Investigation of triaxial cables and microdetectors in small field dosimetry

Background. Small field dosimetry presents unique challenges with source occlusion, lateral charged particle equilibrium and detector size. As detector volume decreases, signal strength declines while noise increases, deteriorating the signal-to-noise ratio (SNR). This issue may be compounded by triaxial cables connecting detectors to electrometers. However, effects of cables, critical for precision dosimetry, are often overlooked. There is a need to evaluate triaxial cable and detector impacts on SNR in small fields. The purpose of this study is to evaluate the influence of triaxial cables and microdetectors on signal-to-noise ratios in small-field dosimetry. This study also aims to establish the importance of cable quality assurance for measurement accuracy. Methods. Six 9.1 m length triaxial cables from different manufacturers were tested with six microdetectors (microDiamond, PinPoint, EDGE, Plastic scintillator, microSilicon, SRS-Diode). A 6 MV photon beam (TrueBeam) was used, with a water phantom at 5 cm depth with 0.5 × 0.5 cm2 to 10 × 10 cm2 fields at 600 MU min−1. Readings were acquired using cable-detector permutations with a dedicated electrometer (except the scintillator which has its own). Cables had differing connector types, conductor materials, insulation, and diameters. Detectors had various sensitive volumes, materials, typical signals, and bias voltages. Results. Normalized field output correction factors (FOFs) relative differences of 13.4% and 4.6% between the highest and lowest values across triaxial cables for 0.5 × 0.5 cm2 and 1 × 1 cm2 fields, respectively. The maximum difference in FOF between any cable-detector combinations was 0.2% for the smallest field size. No consistent FOF trend was observed across all detectors when increasing cable diameter. Additionally, the non-normalized FOF differences of 0.9% and 0.3% were observed between cables for 0.5 × 0.5 cm2 and 1 × 1 cm2 fields, respectively. Conclusions. Regular triaxial cable quality assurance is critical for precision small field dosimetry. A national protocol is needed to standardize cable evaluations/calibrations, particularly for small signals (<pC) from modern detectors. This could enhance measurement accuracy and treatment delivery with advanced small-field radiotherapy techniques that promise improved patient outcomes. Further studies should expand detector and cable models tested across institutions to establish robust quality control guidelines.


