## 1 Introduction  
Stereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT) have become prevalent because of their effectiveness in treating various types of tumors with the simultaneous minimization of exposure to surrounding healthy tissues [1, 2]. SRS and SBRT achieve high tumor control rates and short treatment times by delivering highly focused dose distributions to tumors while ensuring rapid dose fall-off to protect the surrounding organs at risk [3, 4, 5, 6]. To achieve these outcomes, SRS and SBRT rely on the precise control of small fields, multibeam irradiation, and positional accuracy and consistency, which are verified through patient-specific quality assurance (PSQA) prior to treatment [7]. Guidelinesfor PSQA in SRS and SBRT are actively being developed, emphasizing the necessity for strict tolerance in dose delivery and efficient measurement tools, particularly for small-field dosimetry [7; 8; 9].  
Various types of detectors, including film detectors, ionization chambers, diode detector arrays, and amorphous silicon (a-Si) electronic portal imaging devices (EPIDs), have been tested for PSQA [7; 10; 11; 12; 13]. Film detectors achieve the highest spatial resolution and do not exhibit limitations associated with the dose rate, temperature, angle, and energy dependence in the MV beam condition [14]. However, they require long measurement times and are prone to errors due to processing steps such as scaling and scanning, necessitating high technical expertise for accurate measurements [15; 16; 17; 18; 19; 20]. Although ionization chambers exhibit excellent detection performance, their spatial resolution is limited due to size and leakage current, and their outputs exhibit dose rate, pulse, and angle dependence [21; 22]. Similarly, although diode detector arrays contain a low number of embedded detectors, their outputs are still influenced by the dose rate, temperature, and photon incident angles [23; 24; 25]. a-Si EPIDs have emerged as devices for in vivo and high-resolution measurements in PSQA applications, although saturation effects can occur during the measurement of unfiltered high dose rate beams and the over-response to low-energy photons [26; 27; 28]. Additionally, errors exceeding 20% have been reported for the output factors of small field sizes [29].  
In this study, we investigated the feasibility of using a new detector material, cadmium-zinc-telluride (Cd\({}_{0.9}\)Zn\({}_{0.1}\)Te, CZT), for PSQA in SRS and SBRT. Single-crystal CZT detectors are widely used in various applications, such as national security, medical imaging, industrial nondestructive testing, and astronomical observation, because of their high resistivity, wide bandgap, room-temperature operation, and high resolution with short response times [30; 31; 32; 33]. Recently reported CZT detectors demonstrate extremely low leakage currents, high resolution, and stable operation in the \(\gamma\)-ray energy region [34]. Moreover, CZT can boost the miniaturization of the sensor size, which is a critical requirement for detectors in small-field dosimetry. The array-type detector approach, which configures the surface of the CZT detector as a series of small, pixelated detectors, reveals excellent performance, and the fabrication processes are sufficiently mature. The advantage of this array-type detector method is that as the pixel size decreases, the electric field formed near the pixels increases, enhancing spatial resolution. This effect, known as the \"small pixel effect\", is one of the factors that can make the CZT detector particularly suitable for small-field dosimetry [35]. The primary objectives of this study were to apply CZT detectors in PSQA, collect fundamental data, and provide reference information for potential clinical applications. Fundamental data were obtained by evaluating the output characteristics of CZT, including dose linearity, dose rate dependence, energy dependence, angular dependence, depth dependence, and reproducibility.