To study the simplified sphericity index (SSI) of planning target volume (PTV) and correlate it with the gradient index (GI) for stereotactic radiosurgery (SRS)/stereotactic radiotherapy (SRT) treatment of brain metastasis.

A collection of fifteen brain metastasis cases previously treated with SRS/SRT by volumetric-modulated arc therapy (VMAT) technique was included in the analysis. All the previous plan data from Monaco 6.2.1.0 TPS were used for re-planning and computation of SSI and GI. Pearson’s correlation analysis was performed by using OriginPro 8.5 software, and the outcomes were tabulated.

The statistical analysis and linear fitting of data show a negative linear correlation between SSI and GI, taking SSI as the independent variable and GI as the dependent variable. Pearson’s correlation coefficient (r) was found to be -0.91563 with a p-value of 0.0000124 showing strong statistical significance.

It is observed that the GI of the PTV improves as the SSI increases, that is, when the target volume approaches a perfect sphere. Calculating the SSI of the target before planning may help in predicting the GI which may guide making crucial decisions regarding PTV dose prescription and acceptance criteria for organs-at-risk dose tolerance.