Introduction
Advances in radiation treatment, especially in stereotactic radiosurgery (SRS), Gamma knife, Tomotherapy, Cyber-Knife, and intensity modulated radiation therapy (IMRT), require a high degree of precision dosimetry that utilizes small-fields where dosimetry may be uncertain that has been elaborated in many references [1,2].
The use of small fields presents challenges due to the size of detector compared with the field size, the loss of lateral charged particle equilibrium (CPE) and the possibility of partial occlusion of the primary source of radiation [3].The lateral CPE is compromised when the x-ray energy is high or the field dimension is small enough to be comparable to the maximum lateral electron range as elaborated in various references [1,2,4].This can affect the beam profiles and absorbed dose on the central axis.These aspects of small-fields have been discussed in literature where suitable detectors have been recommended [1][2][3][4].
Fowler and Farmer [5,6] provided an initial assessment of insulators in the 1950s for radiation measurement which is an important consideration for modern cables used in radiation dosimetry.Gross [7] provided the importance of Compton current which is often present in most cables.Stem effect became a priority in radiation dosimetry that has been discussed by several investigators [8,9].Various investigators have studied the effects of cables on leakage and extracameral phenomena [10][11][12].Das et al provided some indications as to the quality of cables and length in the radiation beam that impacts the signal in TG-106 [13].The cable effect has been thought mainly problematic in large-field irradiations such as total body irradiation and total skin electron irradiation due to the large amount of cable in the large-fields [14][15][16][17].These effects have been even quantified with Monte Carlo simulation and found to be significant [18].
Electrometers and detectors are calibrated by a national laboratory, accredited dosimetry calibration laboratory (ADCL) every 2 years as required [19].However, the quality of cables has not been given any importance in radiation dosimetry since signals in most reference calibration devices are fairly large (nC).In more contemporary literature, Muir et al [20] emphasized the importance of cable and electrometer where it is stated that modern electrometers can easily measure 100 fC with 0.1% precision.
Contrary to large field irradiation with Compton current, leakage and polarity, small-field creates a different type of problem.The small-field dosimetry detectors are mainly microdetectors with relatively small signals (~fC).In general, most detectors in small-fields produce perturbations that can be corrected but very small-signal combined with high noise cables could produce a significant signal-to-noise ratio (SNR) which has not been addressed.Microdetectors for small field dosimetry are available and possess high precision to disentangle these issues.However, as the sensitive volume size of the detector decreases, the signal also inevitably decreases, making it more difficult to obtain a reliable measurement.'As the sensitive volume or sensitivity of the detector decreases, the signal also inevitably decreases, making it more difficult to obtain a reliable measurement.This is because the smaller the detector volume, the fewer photons or radiation particles are collected, resulting in a weaker signal.Additionally, the smaller the detector volume, the larger the relative contribution of the noise to the total signal, leading to a lower SNR.The choice of detector, electrometer, and triaxial cable connecting the detector to the electrometer can further compound this issue.However, this may not be the case for solid-state detectors, which can provide higher signals despite their smaller sensitive volumes.Nonetheless, the smaller the detector volume, the larger the relative contribution of noise to the total signal, leading to a lower SNR.Thus, using the thinnest triaxial cable may further reduce the SNR of the detector, regardless of the detector type.This is compounded by the choice of detector and electrometer along with triaxial cables that run from detectors to electrometer.
A triaxial cable used in measuring dose minimizes internal signal loss and external noise when connected to the detector and the electrometer.Twin-axial and triaxial cable are both types of coaxial cable used to transmit signals as shown in figure 1.In both cases, the central conductor is surrounded by an insulator, which in turn is surrounded by a braided shield.One of the main differences between the two is the number of conductors.A triaxial cable consists of three conductors; force, shield, and guard.The three conductors are used to carry the extremely small charge/currents generated by detectors without signal degradation, which represent the radiation dose being measured.Leakage current, which is an electromagnetic interference (EMI), can be eliminated by holding the guard at the same voltage as the force line.Figure 1 depicts the differences between the twin-axial and tri-axial cables.
In general, the thickness of the triaxial cable connecting the detector to an electrometer also affects the SNR which is an important measure of the quality of the signals obtained from a detector, as it determines the ability to distinguish between the signal and the background noise.Thicker triaxial cables generally have lower resistance, which reduces the noise from a triaxial cable.Nevertheless, poor cable quality, such as ineffective insulation, high resistance in the signal-carrying portion, physical damage (e.g., kinks or bends), or connector issues, will result in a lower SNR and reduced accuracy in dosimetry.In general, modern triaxial cables can provide a signal on the order of (10 -13 A, 0.1 fA).However, due to factors such as kinks and poor handling, the signal strength could be significantly reduced.In many cases, cables cannot be used due to poor reproducibility of reading or an unacceptable SNR.The quality of cable and its characteristics have been briefly described in AAPM TG-106 [13].In small-field dosimetry, however, the quality of triaxial cable is a critical factor that has not been adequately addressed in modern literature.Additionally, modern high-end electrometers can measure signals in fA (10 -12 A).In small field dosimetry, the magnitude of the 'real' signal from the detector can be comparable to the leakage signal from the cables, potentially leading to a low SNR and compromising measurement accuracy.This can ultimately lead to errors in dose calculations and treatment delivery, compromising patient safety and treatment efficacy.
Detectors and electrometers are periodically calibrated by a national or accredited laboratory with appropriate coefficients for use in various codes of practice [19,21].Unfortunately, there are no guidelines on the quality of triaxial cables.The quality of cable could be a limiting factor, and in many places, maintenance of these cables is not controlled, resulting in significant signal losses.Here, we explore the effect of triaxial cables, microdetectors, and SNR in small-field dosimetry.By evaluating the impact of these factors on measurement accuracy, we aim to identify potential avenues for improvement in small-field dosimetry and contribute to the development of more accurate radiation dose delivery with modern devices delivering small-fields.

Microdetectors with the triaxial cables
Six high-quality triaxial cables, each 9.1 meters long, from different manufacturers were acquired for the experiment.Figure 1  Measurements were performed in a small water tank with a 6 MV photon beam from a Varian True-Beam, ranging from 0.5 × 0.5 cm 2 to a reference field  size of 10 × 10 cm 2 .These measurements were taken at a depth of 5 cm under a 95 cm source-to-distance (SSD) condition in the water tank.Each detector-cable combination was subjected to square field sizes of 0.5 × 0.5 cm 2 , 1.0 × 1.0 cm 2 , 2.0 × 2.0 cm 2 , 3.0 × 3.0 cm 2 , and 10 × 10 cm 2 , with 100 MU at a 600 MU min −1 dose rate.The readings were collected with a dedicated electrometer UNIDOS Romeo (Type TN10053, PTW, Freiburg, Germany), with the exception of W2 plastic scintillator, which used its own high-end electrometer (Supermax, Standard Imaging, W2) with a stated accuracy of 1 fA.The electrometers used in this study have a zeroing or nulling capability, which was employed before each measurement to reduce the leakage contribution to the signal.Each measurement was repeated three times to obtain a mean and standard deviation (SD).All measurements were performed in a single session to minimize potential variations in experimental conditions.For each field size, measurements were taken back-to-back, with only the triaxial cable being changed between measurements.This approach was employed to minimize the potential impact of jaw hysteresis on the results.After each cable change, the cable connections were carefully checked to ensure proper contact and secure fastening, the electrometer was reset, and the leakage current was allowed to stabilize before proceeding with the next measurement, and the detector positioning was verified to ensure consistent alignment with the beam central axis.Following the acquisition of raw data, a field output correction factor (FOF) was applied as defined in TRS483 [1].Relative differences in normalized FOFs were calculated as the percentage difference between the highest and lowest FOF values observed across cables for each field size as shown in table 3. Non-normalized FOF differences, referred to as 'absolute FOF differences', represent the differences in FOF values between cables without normalization to the reference field size.

