Comparison of directly and indirectly estimated entrance skin dose (ESD) for diagnostic radiation qualities (RQR, RQA and RQT) using water phantom and shadowfree diagnostic chamber (SFD)

Background The aim of this study was to find the entrance skin dose (ESD) for diagnostic radiation qualities RQRs, RQAs and RQTs given in IAEA technical report series No. 457 using direct and indirect methods of measurement. Measurements were done for 5 × 5, 10 × 10, 15 × 15, 20 × 20 and 25 × 25 cm 2 field sizes and 70, 80, 90 and 100 cm source to surface distance (SSD) using shadow-free diagnostic (SFD) chamber and water phantom having dimension 30 × 30 × 30 cm 3 . ESD direct measurements were done by placing SFD chamber on the surface of water phantom, while in the case of indirect measurements, air kerma values were obtained. Results ESD values for different selected radiation qualities RQR2, RQR5, RQR8, RQR10, RQA2, RQA5, RQA8, RQA10, RQT8, RQT9 and RQT10 were found to be in the range of 0.0045–5.11 mGy per examination. Conclusions Results obtained were found to be comparable with ESD values published in the literature. The obtained results in this research would help in establishing the national diagnostic reference levels (DRLs) which would help in the optimization of diagnostic imaging procedures. It would also help the radiographers to optimize field sizes and SSDs in order to reduce dose to the patients thereby ensuring good radiological practices, and this would reduce the stochastic risk to the patients caused by the ionizing radiations.


Background
The entrance skin dose (ESD) or surface dose is defined as the absorbed dose (mGy) when radiation reaches the patient.ESD is dependent on beam energy, beam angle, field size, source to surface distance, phantom size and beam modifier devices [1].For skin dose assessment, International Commission on Radiation Units and Measurements (ICRU) and International Commission on Radiological Protection (ICRP) recommend the skin dose at the depth of 0.07 mm which corresponds to the interface between epidermis and dermis layer of the skin [2].Diagnostic radiology practices like general radiography and computed tomography (CT) is increasing day by day in all over the world because of their undoubted clinical benefits [3].ESD is received by the patients during diagnostic radiology practices.Use of ionizing radiation in diagnostic radiology can be linked with the development of cancer, that is, stochastic effects in the patients, but these doses are well below the doses that can cause deterministic effects, so basic radiation protection concept is ALARA which states that all exposures must be as low as reasonably achievable [4,5].ESD is a dose descriptor to quantify diagnostic reference levels (DRLs).Therefore, knowledge of surface dose or ESD is an important consideration in diagnostic radiology [6].
As studied in the literature, there is no single in vivo or in vitro representation for ESD estimations for diagnostic qualities (RQRs, RQAs and RQTs) given in TRS-457.ESD can be measured directly using ion chambers and thermoluminescent dosimeters placed on the patient's surface.It can also be indirectly measured in air using established formulism.IAEA BSS (1996) [7] sets ESD DRLs in the range of (0.4-30 mGy) for different X-ray examinations [8].European Commission (EC) 1996 [9] describes ESD as a quantity to be monitored per radiograph.The ESD recommendations for an adult of average size in plain radiography set by the Australian radiation protection and nuclear safety agency range from a minimum of 0.2 mGy (Chest PA) to a maximum of 26 mGy (Lumber spine spot) [10].Patients exposure to radiation has been increased all over the world due to diagnostic X-ray examinations which may cause stochastic effects to the patients.It is the main responsibility of the radiologists to establish local DRLs and accurately assess this unavoidable dose to the skin to make radiology practices as safe as possible [6,7,9].
Radiation quality in radiation (RQR) shows the radiation beam incident on the patient in fluoroscopy, general radiography and dental radiography.RQR5 is known as reference radiation quality used in general radiography as unattenuated beam.Radiation quality based on aluminum added filtration (RQA) and radiation quality based on copper added filtration (RQT) are established by using added filtration of aluminum and copper, respectively.RQA5 is used as reference radiation quality used in general radiography as attenuated beam.Similarly, RQT9 is the reference radiation quality for CT [11].
The aim of this study was to investigate the ESD per examination for different field sizes and SSDs for diagnostic radiation qualities RQRs, RQAs and RQTs using direct and indirect methods of measurement.These radiation qualities are given in TRS 457.

Methods
All measurements were performed at secondary standard dosimetry laboratory (SSDL), Pakistan.A diagnostic X-ray machine (tube model E7240FX and collimator model 5,129,405) was used to produce collimated beam of X-ray photons.This X-ray machine produces X-rays in the range (40-150) kVp.A PTW SFD chamber (model TM 34060 and Sr No. 00098) in conjunction with PTW-Freiberg electrometer (model TM 4060) was used for the measurements.For X-ray qualities RQAs and RQTs, IBA-made (IBA Dosimetry Inc. Germany) and locally fabricated added filtration of aluminum and copper ( area = 8 × 8 cm 2 , thickness varying from 0.022 to 13.1 mm) was placed at the center of square fields.Direct ESD measurements were made with ion chamber placed on the front surface of IAEA standard PMMA water phantom of dimensions 30 × 30 × 30 cm 3 .This phantom dimension provides full scattering for the field size being used.Indirect measurements were taken in air and ESD was calculated by multiplying air kerma value with backscatter factor and water to air mass energy absorption coefficient ratio.The chamber was positioned at the center of square fields.Irradiation time was 1 s and current was set to 10 mA.Calibrated barometer (all model MK2) and a digital thermometer (model HTC-2) were used for pressure and temperature measurements.
The SFD Ion chamber used in this work has traceability to IAEA primary laboratory at Vienna with mean air kerma calibration coefficient of 0.36 mGy/nC .ESD was determined using the following relations: where K air is air kerma, X = uncorrected charge value, K P,T = pressure temperature correction factor, N k is air kerma calibration coefficient, B air is backscatter factor and ( µ en ρ ) w,air is air to water mass energy absorption coefficient.K P,T , B air and ( µ en ρ ) w,air are unitless quantities.
Values of B air and ( µ en ρ ) w,air for the X-ray qualities were taken from published data [13,14].

