|Year : 2012 | Volume
| Issue : 4 | Page : 528-531
Dose verification to cochlea during gamma knife radiosurgery of acoustic schwannoma using MOSFET dosimeter
Sunil D Sharma1, Rajesh Kumar1, Philomina Akhilesh1, Anil M Pendse2, Sudesh Deshpande2, Basant K Misra2
1 Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, CTCRS Building, Anushaktinagar, Mumbai, India
2 Gamma Knife Centre, P. D. Hinduja, National Hospital and Medical Research Centre, VS Marg, Mahim, Mumbai, India
|Date of Web Publication||29-Jan-2013|
Sunil D Sharma
Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, CTCRS Building, Anushaktinagar, Mumbai - 400 094
Source of Support: None, Conflict of Interest: None
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.
Keywords: Acoustic schwannoma, dose, gamma knife, MOSFET, radiosurgery
|How to cite this article:|
Sharma SD, Kumar R, Akhilesh P, Pendse AM, Deshpande S, Misra BK. Dose verification to cochlea during gamma knife radiosurgery of acoustic schwannoma using MOSFET dosimeter. J Can Res Ther 2012;8:528-31
|How to cite this URL:|
Sharma SD, Kumar R, Akhilesh P, Pendse AM, Deshpande S, Misra BK. Dose verification to cochlea during gamma knife radiosurgery of acoustic schwannoma using MOSFET dosimeter. J Can Res Ther [serial online] 2012 [cited 2019 Sep 20];8:528-31. Available from: http://www.cancerjournal.net/text.asp?2012/8/4/528/106528
| > Introduction|| |
Accuracy in the dose delivery in the Gamma knife is one of the most important component in this sophisticated radiotherapy treatment, which is used for treatment of benign and malignant intracranial diseases.  The number of patients treated with stereotactic radiosurgery increases day by day, it becomes particularly important to define with precision adverse effects on distinct structures of the nervous system.  Cranial nerves are among the critical structures for which Gamma Knife treatment planning team should pay 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) cranial nerves are frequently at risk of radiation injury during Gamma Knife radiosurgery. Acoustic neuroma (i.e., a tumor associated with the 8 th cranial nerve) is treated with highly conformal radiation in its early stage. The minimum tumoricidal dose is recommended in this case to balance the 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 accurate, highly conformal, and high intensity radiation dose is expected to locally control a lesion.  It is well established that the clinical outcome of stereotactic radiosurgery/radiotherapy depends 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 technique, it is very challenging and important to verify the planned dose to cochlea. Present study describes the result of dose verification to cochlea using metal oxide semiconductor field effect transistor (MOSFET) dosimeter and a specially designed multi slice head and neck phantom during the treatment of acoustic schwannoma by Gamma Knife radiosurgery unit (Elekta Instruments, Sweden).
| > Materials and Methods|| |
[Figure 1] shows the basic structure of a P-channel enhancement type MOSFET, which is built on a negatively doped (n-type) silicon.  After a sufficient negative voltage to the gate, a significant number of minority carriers (holes) will be attracted to the oxide/silicon surface from the bulk silicon substrate and the source and drain regions form a conduction channel for allowing current to flow between the source and drain. The voltage necessary to initiate current flow is known as the device threshold voltage. When a MOSFET is exposed to the radiation, electron-hole pairs are generated within the silicon dioxide (SiO 2 ), which is considered as sensitive region. Electrons, whose mobility in SiO 2 at room temperature is very much higher than holes quickly move out of the gate electrode while holes move in a stochastic fashion toward the Si/SiO 2 interface and get trapped, causing a negative threshold voltage shift, which can persist for years. The difference in voltage shift before and after exposure can be measured. This form the basis to MOSFET acts as dosimeter. , The shift in threshold voltage is proportional to the radiation absorbed dose. Standard MOSFET (TN502RD, Best Medical, Canada) with mobile MOSFET dose verification system (Best Medical) were used in this study in standard bias setting. The overall physical size of the sensor is 2.5 × 1.3 × 8 mm 3 and the actual 0.2 mm x 0.2 mm x 0.5 μm. Signals from the MOSFET dosimeter were read out using a wireless mobile MOSFET reader (Best Medical), controlled with remote dose verification software running on a PC. About 2 minute wait periods was maintained between the read-out cycles throughout the study. For linearity measurements, a single MOSFET was irradiated three times in the range of 10 - 800 cGy using 15 × 15 cm 2 field size of 6 MV X-ray beam from a medical linear accelerator (Varian Oncology System, USA). For reproducibility measurements, MOSFETs were repeatedly exposed to 100 cGy five times. The responses from individual dosimeters were averaged and standard deviation was calculated for each of MOSFET dosimeter. The dose response calibration of the MOSFET dosimeters was carried out in 6 MV X-ray beam from a medical linear accelerator by placing them at 10 cm depth in a water equivalent intensity modulated radiotherapy (IMRT) phantom (XWU, Best Medical). The MOSFET dosimeters were irradiated with a predetermined dose value using a 15 × 15 cm 2 field and calibration factors in terms of cGy/mV were obtained.
