**1. Introduction**

External radiotherapy is a well‐accepted and established therapeutic modality for cancer treatment [1]. In this technique, radiation beams, generated by either radiation source or lin‐ ear accelerator, are specifically optimized to cause the death of the tumor cells without having a greater impact on the healthy tissues. It is estimated that dose precision in radiotherapy is approximately ±5%. However, in order to ensure proper dose delivery to the designated area and appropriate intensity, a sophisticated radiation oncology Quality Assurance (QA) pro‐ gram is required [1, 2]. Also, the verification of the final dose delivered to the patient, which can only be carried out by in vivo dosimeters, is very important and should basically be used for all patients undergoing radiation treatment [3].

In vivo dosimetry can be measured by thermoluminescent dosimeters (TLDs) [4, 5], diode dosimeters [6, 7] and MOSFET (Metal‐Oxide‐Semiconductor Field Effect Transistor) dosim‐ eters [8, 9]. TLDs characteristics include the following: cable‐free, accurate, small volume

and tissue‐equivalence. However, an important drawback of TLDs is the reading procedure because information is lost during the reading. Currently, TLDs are most popular dosim‐ eters for QA radiotherapy despite the relatively high cost of the readout equipment and the requirement of a highly trained operator.

Diode dosimeters provide instantaneous readout; however, diodes must be connected to cable for applied voltage during radiation. Even though diode dosimeters are sensitive to the temperature and dependent on the radiation beam, the correction and calibration factors are generally well known.

The concept of radiation sensitive MOSFETs as dosimeter is based on converting the threshold voltage shift as a dosimetric parameter into radiation dose. Ionizing radiation creates positive charge in the MOSFETs oxide and interface trap at silicon dioxide‐silicon interface leading to a transistors threshold voltage shift. In p‐channel MOSFETs, both the positive charge in the oxide and interface traps contributes to threshold voltage shift in the same direction. This is reason why p‐channel MOSFETs instead n‐channel MOSFETs are usually used as dosimeters. p‐channel MOSFETs can be application in low‐field mode (without gate bias during irradia‐ tion) and in high‐field mode (with gate bias during irradiation). High‐field mode leads to the sensitivity increase in MOSFET dosimeters.

The p‐channel MOSFET as integrating dosimeters has been proposed in 1970 [10] and results being verified in 1974 [11]. This further leads to the production of radiation sensitive p‐channel MOSFETs, also known as RADiation‐sensitive Field Effect Transistor (RADFET) or pMOS dosimeter [12]. Besides, radiotherapy pMOS dosimeters could be used for radia‐ tion space monitoring [13, 14], irradiation of food plants [15] and in personal dosimetry [16].

A major advantage of the MOSFET as a radiation sensor is that the radiation‐sensitive region, the oxide film, is very small [11]. The sensing volume is much smaller than competing integral dose measuring devices, such as the ionization chamber or TLD. The MOSFETs sensitive vol‐ ume is typically 1 μm x 200 μm x 200 μm [17] implying that it could be used in vivo dosimetry [18]. This MOSFETs property also makes them attractive for measurements in the gradient radiation field where the gradient mostly depends on a single space coordinate, like resolv‐ ing dose of X‐ray micro beams or dept dose distribution [19]. The advantages of MOSFETs as dosimeters also include real time or delayed reading, non‐destructive and immediate dosimetric information readout, wide dose range, accuracy, competitive price and possible integration with other sensors and/or electronics [20]. Moreover, another field where it is pos‐ sible to explore their advantages is hadron therapy, which is one of the promising radiation modalities in radiotherapy [21]. On the other hand, an important disadvantage of MOSFETs as radiation sensors is the need to separate calibration in fields of different modalities and energies. Furthermore, MOSFET's total accumulated dose range depends on the dosimeter sensitivity and type. The MOSFET needs to be replaced when the upper limit of linearity is achieved. Although, recently, the possibility of MOSFET reuse after recovering for a certain period of time at room or elevated temperature [22] or by current annealing [23] has been studied.

