An evaluation was conducted for HyperArc brain stereotactic radiosurgery (SRS) plans to determine the effect of intra-fractional motion on target coverage and dosimetry to understand the safe limits of zero-PTV-margin HyperArc SRS treatments.

The Steve phantom was CT scanned with an Encompass mask and imported into the Eclipse Treatment Planning System, version 16·01. Two high-resolution spherical contours (GTV1 and GTV2) were generated with various sizes and separations. The rotational shift was transformed into translation based on the distance from the tumours to the isocentre and the rotational angle between the tumours. The effect of translational shifts corresponding to each rotational shift on dosimetric performance was evaluated for various tumour sizes and distances to the isocentre.

The tumour coverage was compared for different tumour sizes and distances to the isocentre due to intra-fractional motion. For tumour sizes less than 6 mm in diameter, the volume coverage changes significantly for separations greater than 5·5 cm from the isocentre. The effects of intra-fractional motion on dosimetry dominate for separations greater than 5·5 cm. Even though the percent volume coverage changes sharply for extreme shifts, the percent dose coverage stays between 70 and 80% for extreme shifts (1·2° in our case) for all lesion sizes.

The target-to-isocentre separation and the size of the lesions are two major factors that contribute to significant dosimetric deviations. When the target-to-isocentre distance is within 3·5 cm, D100% is over 90% coverage. Over 90% target coverage is achievable for zero PTV margin in situations where extreme shifts of 1·2° exist.