Dosimetric impact of rotational errors on the quality of VMAT-SRS for multiple brain metastases: Comparison between single- and two-isocenter treatment planning techniques

Abstract

Purpose: In the absence of a 6D couch and/or assuming considerable intrafractional patient motion, rotational errors could affect target coverage and OAR-sparing especially in multiple metastases VMAT-SRS cranial cases, which often involve the concurrent irradiation of off-axis targets. This work aims to study the dosimetric impact of rotational errors in such applications, under a comparative perspective between the single- and two-isocenter treatment techniques.

Methods: Ten patients (36 metastases) were included in this study. Challenging cases were only considered, with several targets lying in close proximity to OARs. Two multiarc VMAT plans per patient were prepared, involving one and two isocenters, serving as the reference plans. Different degrees of angular offsets at various orientations were introduced, simulating rotational errors. Resulting dose distributions were evaluated and compared using commonly employed dose-volume and plan quality indices.

Results: For single-isocenter plans and 1^⁰ rotations, plan quality indices, such as coverage, conformity index and D_95% , deteriorated significantly (>5%) for distant targets from the isocenter (at> 4-6 cm). Contrarily, for two-isocenter plans, target distances to nearest isocenter were always shorter (≤4 cm), and, consequently, 1^⁰ errors were well-tolerated. In the most extreme case considered (2^⁰ around all axes) conformity index deteriorated by on-average 7.2%/cm of distance to isocenter, if one isocenter is used, and 2.6%/cm, for plans involving two isocenters. The effect is, however, strongly associated with target volume. Regarding OARs, for single-isocenter plans, significant increase (up to 63%) in D_max and D_0.02cc values was observed for any angle of rotation. Plans that could be considered clinically unacceptable were obtained even for the smallest angle considered, although rarer for the two-isocenter planning approach.

Conclusion: Limiting the lesion-to-isocenter distance to ≤4 cm by introducing additional isocenter(s) appears to partly mitigate severe target underdosage, especially for smaller target sizes. If OAR-sparing is also a concern, more stringent rotational error tolerances apply.

Keywords: VMAT; brain metastases; rotational error; single isocenter; spatial uncertainty; stereotactic radiosurgery.

Figure 1

A 3D illustration of selected targets lying in close proximity to OARs (brainstem, optic chiasm, optic nerves) for two indicative cases. Contours legend: targets: red, brainstem: green, optic chiasm: yellow, optic nerves: brown.

Figure 2

Indicative (patient #4) reference treatment plans prepared using both the (a) single‐ and (b) two‐isocenter techniques. Isodose lines, corresponding to reference dose distributions, are superimposed on axial and sagittal slices (top) of the planning CT scan. Arcs configuration and related DVHs are also presented (bottom).

Figure 3

Axial CT slice of patient #5 with isodose lines superimposed for the (a) single‐ and (b) two‐isocenter techniques. Reference dose distributions are represented by solid lines, whereas dashed lines correspond to rotated dose distributions (2°, around all three axes). Contours legend: targets: red, brainstem: green, optic chiasm: yellow, optic nerves: brown.

Figure 4

Box and whisker plots summarizing V_20Gy (top) and D_95% (bottom) deviations induced by simulated rotational errors for the (a) single‐ and (b) two‐isocenter planning techniques. Red lines indicate the median of the data, whereas boxes range from the 1st to 3rd quartile. Whiskers depict the remaining data or extend up to 1.5 times the interquartile range in either direction. Red marks denote any outliers.

Figure 5

Calculated DVHs for two targets for the (a) single‐ and (b) two‐isocenter planning techniques. Rotations occurred around all three axes.

Figure 6

Bar charts of the maximum change in V_20Gy are presented for all 36 targets, grouped according to their volume (<1cc, 1‐2cc, >2cc). The bars in blue color are related to the results for (a) single‐isocenter planning technique, whereas the bars in red color are related to the results for (b) two‐isocenter planning technique. Rotations occurred around all three axes.

Figure 7

Percentage PCI change plotted against target distance to the nearest isocenter, for all patients and metastases (i.e., a total of 36 targets) considered, for both the (I) single‐ and (II) two‐isocenter techniques and simulated angular offset of 0.5° in (a) and (d), 1° in (b) and (e), 2° in (c) and (f), occurring around all three axes. Fitted dashed trendlines (along with calculated slopes) are also shown. PCI, Paddick’s conformity index.

Figure 8

Percentage D_95% changes are plotted against the ratio of target‐displacement to target‐diameter, for all patients and metastases (i.e., a total of 36 targets) considered, and stratified by the degree of rotational error (with different marker color) and the isocenter technique (with different marker shape). Rotations occurred around all three axes. A fitted dashed trendline (along with calculated slope, and R‐square) is also shown.