Significance of Dosimetric Parameters in Patients Undergoing Gamma Knife Radiosurgery for Vestibular Schwannoma.

Vestibular schwannoma (VS) is a benign, encapsulated, and slow-growing tumor of the myelin-forming cells of the 8th cranial nerve. Gamma Knife radiosurgery (GKRS) has become a widely accepted primary treatment modality for small- to medium-sized VSs. In the case of VS, highly conformal, precisely focused radiation is delivered to the acoustic tumor in a single session under the direct supervision of a radiosurgery team. Aim: This study aims to determine the significance of Conformity Index and Gradient Index (GI) in patients undergoing GKRS for VS, retrospectively, and re-assess the plans. Materials and Methods: A dosimetric study of 112 patients of VS (both operated and nonoperated) treated on Gamma Knife Perfexion unit at our hospital, over a 3-year period, was carried out retrospectively. The patients' mean age at the time of GKRS was 48 years and the mean dose to the tumor margin was 13 Gy. The conformality of the treatment plan was determined by Conformity Index. GI determines fall off dose outside the target. Results: The dosimetric parameters such as Conformity Index and GI were calculated using the dose-volume histograms and the volume analysis tools available in the Leksell Gamma Plan using TMR 10 algorithm. The mean Paddick Conformity Index was found to be around 0.80 ± 0.085 and the mean GI was 2.67 ± 0.22. Conclusions: The dosimetric parameters can be used to evaluate the dose coverage and conformity and dose fall off outside the target.

Spontaneous carbonate formation in an amorphous, amine-rich, polymeric drug substance: sevelamer HCl product quality.

Spectral differences among multiple manufacturers/lots of sevelamer HCl were observed by Fourier transform infrared spectroscopy, and further characterization was performed to identify the cause for these differences. The drug substance is a polymer that possesses a large molecular weight, is amorphous, and is practically insoluble in both water and organic solvents. Thus, solid-state characterization methods (spectroscopic and thermal) were required to identify and characterize differences among the samples to assess possible differences in product quality. ¹³C cross-polarization-magic-angle-spinning nuclear magnetic resonance spectroscopy of sevelamer HCl substances demonstrated the presence of a carbonyl-containing species, which was attributed to a carbonate impurity among samples. Stability studies demonstrated that this carbonate impurity formed spontaneously upon exposure of the drug substance to atmospheric water vapor and carbon dioxide, even under ambient conditions. Mechanistically, this behavior likely arises from the large number of primary and secondary amine groups, the hygroscopicity of the HCl salt, and a high degree of molecular mobility due to the amorphous nature of the drug substance.

Dose verification to cochlea during gamma knife radiosurgery of acoustic schwannoma using MOSFET dosimeter.

AIM: Dose verification to cochlea using metal oxide semiconductor field effect transistor (MOSFET) dosimeter using a specially designed multi slice head and neck phantom during the treatment of acoustic schwannoma by Gamma Knife radiosurgery unit. MATERIALS AND METHODS: A multi slice polystyrene head phantom was designed and fabricated for measurement of dose to cochlea during the treatment of the acoustic schwannoma. The phantom has provision to position the MOSFET dosimeters at the desired location precisely. MOSFET dosimeters of 0.2 mm x 0.2 mm x 0.5 µm were used to measure the dose to the cochlea. CT scans of the phantom with MOSFETs in situ were taken along with Leksell frame. The treatment plans of five patients treated earlier for acoustic schwannoma were transferred to the phantom. Dose and coordinates of maximum dose point inside the cochlea were derived. The phantom along with the MOSFET dosimeters was irradiated to deliver the planned treatment and dose received by cochlea were measured. RESULTS: The treatment planning system (TPS) estimated and measured dose to the cochlea were in the range of 7.4 - 8.4 Gy and 7.1 - 8 Gy, respectively. The maximum variation between TPS calculated and measured dose to cochlea was 5%. CONCLUSION: The measured dose values were found in good agreement with the dose values calculated using the TPS. The MOSFET dosimeter can be a suitable choice for routine dose verification in the Gamma Knife radiosurgery.