In radiotherapy, the radiation oncologist determines the radiation dose depending on many factors such as the type and size of tumor, location in the body, how close the tumor is to other radiation sensitive tissues, how deep into the body the radiation need to penetrate, the patient general health and medical history, whether the patient will have other type of cancer treat‐ ments (e.g., chemotherapy) and other factors such as patient age and medical conditions. Cumulative dose range used in radiotherapy ranges from 20 to 70 Gy [24], while typical radia‐ tion dose for one fraction is from 1 up to 5 Gy.

and tissue‐equivalence. However, an important drawback of TLDs is the reading procedure because information is lost during the reading. Currently, TLDs are most popular dosim‐ eters for QA radiotherapy despite the relatively high cost of the readout equipment and the

Diode dosimeters provide instantaneous readout; however, diodes must be connected to cable for applied voltage during radiation. Even though diode dosimeters are sensitive to the temperature and dependent on the radiation beam, the correction and calibration factors are

The concept of radiation sensitive MOSFETs as dosimeter is based on converting the threshold voltage shift as a dosimetric parameter into radiation dose. Ionizing radiation creates positive charge in the MOSFETs oxide and interface trap at silicon dioxide‐silicon interface leading to a transistors threshold voltage shift. In p‐channel MOSFETs, both the positive charge in the oxide and interface traps contributes to threshold voltage shift in the same direction. This is reason why p‐channel MOSFETs instead n‐channel MOSFETs are usually used as dosimeters. p‐channel MOSFETs can be application in low‐field mode (without gate bias during irradia‐ tion) and in high‐field mode (with gate bias during irradiation). High‐field mode leads to the

The p‐channel MOSFET as integrating dosimeters has been proposed in 1970 [10] and results being verified in 1974 [11]. This further leads to the production of radiation sensitive p‐channel MOSFETs, also known as RADiation‐sensitive Field Effect Transistor (RADFET) or pMOS dosimeter [12]. Besides, radiotherapy pMOS dosimeters could be used for radia‐ tion space monitoring [13, 14], irradiation of food plants [15] and in personal dosimetry

A major advantage of the MOSFET as a radiation sensor is that the radiation‐sensitive region, the oxide film, is very small [11]. The sensing volume is much smaller than competing integral dose measuring devices, such as the ionization chamber or TLD. The MOSFETs sensitive vol‐ ume is typically 1 μm x 200 μm x 200 μm [17] implying that it could be used in vivo dosimetry [18]. This MOSFETs property also makes them attractive for measurements in the gradient radiation field where the gradient mostly depends on a single space coordinate, like resolv‐ ing dose of X‐ray micro beams or dept dose distribution [19]. The advantages of MOSFETs as dosimeters also include real time or delayed reading, non‐destructive and immediate dosimetric information readout, wide dose range, accuracy, competitive price and possible integration with other sensors and/or electronics [20]. Moreover, another field where it is pos‐ sible to explore their advantages is hadron therapy, which is one of the promising radiation modalities in radiotherapy [21]. On the other hand, an important disadvantage of MOSFETs as radiation sensors is the need to separate calibration in fields of different modalities and energies. Furthermore, MOSFET's total accumulated dose range depends on the dosimeter sensitivity and type. The MOSFET needs to be replaced when the upper limit of linearity is achieved. Although, recently, the possibility of MOSFET reuse after recovering for a certain period of time at room or elevated temperature [22] or by current annealing [23] has been

requirement of a highly trained operator.

sensitivity increase in MOSFET dosimeters.

generally well known.

232 Radiotherapy

[16].

studied.

This chapter presents some of the results obtained in our laboratory, which considers the influ‐ ence of some parameters to pMOS dosimeters sensitivity and fading. Dosimeters were manu‐ factured in Tyndall National Institute, Cork, Ireland. Sensitivity results are also presented for commercial MOSFETs in order to investigate their possible application in radiotherapy.
