Characterization of a new MOSFET detector configuration for in vivo skin dosimetry

Abstract

The dose released to the patient skin during a radiotherapy treatment is important when the skin is an organ at risk, or on the contrary, is included in the target volume. Since most treatment planning programs do not predict dose within several millimeters of the body surface, it is important to have a method to verify the skin dose for the patient who is undergoing radiotherapy. A special type of metal oxide semiconductors field-effect transistors (MOSFET) was developed to perform in vivo skin dosimetry for radiotherapy treatments. Water-equivalent depth (WED), both manufacturing and sensor reproducibility, dependence on both field size and angulation of the sensor were investigated using 6 MV photon beams. Patient skin dosimetries were performed during 6 MV total body irradiations (TBI). The resulting WEDs ranged from 0.04 and 0.15 mm (0.09 mm on average). The reproducibility of the sensor response, for doses of 50 cGy, was within +/-2% (maximum deviation) and improves with increasing sensitivity or dose level. As to the manufacturing reproducibility, it was found to be +/-0.055 mm. No WED dependence on the field size was verified, but possible variations of this quantity with the field size could be hidden by the assessment uncertainty. The angular dependence, for both phantom-surface and in-air setups, when referred to the mean response, is within +/-27% until 80 degree rotations. The results of the performed patient skin dosimetries showed that, normally, our TBI setup was suitable to give skin the prescribed dose, but, for some cases, interventions were necessary: as a consequence the TBI setup was corrected. The water-equivalent depth is, on average, less than the thinnest thermoluminescent dosimeters (TLD). In addition, when compared with TLDs, the skin MOSFETs have significant advantages, like immediate both readout and reuse, as well as the permanent storage of dose. These sensors are also waterproof. The in vivo dosimetries performed prove the importance of verifying the dose to the skin of the patient undergoing radiotherapy.

Chest-wall Surface Dose During Post-mastectomy Radiation Therapy, with and without Nonmagnetic Bolus: A Phantom Study

“…In addition, the difference between the surface doses on the chest wall of the computerized treatment plan and those of the actual measurement was reported in the previous experiment, and there is a MOSFETderived ± 6 % surface-dose-measurement difference that confirms a similarity to this study [22,23]. It was shown that as the curvature of the region that the MOSFETmeasuring position is located in increases, the radiation dose increases in the same cross-section, and this tendency is attributed to the angle dependency of the MOSFET itself according to the attachment position [18,24]. In addition, we confirmed that, although the absorbed dose in the build-up area is very important for tangential irradiation, one of the radiation-therapy techniques for breast cancer, there is a radiation-dose difference that depends on the radiation-dose-calculation algorithm of the instrument that is used for the computerized treatment plan [25,26]; therefore, an additional verification of the treatment plan that uses algorithms that are different from the adaptive convolution algorithm that was used in this experiment is needed.…”

“…There is no energy dependency here, and the dependency of the radiation-incidence angle is ± 2 % at an energy range of 1 MeV to 20 MeV with a 360° rotation. Also, a temperature dependency of 0.5 % exists at 20° to 40° and unlike the TLD, the influence of the temperature change is low, which is useful for the measurement of the accumulated radiation dose [18,19]. Threshold voltage (V T ) is required for a current to flow into the MOSFET and the general threshold voltage is the sum of the flat band voltage (VFB) and the ideal threshold voltage (V ideal T ), as follows:…”

In-vivo dosimetry for conformal arc therapy using several MOSFET in stereotactic radiosurgery computed by an inverse model

“…Therefore, MOSFETs could be suitable for in-vivo assessment of radiosurgery microbeams [7,6,8]. In the literature their use has been described extensively for such in vivo procedures as: in vivo dosimetry for external radiotherapy [9,10,11], dose verification in intensity modulated radiotherapy (IMRT) [12,13,14], measurements in head and neck Tomotherapy [15], skin dose measurement [16], entry dosimetry [17], implantable detectors for in-situ testing during radiation therapy treatment [18], dose verification of permanent low-dose-rate implants [19] and as a dosimeter for imaging in radiological procedures [20]. Additional studies have reported excellent linearity, dosimetric accuracy in the build-up region and directional independence [9,21].…”

Angular Dependence of the MOSFET Dosimeter and its Impact on In vivo Surface Dose Measurement in Breast Cancer Treatment

“…Therefore, it is critical that the response of the in vivo skin dose verification system is angle-independent or its dependence is well-understood. Much research has been conducted to characterize different brands of MOSFET dosimeters (7)(8)(9)(22)(23)(24)(25)(26)(27). Manufacturer reports as well as some research has shown that for MOSFETs without a buildup cap performance is relatively independent of beam incidence angle and that for MOSFETs with a buildup cap readings are relatively independent of beam incidence angle up to 45 degrees but suffer a sudden discontinuity at around 45 degrees as the buildup cap moves out of the beam path.…”