Problems Affecting Accurate Dose Measurement in Small-Field for Linear Accelerator

Background: The use of relatively narrow fields has become necessary with the advent of intricate and accurate radiation therapy delivery dose to patients; therefore, small-field dosimetry faces several difficulties. Both dose calculations and measurements require to be performed with extra care, due to the uncertainty that might be increased by using such small field sizes. Material and Methods: In this study, we investigated the effect of detectors size on the dosimetry of small fields [starting with radiation fields from (1cm x 1cm), (2cm x 1cm), and (3cm x 1cm)...etc., up to (4cm x 5cm) and (5cm x 5cm)]. We used the linear accelerator and different types of ionization chambers i.e. [Farmer FC65-P, CC13, and CC01 (pinpoint)] an addition to semiconductors i.e. (IBA Razor diode)], and we investigated all detectors to read the absorbed dose in water under the reference conditions (field 10cm x10cm, SSD 100cm and depth 10cm). Results: While measuring the absolute dose under reference conditions, all detectors had a non-significant difference of less than ±2%, except for the Razor diode, which showed a significant difference of ± 5%. On the other hand, when small fields were measured, we discovered a significant difference of 48%, compared to the Razor diode. Conclusion: The Razor diode is more stable in small-field dosimetry than other detectors. Also, the Razor Diode is intended for relative dosimetry but, it shall not be used for absolute dose measurements.


Introduction
The majority of cancer patients will require radiotherapy at some time during their illness, either for curative or palliative purposes, making it an important part of a multidisciplinary cancer treatment approach (Thakur et al., 2022).
In radiotherapy, we can define the small field as that field with dimensions smaller than the lateral range of the electrons that contribute to the dose, the Multi-leaf Collimators (MLC) roughly can be made in narrow field sizes up to 1cm x 1cm or less, so at least one of the following three physical conditions are generally considered to determine if an external photon beam can be designated small: (a) Lack of charged particle (Loss of lateral charged particle equilibrium) LCPE. (b) There is partial occlusion of the primary photon source by the collimating devices on the beam axis. (c) The size of the detector like or larger than the cross-sectional beam dimensions at the depth of measurement .
The selection of an appropriate detector for dosimetry in small fields is challenging, and it is necessary to choose a suitable detector with the best performance, and not all detections of ionization chambers are sensitive enough to radiation dosimetry (Zhu et al., 2009), also, there is no common agreement among researchers on the use of specific detector types. Some studies have investigated the effect of the construction and size of detectors in small radiation fields (Scott et al., 2012;Underwood et al., 2013). Researchers have frequently evaluated the effectiveness of various detectors at dose distribution measurements, but most of them focused only on the stereotactic radiation field created by radiosurgery devices and circular cones, a few studies focused on small fields used in beamlets of linear accelerators.
IMRT and VMAT fields use small segments shaped by MLCs of Linac for dose delivery, so a consensus has been that suitable detectors must be considerably smaller than the field size as it impacts detector readings when charged particle equilibrium (CPE) breaks down around the sensitive volumes of detectors (Das et al., 2008a), which could lead to significant uncertainty in the accuracy of clinical dosimetry (Bich, 2014) compared to traditional radiotherapy, as we pointed out that accurately measuring absolute dose or output factors at an absorbed dose to the water at sub-centimeter fields was very difficult, so the IAEA recommended appropriate detector systems and measurement methodologies at TRS-483 . An overview of the issue of LCPE and the changes in photon beam perturbations with decreasing field size were provided. The dosimetry and commissioning of traditional large field sizes starting from 5cm x 5cm and more up to 40cm x 40cm used high photon energy in radiotherapy has been addressed in several reports of the TG-106 (Das et al., 2008b). These and other dosimetry protocols are based on measurements using an ionization chamber of absorbed dose to water, traceable to International Standards of units dosimetry laboratory (PSDL) at reference conditions, such as a conventional field size of 10cm x 10cm (Allisy et al., 2009).
In conventional broad beams, it is derived from a ratio of detector readings due to the practical independence of perturbation correction factors on field size (Alfonso et al., 2008). In small-field dosimetry, however, such independence does not exist, notably for perturbation factors, and an output factor of absolute dose to water measurements will in almost all cases require an output correction factor to the measured detector reading ratio relative to the machine-specific reference field (fmsr). The symbol for a field output factor in IAEA TRS-483 is clinic field (fclin); (fmsr) (Qclin); (Qmsr) and Report of AAPM TG 155 (O'Brienet al., 2016;. Field output correction factor. A field output correction factor is a correction factor that accounts for the differences between the response of a detector in a non-reference (clinical) field and that in an msr reference field (Das et al., 2021), and accordant to TRS-483  and Joint Committee (Prieto et al., 2015) in uncertainties for small field measurements. It has been shown that measurement uncertainty increases from ±2% at 10cm x 10cm to ±5% at 5cm x 5cm and is smaller than that (Zhao et al., 2022).

