On the reduction of aperture complexity in kidney SABR

Abstract

Background: Stereotactic ablative body radiotherapy (SABR) of primary kidney cancers is confounded by motion. There is a risk of interplay effect if the dose is delivered using volumetric modulated arc therapy (VMAT) and flattening filter-free (FFF) dose rates due to target and linac motion. This study aims to provide an efficient way to generate plans with minimal aperture complexity.

Methods: In this retrospective study, 62 patients who received kidney SABR were reviewed. For each patient, two plans were created using internal target volume based motion management, on the average intensity projection of a four-dimensional CT. In the first plan, optimization was performed using a knowledge-based planning model based on delivered clinical plans in our institution. In the second plan, the optimization was repeated, with a maximum monitor unit (MU) objective applied in the optimization. Dose-volume, conformity, and complexity metric (with the field edge metric and the modulation complexity score) were compared between the two plans. Results are shown in terms of median (first quartile - third quartile).

Results: Similar dosimetry was obtained with and without the utilization of an objective on the MU. However, complexity was reduced by using the objective on the MUs (modulation complexity score = 0.55 (0.50-0.61) / 0.33 (0.29-0.36), P-value < 10^-10 , with/without the MU objective). Reduction of complexity was driven by a larger aperture area (area aperture variability = 0.68 (0.64-0.73) / 0.42 (0.37-0.45), P-value < 10^-10 , with/without the MU objective). Using the objective on the MUs resulted in a more spherical dose distribution (sphericity 50% isodose = 0.73 (0.69-0.75) / 0.64 (0.60-0.68), P-value < 10^-8 , with/without the MU objective) reducing dose to organs at risk given respiratory motion.

Conclusions: Aperture complexity is reduced in kidney SABR by using an objective on the MU delivery with VMAT and FFF dose rate.

Keywords: FFF; SABR; VMAT; interplay; kidney.

Fig. 1

Axial view of a typical dosimetry obtained for one patient with original KBP (top row) and one patient with modified KBP (bottom row). Isodoses 50% (yellow), 100% (magenta), and 120% (blue) are shown for plan with the MU objective (a) and (c) and without the MU objective (b) and (d). The structures ITV (red), PTV (cyan), nontumor ipsilateral kidney (brown), small bowel within 5 cm of the ITV (orange), and large bowel (green) are shown.

Fig. 2

Comparison of dose metrics with (MUO) and without (NMUO) the MU objective for patients with fractionation (a) 26 Gy/1 Fx (24 patients) and (b) 42 Gy/3 Fx (38 patients). Red dotted lines indicate dose limit.

Fig. 3

Conformity index (a) CI95 and (b) CI50 shown with (MUO) and without (NMUO) the MU objective.

Fig. 4

Isodoses 50% (yellow) and 100% (magenta) for a plan (a) with and (b) without the MU objective. The large bowel (green) and small bowel (orange) are adjacent to the PTV (cyan) resulting in the 50% isodose line splaying in between bowel loops. Dose splay is reduced by using an objective on the monitor unit. Sphericity of the (c) 50% isodose and (d) 100% isodose.

Fig. 5

(a) Modulation complexity score (MCS) vs edge metric (EM) in mm⁻¹ with (MUO) and without (NMUO) the MU objective. Complexity of DCAT fields is shown for comparison. (b) Modulation complexity score (MCS), area aperture variability weighted by monitor unit (AAVw), and leaf sequence variability weighted monitor unit (LSVw) with and without the MU objective and for DCAT fields.