Results
Figure 2 presents the FOF per field size when the detector is switched out but the same triaxial cable is used for measurement.We observed an average difference of 13.4% between the maximum and minimum values of normalized FOF among the six detectors, each using different triaxial cables, in 0.5 × 0.5 cm 2 field size.Additionally, the averaged values for a single detector and over all triaxial cables in the same field size showed a difference of 0.9%.While seemingly small, this difference is particularly notable in small field dosimetry, where even sub-1% variations are significant and can represent substantial changes in measurement accuracy.In the same field size, the PinPoint detector, with a sensitive volume of 2.9 mm, recorded a measurement of 0.267 ± 0.001, among all triaxial cables, while the EDGE detector, with a sensitive volume of 0.8 mm 3 , showed a measurement of 0.401 ± 0.000.In a field size of 1.0 × 1.0 cm 2 , the PinPoint detector registered the lowest FOF value of 0.691, and the microDiamond detector, with a sensitive volume of 2.2 mm 3 , recorded the highest FOF value of 0.736 ± 0.001.The FOF among the six detectors varied by 4.6% in the field size of 1.0 × 1.0 cm 2 are shown in figure 2. Figure 3 illustrates the relationship between the diameter of a triaxial cable and the normalized relative FOF for each detector and field size.There was a variation of 0.009 ± 0.004 in the normalized relative FOF values across different cable-detector combinations at a field size of 0.5 × 0.5 cm 2 .This suggests that there is no consistent trend indicating that the thickness of the cable increases the average FOF values across all detectors.This is due to the choice of detectors, as not all of them are ion-chamber.In general, solid-state detectors provide a higher signal.However, a deviation was noted within each detector.The difference in FOF between cable 2 (with a diameter of 3 mm) and cable 4 (with a diameter of 3.5 mm) was 0.136 ± 0.047.Likewise, in the field size of 1.0 × 1.0 cm 2 , no consistent trend was observed in the discrepancy between detector and cable values, with a discrepancy of 0.003 ± 0.001 observed when the cable thickness increased the average FOF values among all detectors.However, there was a difference of 0.047 ± 0.15 in FOF between the maximum and minimum values for cables 2 and 4. Furthermore, we calculated the variation in FOF between cable 2 and cable 3, which have the same diameter, at the 0.5 × 0.5 cm 2 field size.The mean and standard deviation values were found to be 0.006 and 0.003, respectively, indicating a relatively small variation between these two cables despite their identical diameters.