Measurements of ESD for RQRs
Measurements were taken for the charge values given in Table 1, and ESD (direct and indirect) was calculated using Eqs. 1 and 2. ESD measurements for RQRs for different field sizes and SSDs are shown in Table 1.Exposure time (1) ESD (Direct method) = X(nC) × K P,T × Nk mGy/nC (2) ESD (In-direct method) = Kair mGy × Bair × µ en ρ w, air Similar graphs were observed for RQR2 and RQR8.These graphs show increase in ESD with field size which is due to backscatter factor (BSF).There is sharp increase in ESD value from field size × 5 cm 2 to 10 × 10 cm 2 because there is a large increase in BSF value in this range.For other square field sizes 15 × 15 cm 2 , 20 × 20 cm 2 and 25 × 25 cm 2 , variation in ESD value is little because BSF variation is very little between these successive field sizes [13].

Measurements of ESD for RQAs
For RQA measurements, added filtration of aluminum was used.Exposure time and tube current were 1 s and 10 mA, respectively.ESD measurements for RQAs for different field sizes and SSDs are shown in Table 2. Exposure time and tube current were 1 s and 10 mA, respectively.Figures 3 and 4 show variation of ESD with field   size and SSD for radiation qualities RQA2 and RQA10, respectively.These radiation qualities have application in measurements behind the patients.RQA5 (70 kV and 21 mm Al added filtration) is chosen as a reference radiation quality for attenuated beams for general radiography applications [11].Explanation of the graphs of RQAs (Figs. 3, 4) is the same as described for RQRs.Further, we see that ESD values for RQAs are lower than RQRs because BSF values for all field sizes increase with incident beam energy and reach a maximum value between 50 and 70 keV; for higher energies (> 70keV), it decreases [13].

Measurements of ESD for RQTs
For RQTs, added filtration of copper (Cu) was used for measurements.Exposure time and tube current were 1 s and 10 mA, respectively.Table 3 shows ESD measurements for RQTs for different field sizes and SSDs. Figure 5 shows variation of ESD with field sizes and SSDs for radiation qualities RQT9.RQT series represents unattenuated beam used in computed tomography.RQT9 (120 kV and 0.25 mm Cu added filtration) is chosen as a reference radiation quality for CT [11].
ESD values for RQTs are lower than RQRs because added filtration (Cu) absorbs the soft X-rays which would otherwise contribute to ESD.Deviation between direct   and indirect measurements of ESD is found to be in the range of 0.032% to 29.4% which is due to combined uncertainties in the values of backscatter factors and mass energy absorption coefficient ratios [3].In the literature, this deviation was found to be 0% to 40% [4].

Discussion
Entrance skin dose (ESD) is considered to be an important parameter in assessing the patient dose in diagnostic radiology [15].Medical diagnostic procedures are the largest contributor of patient radiation dose because globally every year large number of X-ray examinations   Comparison of present data with some published literature is shown in Table 5.As present study was performed on water phantom, exact comparison cannot be made with the clinical scenarios which is the limitation of this study, but present work can be used by the radiology departments who are setting their diagnostic reference levels (DRLs) in the field of diagnostic radiology.Deviation between direct and indirect method for ESD measurement is shown in Table 4 for different radiation qualities given in TRS-457.Tables 1, 2, 3, 4 and 5 show that ESD depends on beam energy, tube voltage, tube current, exposure time, field size and SSD.ESD increases with increasing filed sizes and decreasing SSDs.This study for the calculation of ESD for diagnostic X-rays (RQRs, RQAs and RQTs) will help in establishing diagnostic reference levels (DRLs) in diagnostic radiology department.It will also help the radiographers to optimize field sizes, SSDs, tube voltage and tube current which will help in optimizing the dose to the patients, ensuring good practice and thereby reducing the stochastic risk through ionizing radiations.

Conclusions
This study takes into account ESD variations with SSD and field size for diagnostic X-ray machine available at SSDL of Pakistan.Results of this study show good agreement with relevant studies, and these may be used as a baseline data for establishment of local diagnostic reference levels.Choice of X-ray examination parameters like tube voltage (kV), current (mA), exposure time (sec), SSD and field size during planer X-ray examinations has to be optimized so as to minimize ESD, and this will reduce the stochastic risk to the patients.

Recommendations
It is recommended that ESD data of present study can be used for the calculation of effective dose (ED) using corresponding mGy to mSv conversion factors for use in radiation protection purposes in radiology departments.It is also recommended that ESD values given in this work can be used for comparison with corresponding values obtained either with torso and anthropomorphic phantoms or planar X-ray examinations.ESD measurements with thermoluminescent dosimeters (TLDs) are also recommended which are cheap and easily available.

Fig. 1 Fig. 2
Fig. 1 Variation of ESD with field size for different SSDs using direct and indirect method (RQR5)

Fig. 3 Fig. 4
Fig. 3 Variation of ESD with field size for different SSDs using direct and indirect method (RQA2)

Fig. 5
Fig. 5 Variation of ESD with field size for different SSDs using direct and indirect method (RQT9)

Table 1
ESD measurement of RQRs

Table 2
ESD measurements for RQAs

Table 3
ESD measurements for RQTs

Table 5
Comparison of some selected present data with published data