|Figure 1: Basic structure of a P-channel enhancement type metal oxide semiconductor fi eld effect transistor (MOSFET) which is built on a negatively doped (n-type) silicon|
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[Figure 2] is a photograph of specially designed polystyrene multi slice head and neck phantom for dose verification during Gamma Knife radiosurgery. The physical density and effective atomic number of the phantom material is 1.05 g/cm 3 and 5.7, respectively. 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 portions of phantom is made up of slices of thickness 2 mm. Variable slice thickness was selected to get the dosimetry data at a reasonably good spatial resolution in the central part, which contains the target lesion for dose delivery. The phantom has the facility to hold different types of detectors such as radiochromic films, thermoluminescent dosimeters (TLDs) discs of diameter 4.5 mm and thickness of 0.8 mm (or powder packed in small pouches) and MOSFET dosimeters. The phantom also has facility to fit the Leksell frame for localization of point of measurement.
|Figure 2: Photograph of specially designed multi slice head and neck polystyrene phantom used for the dose verifi cation measurements during Gamma Knife radiosurgery|
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The CT images (slice thickness of 1.25 mm) of the phantom were taken with Leksell frame and these images were subsequently transferred to the Leksell Gamma Knife treatment planning system (TPS) (Gamma Plan, version 5.34, Elekta, Sweden). 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 five randomly selected patients from a group of 120 patients treated for acoustic schwannoma using Gamma Knife at P.D. Hinduja National Hospital and Medical Research Centre, Mumbai during January 1997 to December 2008 were used in the dose verification of cochlea. Depending on the volume of the tumor, the patients of acoustic schwannoma were classified in five groups, namely, very small (tumor volume = 0.1 - 0.2 cm 3 , e.g., intracanalicular), small (tumor volume = 0.2 - 0.5 cm 3 ), medium (tumor volume = 0.5 - 2.0 cm 3 ), large (tumor volume = 2.0 - 5.0 cm 3 ), and larger (tumor volume > 5.0 cm 3 ). The anatomical position of the cochlea is nearly constant for all the patients. One patient from each of the five groups of the patients treated was selected in this dosimetric study. The treatment plans of these patients 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 the maximum dose points. Points of measurement were located at about 3 - 5 mm from the target volume. [Figure 3] shows the photograph of experimental set up used for irradiating the phantom with MOSFET dosimeters in situ using the planned irradiation of acoustic schwannoma of a patient. The dose delivered to the point of interests was determined using the read out of the MOSFET dosimeters and the respective dose response calibration factor.
|Figure 3: Photograph of experimental set up used for irradiating the phantom along with MOSFET dosimeters|
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| > Results|| |
[Figure 4] shows the dose response curve of the MOSFET dosimeters in the range of 50 - 800 cGy in a 6 MV photon beam. The analysis of this curve indicates that the response of the MOSFET dosimeter fulfils the required limit of linearity in this dose range. In addition, the reproducibility for a given MOSFET was found to be ± 2% at two standard deviations. [Table 1] shows the calibration factor of MOSFET dosimeters measured in 6 MV photon beam from a medical linear accelerator using 15 × 15 cm 2 field size. These individualized calibration factors were used for estimating the radiation absorbed dose at the point of interests for the purpose of clinical dose verification. [Table 2] shows the measured dose to cochlea and their comparison with calculated dose at the same location by the TPS. It can be seen from this table that the difference between calculated and measured dose is in the range of 3.6 - 5%.
|Figure 4: The linearity in response of MOSFET dosimeters in the dose range of 50 – 800 cGy measured in 6 MV photon beam from a medical linear accelerator using 15 × 15 cm2 field size|
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|Table 1: Calibration factor of MOSFET dosimeters determined in 6 MV therapeutic photon beam from a medical linear accelerator using 15 × 15 cm2 field size|
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|Table 2: Comparison of treatment planning system computed and MOSFET dosimeter measured dose value of cochlea|
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The majority of patients come for follow up after every 6 months for nearly 2 - 5 years after the treatment. The consultant notes of the follow-ups were referred to for knowing the status of hearing. The follow-up results of 120 patients treated at our center for acoustic schwannoma have not reported any detritions of hearing, which also confirms the reliability of the measured dose value by the MOSFET dosimeters.
| > Discussions|| |
It is evident from the graph shown in the [Figure 4] that response of MOSFET dosimeter is linear in the dose range used in this study. The linearity of MOSFET in this range is comparable to the linearity of an ionization chamber, which is considered as gold standard in the field of medical radiation dosimetry. These observations indicate that the mobile MOSFET dosimeter used in this study is an acceptable relative dose verification device in clinical set up. The difference in the calculated and measured dose is marginally at higher side due to the influence of positional uncertainty in the placement of the MOSFET dosimeters at the planned location. It can be noted that the dose gradient in the dose distribution of Gamma Knife plan is significantly high and hence positioning of the dosimeter at the required location in a phantom during Gamma Knife irradiation is very crucial. In this study, the CT scan of the multi slice head and neck polystyrene phantom along with MOSFET dosimeters and Leksell frame were taken to minimize the uncertainty in the measured dose due to positional uncertainty of the dosimeter. It can also be noted from the table that the measured dose values are lower than the 9 Gy, which is considered as the limiting dose value for preserving the hearing loss of the patients at our centre.
| > Conclusions|| |
The dose to cochlea of the patients treated for acoustic schwannoma by Gamma Knife was verified by conducting in-phantom dosimetry using miniature mobile MOSFET dosimeters. 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. The patients treated at our centre for acoustic schwannoma have not reported any detritions of hearing during follow-ups, which also confirms the reliability of the MOSFET dosimetry.
| > References|| |
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2]