Detectors specification
Dose distribution measurements were done for different radiation fields from 1cm x 1cm to 5cm x 5cm, in addition to the reference field size of 10cm x 10cm. In measurements, we used the Elekta Synergy linear accelerator with photon energy beams of 6 MV & 10 MV and different types of ionization chambers (FC65-P, CC13, and CC01) and IBA Razor diode detector, with specifications, summarized in (Table 1).
The detectors were calibrated before this study; to insure the high accuracy of the machine output measurements.

Phantom specification
In the present work, the measurements were performed in a water phantom (IBA Blue Phantom), threedimensional scanning (48 x 48 x 48 cm 3 ). A common control unit is integrated with the phantom that acts as an interface between the phantom and computer software which allows the ion chamber and IBA Razor (diode) to place remotely controlled.

Electrometer specification
Electrometer: (IBA DOSE-2) is a portable, dualchannel, high-precision reference class electrometer for measurements of absorbed dose.

Methods
The setup was first benchmarked with a set of measurements of the absolute dose under reference conditions in high energy photon beams at10cm depth in a water phantom using 110cm source-to-chamber (SCD) distance or 100cm source-to-surface distance (SSD) and field size (10cm × 10cm), that's showing in Figure 1.
The detector placement is in the parallel direction of radiation field at measuring with the Razor diode and perpendicular direction at other detectors as shown in Figure 2. The measurements based on the IAEA TRS-398 protocol (Musolino, 2001).
The characteristics curves of PDD curve and lateral profiles (at different depths of dmax, d50 mm, d100 mm, and d200 mm) was measured at reference parameters for photon beam energies 6MV and 10 MV as a calibration test of the machine output before beginning of this study as shown in Figure 3, Figure 4, and Figure 5.

Formalism for fields
In this study, we have used formalism based on Alfonso et al., 2008 proposed a new formalism for reference dosimetry of small and non-standard fields that establishes a relationship of Codes of Practice (CoP) applicable to conventional radiotherapy techniques. According to newly proposed techniques, the total dosimetry process is performed in fmsr matched with the conventional 10 cm× 10 cm reference field.
The measurement of absorbed dose to water in reference condition (1)  DOI:10.31557/APJCP.2023.24.8.2757 For 6 MV For 10 MV b-The IBA Farmer FC 65-P, CC13, and CC01 (Ionization chamber) are calibrated at the National Institute of Standards (NIS). We measured the crosscalibration factor for IBA Razor (diode) at 100cm SSD, 10cm depth in water phantom which has been measured in ten measurements and then took the average measurements for taken the cross-calibration according to TRS-398 (Musolino, 2001).
c-In the study of the variation in different detectors with different narrow field sizes, we have found the following: In the beginning, when we measured the absorbed dose rate to water at Percentage depth dose (PDD) at Z reference for a field size of 10cm x 10cm, we found that's the doses are almost the same approximately 67% at energy 6 MV and 72 % at energy 10 MV for all detectors except for IBA Razor (diode) and which has a greater doses than the one that was 71.8% at energy 6 MV and 74.8% for energy 10 MV. Secondly, at fields smaller than 4 cm x 4 cm, the detector becomes too larger than the diameter of the radiation field, which leads to LCPE, and, the closer we get to the 1cm x 1cm (Bouchard et al., 2015), the clearer the vision on the loss of charged particle and high overlapping radiation beams, on the other hand, the small detectors size have enough diameter with low overlapping these are shown in Figure 6 for energy 6 MV all detectors, there is a substantial difference as a function of LCPE in small fields from 1 cm x 1 cm bottom up 4cm x 4 cm with a large substantial difference in LCPE in Farmer 0.65 about 48% compared with Razor reads.
In Figu 7 for energy 10 MV we found the same substantial difference as a function of LCPE, with higher substantial at farmer 0.56 compared with energy 6 MV.
Measured absorbed dose to water in reference field dosimetry (msr) with corrected (pressure, temperature, ion recombination, polarity effects, electrometer factor, and beam quality factor).
Is the calibration factor of the ionization chamber in terms of absorbed dose to water, performed in Co 60 beams at the national standards laboratory, is beam quality correction factor, and is another correction factor arising from the change of field size, shape, phantom material, and beam quality from the reference condition.
The absorbed dose in small field f clin is different from the reference field f msr so we have a new output factor of small field Ω that's depending on the field size.
IBA Razor diode (chamber) calibrated with measured using the following formula: Monitor unit; ND,W,Q0 this isN_(D,W) in protocol TRS-398 (Laub and Wong, 2003) and AAPM TG-51 (Almond et al., 1999) cross-calibration factor is used reference ionization chamber to measure the dose at the reference field (10cm × 10cm), then used the IBA Razor diode chamber at the same condition obtained by electrometer reading (nC).
In output factor measurement, the typical normalized reference field was much larger than the small fields; to minimize the error one option is to be correct factors are the basic requirement for the calibration (O'Brien et al., 2016;Francescon et al., 2020).
So, the correction factors are implemented using the following equation:

