Is intra-fraction motion of clinical relevance for patients with multiple intracranial metastases when treated with single isocentre multi-target volumetric arc radiosurgery? A retrospective service evaluation to assist the implementation of single isocentre radiosurgery

Introduction: Inherent to SRS and the small treatment fields used are the tight margins and resulting steep dose gradient. Positioning errors are of extreme importance, as any deviation, commonly known as intrafraction motion, could result in geographical miss. When multiple lesions are treated with a single isocentre, a new potential for error is created. The purpose of this study was to investigate intrafraction motion and rotational deviations in patient position during frameless stereotactic radiosurgery for intracranial metastases to aid the implementation of single isocentre stereotactic radiosurgery for multiple targets into routine practice. Method and materials: Retrospective study of 21 SRS/SRT patients with 25 intracranial metastases. For patients with multiple metastases, discrete isocentres were placed in each lesion and treated sequentially. A non-invasive thermoplastic mask was used for immobilisation and ExacTrac/Novalis Tx 6DOF system used for kV image pre-positioning and verification at each floor angle to correct for intrafraction motion. Rotational deviations after initial set up correction were recorded. Additionally, floor twists were recorded and analysed using Pearson’s correlation coefficient to determine how rotational deviations were related to floor movement. Results: From 163 rotational deviations the mean and mode of all rotations in three rotational axis was 0.4o, 0.3o and 0.2o and 0.0o, 0.1o and 0.1o for lateral, longitudinal and vertical axis respectively. The maximum rotation was 1.7o, 1.0o and 1.5o (lateral, longitudinal and vertical respectively). Larger rotations were seen in patients treated over 3 fractions. From 93 floor twists a correlation of -.029, -.0.34 and -.262 for lateral, longitudinal and vertical rotational axis was recorded. Conclusion and Discussion: It was determined that 149/163 (91%), 152/163 (93%) and 152/163 (93%) rotations (lateral, longitudinal and vertical respectively) are within current tolerance (0.7o) indicating that current set up and immobilisation allows for safe implementation of the novel single isocentre technique when. A conservative approach to implementation is recommended. No relationship between floor twist was found. Future investigation into the placement of a single isocentre at the planning stage and direct evaluation and quantification of geographical miss is needed.