In silico assessment of out-of-field dose in multiple brain metastases stereotactic radiosurgery

Abstract In stereotactic radiosurgery (SRS) for multiple brain metastases (MBM), there is higher out-of-field (OOF) dose contribution than that experienced in single metastasis. This study seeks to assess the impact of the introduced error in OOF dose in MBM SRS. CyberKnife Off-Centre-Ratios (OCRs) were modified by altering the measured umbra dose by an average 0.3 and 0.6% of the maximum in-field dose for 12 cones at 5 depths. An existing MATLAB Independent Dose Calculation (IDC) code was used to calculate dose at the Gross-Target-Volume (GTV) centre-of-mass (COM) and to surrounding voxels. GTV COM dose was calculated for 126 GTVs (36 patients) and dose deviations between unmodified and modified OCR were compared. Treatment plans in the Precision treatment planning system were analysed for potential causes of dose uncertainties. Up to 7.2% GTV COM dose deviation was observed for an average 0.6% change in umbra dose. Patients with many GTVs that were close to each other, with GTVs through which many beam passed or were proximal, and patients with large GTVs had higher dose deviations due to OOF dose contributions. Patients with multiple dose prescription levels (MDPLs) had larger dose deviations for GTVs with lower prescribed dose. Introducing known errors in umbra dose showed that MBM doses are impacted by OOF dose non-uniformly. OOF dose from one GTV beams can contribute significant dose to other GTVs and can be influenced by GTV-to-GTV proximity, geometrical distribution of GTVs, beam orientations, GTV sizes, and MPDLs. MBM SRS dose calculation demands accurate estimates of OOF dose.