Results
a-The absorbed dose in the water phantom measured at reference parameters [field size 10 cm × 10 cm,100 cm SSD, 10cm depth in water, and 100 Monitor Unit (MU)]. IBA Farmer 0.65 (ionization chamber) was used for photon energies (6 MV and 10 MV), and the beam quality index (Q0) for high energy photon beams was used according to the relation with the Tissue Phantom Ratio as reported in TPR 20,10

d-Correction factor
The correction factor is calculated from this experiment for Razor (diode) chamber by following equation 5 is represented in Figure 8 for energy 6 MV and in Figure 9 for energy 10 MV. The correction factor measured from an experiment was found excellent and in agreement with the literature values for the small field.

Discussion
Radiotherapy has been considered as one of the principal modalities in cancer treatment. To begin treatment procedure, several steps should be taken, such as taking a history of patient, considering different modalities to treat, prescription of treatment dose, imaging and target volume definition, treatment planning, quality assurance (QA) and verification of the plan, patient set up, treatment delivery, and verification of its precision (Zarei, and Sheikholeslami, 2017).
The study revolves around the validation process of matching the percent depth dose (%dd) to the beam profile, measuring the absolute dose at the reference condition and small field to different detectors with the study of the Problems affecting accurate Dose measurement in small-field for a linear accelerator.
After the machine validated (%dd, profile beam, TPR 20,10 and ion-chamber calibrated), we noticed that measurement results of all detectors are nearly the same when measuring at reference field size, which implies that the lateral charged particle equilibrium of the radiation field is larger than the diameter of the detector, except Razor diode which higher reading because of its extremely small active size and highly sensitive to radiation, so it's designed to measure relative doses and it's not designed to measure absolute doses.
At the small field size the detector size becomes too larger than the diameter of the radiation field, which leads to loss of lateral charged particle equilibrium, but with the small detectors size have enough diameter with low overlapping these are shown in Razor (diode) read, so it's very suitable for relative dosimetry of photon fields in radiotherapy, particularly at small field sizes; therefore, it becomes necessary to determine the correction factor under different experimental conditions to account for the diode's sensitivity to these parameters and ensure accurate dose measurements.
As the diode is not water equivalent and sensitive to different parameters such as SSD, field size, and angle of incidence. So, we measured the correction factor under different experimental conditions.
In summary, it underscores the similarity in measurement results among the detectors at the reference field size, except for the Razor diode. The Razor diode, with its small size and high sensitivity to radiation, is designed for relative dose measurements and demonstrates higher readings. At small field sizes, the Razor diode remains suitable for relative dosimetry due to its small size and limited overlap with the radiation field. However, the measuring necessitates the determination of correction factors under different experimental conditions for accurate dose measurements.
In conclusion, the variations in the absorbed dose measurements in the water phantom refer to; all the large detectors (in state of the reference field size10cm x 10cm) are suitable values with a non-significant difference of less than ± 2% because the detector is considerably smaller than the field size and the readings of (CPE) didn't break down around the sensitive volumes of detectors, except the measurements performed by IBA Razor diode detector which, show a significant difference ± 5%, for measured quantities. While the dose measurements in the case of small field sizes; showed significantly ±48% compared to the IBA Razor diode; so, the Razor is more stable in small field dosimetry because the size of the detector is small enough diameter with low overlapping.

Author Contribution Statement
All authors actively participated in the work presented in this manuscript. A. M. carried out the practical measurements, writing methodology, and instrumentation. G. M. H. carried out the calibration of the dosimetry tools, writing methodology and analysis of data and results. K. T. E. participated in design, coordination and helping to draft the manuscript. E. M. A. participated in design and writing basic parts of a manuscript. All authors read and approved the final manuscript.