Discussion
Triaxial cable assemblies are designed for accurate measurement of current or charge with minimal signal degradation.The triaxial cables are composed of lownoise, high-performance, and radiation-resistant materials.The outer braid functions as a low-impedance transmission line, and the inner conductor and first braid are connected in parallel at the transmitting end to form a grounded shield.This arrangement reduces cable capacitance and eliminates the need for the thin shield to connect to the chassis as a signal return.When selecting the length of a triaxial cable for small field dosimetry, considerations include measurement setup requirements, such as field size and precision, and the trade-off between signal strength and noise level.However, even though longer cables present more resistance, the impact on the SNR is generally negligible when using commercially available triaxial cables.
It is important to note that, for FOF measurements, the leakage contribution is expected to be the same for both the reference field size and the smaller field size, assuming that the measurements are performed at the same dose rate.When calculating the FOF as the ratio of the detector readings at these two field sizes, the leakage contribution should theoretically cancel out.However, other factors, such as cable insulation quality, resistance, and physical integrity, may still affect the signal quality and reproducibility of measurements, even if the leakage contribution is minimized through the FOF calculation.Therefore, it is crucial to ensure that triaxial cables used in smallfield dosimetry are of high quality and are regularly inspected for any signs of damage or deterioration.
Although no consistent FOF trend was observed across all detectors when increasing cable diameter, it is important to note that the study design allowed for the evaluation of different cable diameters while keeping the detector constant.Further analysis of the data for individual detectors may reveal detector-specific trends or sensitivities to cable diameter variations.Future research could focus on investigating these detector-specific relationships to better understand the impact of cable diameter on FOF measurements for each detector type.
Ionization chambers and electrometers are periodically calibrated by a national or accredited laboratory with appropriate coefficients for use in various codes of practice [19,21].However, there are currently no guidelines or standardized procedures for evaluating the quality and performance of triaxial cables used in small-field dosimetry measurements.We suggest that standards laboratories consider providing a service for testing and characterizing the performance of triaxial cables used in conjunction with ionization chambers and electrometers.This could involve developing protocols for assessing cable quality, leakage current, and signal integrity, as well as establishing acceptance criteria for cable performance.Additionally, we recommend that triaxial cables be sent along with ionization chambers and electrometers for calibration, allowing the system to be calibrated as a whole.By implementing these measures, we can ensure that the entire measurement system, including triaxial cables, is performing optimally and providing reliable results for small-field dosimetry.Variation in field output factor (FOF) for small fields using different detectors with the same triaxial cable, normalized to percentage differences to highlight relative changes.The bar plots represent the normalized relative FOF, calculated by dividing each detector's FOF value for a given cable by the average FOF value across all cables for that specific detector and field size.This normalization allows for a clear comparison of the relative impact of different cables on each detector's FOF.The red data points plotted on the secondary y-axis represent the diameter of each triaxial cable in millimeters, with one data point per cable.The solid red line connecting these points illustrates the trend in cable diameter across the different detectors.The 'Triaxial Cable' label on the legend indicates the correspondence between cable diameter and FOF variation for each field size tested.The field sizes are: (A) 0.5 × 0.5 cm 2 , (B) 1.0 × 1.0 cm 2 , (C) 2.0 × 2.0 cm 2 , and (D) 3.0 × 3.0 cm 2 .
Our study has limitations since we used only 6 cables and 6 detectors.Currently, there are many more types of cables and detectors.It was not our intent to provide data on SNR of each combination but rather a general awareness of the importance of cable in radiation dosimetry.Like other parameters, it is prudent to acquire a quality cable and inspecting them periodically for abuse and kinks.A national recommendation on the quality of cable is also desired.

Conclusions
Regular inspection of triaxial cables is of paramount importance to maintaining precision in small field dosimetry.The lack of a national protocol for the evaluation and calibration of these cables, particularly for small volume chambers, can present challenges to achieving accurate dosimetry in small fields.The establishment of such a protocol would greatly enhance the accuracy and reliability of small field dosimetry, thereby improving patient outcomes and the efficacy of treatments.

Figure 1 .
Figure 1.Cut-through views of coaxial and triaxial cables.Coaxial cables, also known as twin-axial cables, consist of two conductors: a central conductor surrounded by an insulating layer and an outer conductive shield.BNC (Bayonet Neill-Concelman) connectors are commonly used with coaxial cables and are a type of miniature radio frequency (RF) connector.Coaxial cables have a leakage current generated by the insulation between the conductors, which is typically 10 giga-ohms.Triaxial cables, on the other hand, have three conductors: a central conductor (force), an inner conductive shield (guard), and an outer conductive shield (shield).The guard conductor is held at the same potential as the force conductor, effectively eliminating the leakage current.In the example shown, a 10 V potential is applied to illustrate the concept of leakage current elimination in triaxial cables.Triaxial cables provide better shielding and lower noise compared to coaxial cables.

Figure 2 .
Figure2.Variation of the field output factor (FOF) for small fields, illustrated for different detectors while maintaining the same triaxial cable for each measurement.The figure plots the FOF against the equivalent squared field size, with each color representing a different detector.Insets highlighting the data for 0.5 × 0.5 cm 2 and 0.5 × 0.5 cm 2 field sizes to showcase the range of FOF values obtained across the six triaxial cables used.

Figure 3 .
Figure 3. Variation in field output factor (FOF) for small fields using different detectors with the same triaxial cable, normalized to percentage differences to highlight relative changes.The bar plots represent the normalized relative FOF, calculated by dividing each detector's FOF value for a given cable by the average FOF value across all cables for that specific detector and field size.This normalization allows for a clear comparison of the relative impact of different cables on each detector's FOF.The red data points plotted on the secondary y-axis represent the diameter of each triaxial cable in millimeters, with one data point per cable.The solid red line connecting these points illustrates the trend in cable diameter across the different detectors.The 'Triaxial Cable' label on the legend indicates the correspondence between cable diameter and FOF variation for each field size tested.The field sizes are: (A) 0.5 × 0.5 cm 2 , (B) 1.0 × 1.0 cm 2 , (C) 2.0 × 2.0 cm 2 , and (D) 3.0 × 3.0 cm 2 .

Table 1 .
Characteristics of triaxial cables used in this study.The outer diameter refers to the overall diameter of the triaxial cable, including insulation and shielding layers.

Table 2 .
Characteristics of microdetectors used in this study.

Table 3 .
Field output factor (FOF) k Q Q [1]n msr clin msr for fields for 6 MV, as a function of the equivalent square field size[1].