Non-coplanar Arc-Involved Beam Arrangement With Sufficient Arc Rotations Is Suitable for Volumetric-Modulated Arc-Based Radiosurgery for Single Brain Metastasis

Abstract

Introduction In linac-based stereotactic radiosurgery (SRS) leveraging a multileaf collimator (MLC) for brain metastasis (BM), volumetric-modulated arcs (VMAs) enable the generation of a suitable dose distribution with efficient planning and delivery. However, the arc arrangement, including the number of arcs, allocation, and rotation ranges, varies substantially among devices and facilities. Some modalities allow coplanar arc(s) (CA(s)) or beam(s) alone, and some facilities only use them intentionally despite the availability of non-coplanar arcs (NCAs). The study was conducted to examine the significance of NCAs and the optimal arc rotation ranges in VMA-based SRS for a single BM. Materials and methods This was a planning study for the clinical scenario of a single BM, including 20 clinical cases with a gross tumor volume (GTV) of 0.72-44.30 cc. Three different arc arrangements were compared: 1) reciprocating double CA alone of each 360º rotation with different collimator angles of 0 and 90º, 2) one CA and two NCAs of each 120º rotation with the shortest beam path lengths to the irradiation isocenter (NCA_L), and 3) one CA of 360º rotation and two NCAs of each 180º rotation (NCA_F). The three arcs were allocated similarly to equally divide the cranial hemisphere with different collimator angles of 0, 45, and 90º. Three VMA-based SRS plans were generated for each GTV using a 5 mm leaf-width MLC with the identical optimization method that prioritized the steepness of dose gradient outside the GTV boundary without any constraints to the GTV internal dose. A prescribed dose was uniformly assigned to the GTV D _{V-0.01 cc}, the minimum dose of GTV minus 0.01 cc. The GTV dose conformity, the steepness of dose gradients both outside and inside the GTV boundary, the degree of concentric lamellarity of the dose gradients, and the appropriateness of the dose attenuation margin outside the GTV boundary were evaluated using metrics appropriate for each. Results The arc arrangements including NCAs showed significantly steeper dose gradients both outside and inside the GTV boundary with smaller dose attenuation margins than the CAs alone, while NCAs showed no significant advantage on the GTV dose conformity. In the NCA-involved arc arrangements, the NCA_F was significantly superior to the NCA_L in terms of the GTV dose conformity, the steepness of dose gradient outside the GTV, the degree of concentric lamellarity of the dose gradients outside and inside the GTV boundary, and the appropriateness of dose attenuation margin. However, the NCA_F showed no significant advantage on the steepness of dose increase inside the GTV boundary over the NCA_L. The dose increase just inside the prescribed isodose surface to the GTV boundary was significantly steeper with the NCA_L than the NCA_F. Conclusions In VMA-based SRS for a single BM, an arc arrangement including NCAs is indispensable, and sufficient arc rotations are suitable for achieving a dose distribution that maximizes therapeutic efficacy and safety in comparison to limited ones which are appropriate for dynamic conformal arcs. Although VMA with CAs alone can provide a non-inferior GTV dose conformity to NCAs, CA(s) alone should be applied only to situations where shorter irradiation time is prioritized over efficacy and safety.

Keywords: brain metastasis; dose conformity; non-coplanar arc; stereotactic radiosurgery; volumetric-modulated arc therapy.

Figure 1. Three different arc arrangements compared.

The images show head computed tomography (CT) images of a patient harboring a single brain metastasis (BM) in the right parietal lobe (A-F); the location of a gross tumor volume (GTV), the irradiation isocenter position, and three arc arrangement patterns (A, D; B, E; C, F); axial views (A-C); and coronal views (D-F). The three arc arrangements consist of two coplanar arcs (CAs) alone with to-and-fro rotations of 360º and each collimator angle of 0 and 90º (A, D); one coplanar arc (CA) and two non-coplanar arcs (NCAs) with each arc rotation of 120º to minimize the beam path lengths and the collimator angle setting of 0, 45, and 90º (B, E); and one CA with 360º rotation and the collimator angle of 0º and two NCAs with each 180º rotation and the collimator angles of 45 and 90º (C, F). The couch was rotated 60º clockwise and counterclockwise so that NCAs trisected the cephalad hemisphere (B, C, E, F). CA: coplanar arcs; NCA_L: non-coplanar arcs with limited rotations; NCA_F: non-coplanar arcs with full rotations.

Figure 2. Comparisons of the GTV dose inhomogeneity and conformity.

The images show box-and-whisker plots (BWPs) along with the results of Friedman’s test (FT), Scheffe’s post hoc test (SPHT), and Wilcoxon signed-rank test (WSRT) (A, B); the GTV D_{V-0.01 cc} (%) relative to the GTV D_{0.01 cc} (100%) to indicate the dose inhomogeneity (A); and the spillage volume (cc) of the irradiated isodose volume (IIV) of GTV D_{V-0.01 cc} outside the GTV to demonstrate the dose conformity (B). GTV: gross tumor volume; D_{V-0.01 cc}: a minimum dose to cover a target volume (TV) minus 0.01 cc (D_>95% for TV >0.20 cc, D_95% for TV ≤0.20 cc); IDS: isodose surface; PIV: prescribed isodose volume; WSRT: Wilcoxon signed-rank test; CA: coplanar arcs; NCA_L: non-coplanar arcs with limited rotations; NCA_F: non-coplanar arcs with full rotations; D_{0.01 cc}: a minimum dose covering 0.01 cc of a TV (D_{0.01 cc} for TV ≥0.20 cc and D_5% (D_{<0.01 cc}) for TV <0.20 cc).

