 3.3 cm³ ). Therefore, downsizing the leaf width of a multileaf collimator (MLC) may provide higher spatial resolution and better plan quality. Volumetric modulated arc therapy (VMAT) could simulate the half MLC leaf width through couch shifting between arcs. This study investigated changes in VMAT quality for HA-WBRT with a simulated fine MLC leaf width.

We included 18 patients with brain metastasis. All target and avoidance structures were contoured by an experienced radiation oncologist. The prescribed dose was 30 Gy in 10 fractions. For each patient, three different treatment plans were generated for comparison: VMAT with couch-shift, VMAT without couch-shift, and TomoTherapy. All treatment plans fulfilled Radiation Therapy Oncology Group (RTOG) 0933 criteria for HA-WBRT. The Wilcoxon paired signed-rank test was used to compare different treatment plans.

VMAT with couch-shift had the better average conformity index (0.823) with statistically significant difference compared to VMAT without couch-shift (0.810). VMAT with couch-shift (0.219) had a more favorable average homogeneity index (HI) than did VMAT without couch-shift (0.230), although the difference was not significant. TomoTherapy had an optimal average HI of 0.070. In terms of the hippocampus, all three treatment plans met the RTOG 0933 criteria. VMAT with couch-shift had a lower average D_max (15.2 Gy) than did VMAT without couch-shift (15.3 Gy, p = 0.071) and TomoTherapy (15.5 Gy, p = 0.133). The average D_100% of hippocampus was the same for both VMAT with and without couch-shift (8.5 Gy); however, TomoTherapy had a lower average D_100% value of 7.9 Gy. The treatment delivery time was similar between VMAT with and without couch-shift (average, 375.0 and 369.6 s, respectively). TomoTherapy required a long average delivery time of 1489.9 s.

The plan quality of VMAT for HA-WBRT was improved by using the couch-shift technique to simulate the half MLC leaf width. However, the improvement was not statistically significant except conformity index. The downsizing effect decreased with the use of the sophisticated grade of VMAT. TomoTherapy offered superior plan quality but required the longest delivery time.