As the number of patients treated with stereotactic radiosurgery increases, it becomes particularly important to define with precision adverse effects on distinct structures of the nervous system. Accuracy in the dose delivery in the Stereotactic Radiosurgery is one of the most important component in this sophisticated radiotherapy treatment of benign and malignant intracranial diseases. Cranial nerves are among the critical structures for which Gamma Knife treatment planning teams pay very close attention. The risk of cranial nerve neuropathies following Gamma Knife radiosurgery depends on treatment site, dose, and length of irradiation. The eighth (vestibulocochlear) and second (optic) and cranial nerves are not infrequently at risk of radiation injury during Gamma Knife radiosurgery. Fortunately the dose tolerance of cranial nerves has been well-studied. Acoustic neuroma (i.e., a tumor associated with the 8th cranial nerve) treatment volume are treated with highly conformal radiation while the tumor is small, rather than waiting until it becomes large and riskier to treat. The minimum tumoricidal dose is recommended, as this balances treatment with the desire to preserve the patient’s hearing and the avoidance of facial paralysis or sensory neuropathies. Gamma Knife radiation dose which extend over the area of the cochlea are recommended to be less than 9 Gy to preserve the hearing loss. The maximum dose delivered to the cochlear nucleus was the most significant prognostic factor of hearing deterioration. Conventional radiobiological concepts may not be directly applicable to Gamma Knife radiosurgery. However, it is clear that the Gamma Knife is well suited for cases where accurately-delivered, highly conformal, and high intensity radiation dose is expected to locally control a lesion. It is well established that the clinical outcome of radiation surgery /therapy is depend upon the accuracy of dose delivered. Considering the spatial location of cochlea with lesion in the acoustic schwannoma and steep dose gradient in the Gamma Knife treatment techniques, it is very challenging and important to verify the plan dose to cochlea.

     In the present study, plan dose to cochlea were measured using the MOSFET in an especially design polystyrene multi sliced head phantom by simulating acoustic schwannoma gamma knife radio surgery for sole purpose dose verification.

      

     The phantom and dosimeters Multi sliced polystyrene head phantom was designed and fabricated. The outer dimension of the phantom was chosen equivalent to outer dimension of the head of a reference Indian man. The central region of this phantom consists of 1 mm thick slices while the lower and upper portion of phantom is made up of slices of thickness 2 mm. The head phantom has the facility to hold different types of detectors such as films, TLDs of 4.5 mm in diameter and a thickness of 0.8 mm (or powder packed in small pouches) and MOSFETs. Standard MOSFET (TN502RD, Best Medical Canada) with mobile MOSFET Dose Verification System (Best Medical, Canada) were used in this study in standard bias setting. The overall physical size of the sensors is 2.5 x 1.3 x 8 mm3, and m. Signals were read out(the actual sensitive volume is 0.2 mm x 0.2mm x 0.5 using a wireless mobile MOSFET reader (Best Medical Canada), controlled with remote dose verification software running on a PC. Dosimetric characteristics such as reproducibility and linearity of MOSFET were studied. Individual MOSFETs were calibrated by first placing the MOSFET at 10 cm depth in XWU-IMRT Phantom (Best Medical Canada). The MOS-FETs were then irradiated with a predetermined dose value using a 15 x 15 cm2 field, and calibration factors in cGy/mV were obtained. B. Planning and irradiation The CT images (slice thickness 1.25 mm) of the sliced polystyrene head phantom were taken with Leksell frame as shown in Fig. 1 and these images were subsequently transferred to the Leksell Gamma Knife planning system (Gamma Plan, version 5.34) As CT images are assumed to have better spatial accuracy and precision in comparison to MRI images and hence CT images were used in this work. The treatment plans of patients treated earlier for acoustic Schwannoma were transferred to the phantom and coordinates of maximum dose point inside the cochlea was derived. The MOSFET dosimeters were placed at the maximum dose point in the cochlea and points located 4 mm away from this maximum dose points. Points of measurement were at about 3-5 mm from the target. The irradiation of the phantom containing dosimeters was carried out using the simulated treatment plans of the patients.

     Table 1 shows the calibration factor of MOSFETs used for the dose verification. Measured dose with different detectors were compared with calculated dose and given in the Table 2. All the detectors had shown good agreement with calculated dose. All detectors had shown lower value as compared to calculated dose by planning system. The points of measurement and calculated points were matched accurately because of the Leksell frame coordinates. Table.1: Calibration Factor of the MOSFET used. Sr No. Calibration Factor #1 0.890 #2 0.910 #3 0.903 #4 0.890 #5 0.902 Table 2: Comparison of TPS calculated and MOSFETs dosimeter measured dose values. Case No Dose (Gy) TPS value MOSFET #1 7.5 6.5 #2 8.5 7.1 #3 7.7 6.8 #4 8.0 7.3 #5 8.5 7.5 Fig 2 Photograph showing the experimental set up of irradi-ating the polystyrene head phantom.

     This is a retrospective study.

     All the five patients treated did not shown any detritions of hearing. The measured doses are lower than the value predicted by Leksell Treatment planning system.

     Difference in measured and calculated value needs further detail investigation. To set dose limit constraints for different critical organ, it is recommended to take necessary caution while evaluating treatment plans.