Figure 3. Comparisons of the GTV dose conformity by an alternative metric and the steepness of dose increase just inside the GTV DV-0.01 cc IDS.

The images show BWPs along with the results of FT, SPHT, and WSRT (A, B); the GTV D_eIIV (%) relative to the GTV D_{V-0.01 cc} (100%) (A); and the GTV coverage value by the GTV D_eIIV to indicate the dose conformity alternatively (B). GTV: gross tumor volume; D_{V-0.01 cc}: a minimum dose to cover a target volume (TV) minus 0.01 cc; IDS: isodose surface; D_eIIV: the minimum dose to cover the irradiated isodose volume equivalent to a target volume (on the dose-volume histogram); WSRT: Wilcoxon signed-rank test; CA: coplanar arcs; NCA_L: non-coplanar arcs with limited rotations; NCA_F: non-coplanar arcs with full rotations; BWPs: box-and-whisker plots; FT: Friedman’s test; SPHT: Scheffe’s post hoc test.

Figure 4. Comparison of the appropriateness of dose attenuation margin outside the GTV.

The images show BWPs along with the results of FT, SPHT, and WSRT (A, B); D_eIIV (%) of the GTV + 2 mm relative to the GTV D_{V-0.01 cc} (100%) (A); and the coverage value of GTV + 2 mm by the D_eIIV to demonstrate the degree of concentric lamellarity of dose gradient outside the GTV boundary (B). GTV: gross tumor volume; GTV + 2 mm: GTV evenly expanded by 2 mm; D_eIIV: a minimum dose to cover the irradiated isodose volume equivalent to a target volume; WSRT: Wilcoxon signed-rank test; CA: coplanar arcs; NCA_L: non-coplanar arcs with limited rotations; NCA_F: non-coplanar arcs with full rotations; BWPs: box-and-whisker plots; FT: Friedman’s test; SPHT: Scheffe’s post hoc test; D_{V-0.01 cc}: a minimum dose to cover a target volume (TV) minus 0.01 cc; IDS: isodose surface.

Figure 5. Comparison of the characteristics of the dose gradient inside the GTV boundary.

The images show BWPs along with the results of FT, SPHT, and WSRT (A-D); D_eIIV (%) of the GTV - 2 mm (A) and GTV – 4 mm (C), relative to the GTV D_{V-0.01 cc} (100%), to indicate the steepness of dose increase inside the GTV boundary; and the coverage values of the GTV – 2 mm (B) and GTV – 4 mm (D) to demonstrate the degree of concentric lamellarity of dose increase. GTV: gross tumor volume; GTV - X mm: GTV evenly reduced by X mm; D_eIIV: the minimum dose to cover the irradiated isodose volume equivalent to a target volume; WSRT: Wilcoxon signed-rank test; CA: coplanar arcs; NCA_L: non-coplanar arcs with limited rotations; NCA_F: non-coplanar arcs with full rotations; BWPs: box-and-whisker plots; FT: Friedman’s test; SPHT: Scheffe’s post hoc test; D_{V-0.01 cc}: a minimum dose to cover a target volume minus 0.01 cc.

Figure 6. Comparison of the steepness of dose gradient outside the GTV boundary.

The images show BWPs along with the results of FT, SPHT, and WSRT (A, B); and the spillage volumes (cc) of the IIVs with 75% (A) and 50% (B) of the GTV D_{V-0.01 cc} outside the GTV to demonstrate the steepness of dose gradient outside the GTV. GTV: gross tumor volume; IIV: irradiated isodose volume; WSRT: Wilcoxon signed-rank test; CA: coplanar arcs; NCA_L: non-coplanar arcs with limited rotations; NCA_F: non-coplanar arcs with full rotations; BWPs: box-and-whisker plots; FT: Friedman’s test; SPHT: Scheffe’s post hoc test; D_{V-0.01 cc}: a minimum dose to cover a target volume (TV) minus 0.01 cc.

Figure 7. Comparison of the dose distributions for the GTV of 9.54 cc.

The images show head CT images of a patient with a single BM (A-I), onto which the GTV contoured in red, arc arrangements, and representative isodoses are superimposed; coronal views with the most irregular GTV shape (A-F); and axial views (G-I). The isodose lines are shown as relative values with the GTV D_{V-0.01 cc} as 100% (yellow). GTV: gross tumor volume; CA: coplanar arcs; NCA_L: non-coplanar arcs with limited rotations; NCA_F: non-coplanar arcs with full rotations; CT: computed tomography; BM: brain metastasis; D_{V-0.01 cc}: a minimum dose covering a target volume minus 0.01 cc.