Dosimetric comparison of hypofractionated stereotactic radiotherapy by three different modalities for benign skull base tumors adjacent to functioning optic pathways

Background: As the optic pathways are thought to be the structures most vulnerable to irradiation, skull base tumors involving them are especially challenging to treat. Stereotactic radiosurgery (SRS) / stereotactic radiotherapy (SRT) is an effective and safe option for the treatment of them. Characteristics of dosimetry of SRT for skull base tumors by Gamma Knife were evaluated in comparison with those by other modalities. Materials and Methods: Original Novalis (NV) multi-beam-intensity-modulated-SRT(MB-IM-SRT) plan and additional simulation plans of Gamma Knife (GK) and TomoTherapy (TT) were compared in 20 cases. For target covering, 95% dose was assigned for 95% of the planning target volume (PTV) (D[95%]=28.5 Gy / 5 fractions). Conformity index (CI), homogeneity index (HI=D95% / maximum dose of PTV)), gradient index (GI=V[47.5% dose] / V[95% dose] of body), and the doses to organs at risk (OARs) were evaluated. Results: CI and GI were significantly better with GK than NV or TT. HI was significantly smaller (less homogeneous) with GK. D[1 ml] and V[20 Gy] of brainstem were significantly smaller with GK than NV or TT. V[20Gy] of whole brain was also significantly smaller with GK. D[0.1 ml] and V[20 Gy] of optic pathways were smaller with GK than NV or TT, though the differences did not reach statistical significance. Conclusion: If a higher internal dose gradient is interpreted as an advantage for tumor ablation, GK SRT might be expected to be a more effective and safer treatment for skull base benign tumors adjacent to the optic pathways and brainstem when they are not large.