Gamma Knife radiosurgery for vestibular schwannomas: Evaluation of planning using the sphericity degree of the target volume

Introduction: This study explores the possibility of a relationship between the sphericity degree of a target volume with the dose distribution. This relationship is evaluated based on the ratio isodose volume / target volume (IV/TV) and the metrics coverage, i.e., selectivity, gradient index, conformity index and mean dose when planning radiosurgery for vestibular schwannoma. Methods: Sphericity degree (φ) was calculated for each target volume (TV) of 64 patients who underwent stereotactic radiosurgery (SRS) for vestibular schwannoma. The calculation of this parameter was developed using the theoretical definition for operational sphericity φ = VP/VCS. The values found are evaluated considering the following metrics:—Coverage (C), selectivity (S), gradient index (GI), Paddick conformity index (CIPaddick) and dose distribution (IV/TV). The planning was also carried out considering a spherical target volume defined in a spherical phantom. The spherical volume is the same as the target used in the treatment plan. The planning of the spherical target was considered as a reference plan to evaluate the dose distribution inside and outside the volume. Results: It was possible to observe that the majority of target volumes has (ϕ) around 0,66–0,77, corresponding to 54,7% of the total. Considering the mean values for metrics, the results are: C = 0,98, S = 0,78, GI = 3,11 and CI = 0,81. The dose distribution was equivalent for treatment plans and reference plans. Quantitative analysis for IV/TV shows that these values are higher than 30% for treatment plans where shot density is large. Conclusion: This study demonstrates that de sphericity degree (φ) can be related to the dose distribution (IV/TV). Therefore the sphericity degree is a good parameter to evaluate the dose distribution of a plan for vestibular schwannoma treatment, considering the reference plan as being a spherical target using a leksell gamma knife® perfexion (LGKP). This study shows that the sphericity degree offers important information of the dose distribution outside and inside the target volume. This is not evaluated by the other parameters already implemented as metric to analyzing the GKP plans.