Introduction:
Starting around 1972, Computed Tomography is consistently being created phenomenally, eminently with the assistance of software engineering, which has permitted making exceptionally exact diagnostics. CT advancement went through a few phases, from the model of Hounsfield to going through the consecutive and helical modalities. This advancement made CT a vital assessment in radiology (Kanne & Lin, 2018). In any case, this radiological strategy is the most lighting contrasted with different procedures; it can convey a portion of 50-500 times more prominent than a standard radiological assessment. Some patients can have life-altering effects after having a CT scan, but others oppose having the scan done, due to cancer risk. Studies have shown that the scan has a low risk of cancer-causing agents in the body (Pontone, G). To assess the danger to patients from CT checks, a gauge of the dose delivered to the skin and organs of a patient is fundamental. A need in this way exists to decide proper dosimetric amounts, for example, the organ dose and peak skin dose (PSD) (De las Heras).
The Federal Drug Administration (FDA) states that the Peak Skin Dose is the “highest radiation dose accruing actually at a single site on a patient’s skin.” Knowing the appropriate highest dosage is vital so that no harm is caused to the patient. The United States has regulated that the” fluoroscopic system provides a display of the irradiation time, dose rate at the interventional reference point during irradiation, and the cumulative dose for the procedure upon completion of irradiation” (Pontone, G). In preparation for actual patients, technologists and physicists would revert to the manufactured dose estimation which is called the Computed Tomography Dose Index (CTDI). The CTDI is generally utilized for quality control including the radiation output of CT machines. Specifically, the volume CTDI is shown on the control center of all CT machines and is promptly accessible to the administrator. In any case, the CT Dose Index (CTDIvol) was originally designed as an index of dose associated with various CT diagnostic procedures, not as a direct dosimetry method for individual patient dose assessments.
Moreover, CTDIvol is reported in two units: a 16-cm phantom for head exams or 32-cm phantom or body exams. The relationship between the CTDIvol and airiest dose depends on various factors, two of which are the patient size and composition. CTDIvol is displayed on the console of CT scanners, and it gives genuine estimates of the dose being delivered to patients and can serve to approve Monte Carlo recreations (Jones, A. Kyle). Specifically, estimating Peak Skin Dose is ideal since it is a surely known dosimetric amount that directly identifies with radiation-incited skin wounds. Besides, estimation estimates of PSD values, utilizing appropriate phantoms can without much of a stretch be made across all types of CT units and scan protocols accessible in clinics (Tack & Gevenois, 2018). This is significant for comparing doses for a similar CT examination in different facilities, which can change fundamentally. More recently, modifications to the original CTDI concept have attempted to convert it into to patient dosimetry method, but have mixed results in terms of accuracy.
Nonetheless, CTDI-based dosimetry is the current worldwide standard for estimation of patient dose in CT. Therefore, CTDIvol is often used to enable medical physicists to compare the dose output between different CT scanners. Also, since CTDIvol estimates the patient’s radiation exposure from the CT procedure, the exposures are the same regardless of patient size, but the size of the patients is a factor in the overall patient’s absorbed dose (SSDE). The size-specific dose estimate (SSDE) is measured in mGy, and it is a method of estimating CT radiation dose that takes a patient’s size into account.
From a radiation protection point of view, determining the maximum dose delivered to the skin would allow deriving quantities that can be compared with dose reference levels set by national and international standards. The most important outcome from a radiation safety perspective is evaluating if a radiation injury had occurred quickly (NCRP Report 116.) In this research, the peak skin dose delivered to a patient was estimated experimentally by measuring the dose delivered to the surface of the NEMA phantom and 32 cm CTDI phantom using external dosimeters. These dosimeters will provide PSD values for a given protocol and its related CTDIvol. From this, a relationship can be evaluated between both quantities. The aim of this project was to test the hypothesis that the size-specific dose estimate (SSDE) has a sufficiently strong linear relationship with PSD to allow direct calculation of the PSD directly from the SSDE.
Materials and Methods:
The measurements were performed with a Siemens 64 slices, Biograph mCT. A comparison was made between the CTDIvol value displayed on the CT console and the measured CTDIvol value using the AAPM protocol. For every examined scanner, the CTDIvol was obtained from scans in an axial mode for head scans and helical mode of the routine pelvis, cervical spine, abdomen, and thoracic scans using the scan parameters as shown in Table 1. The corresponding CTDIvol displayed on the console was recorded as shown in Table 1.
Peak Skin Dose was estimated by using Nanodots dosimeters (International Specialty Products, Inc., Wayne, NJ, USA) which have optically stimulated luminescence (OSL) technology which is a single point radiation monitoring dosimeter. It is a useful tool in measuring the patient dose, and it is an ideal solution in multiple settings, including diagnostic radiology, nuclear medicine, interventional procedures and radiation oncology (LANDAUER).
Nanodots dosimeters also have minimal angular or energy dependencies with appropriate calibration which can be used to measure skin dose at a point of interest. Moreover, LANDAUER provides a set of calibration dosimeters exposed at a beam quality of 80 kVp on a PMMA phantom at normal incidence for conventional (non-mammography) diagnostic radiology applications. For radiation oncology applications, LANDAUER provides a set of screened, unexposed calibration dosimeters that can be irradiated using a radiation therapy beam quality. Another way for calibration is to request a dosimeter set exposed to a 662 keV beam quality (Cs-137).
The Nanodot dosimeters were placed on three different locations (Anterior-Posterior, Lateral (LAT) and Posterior-Anterior) as shown in figure 1, and the dose to the skin was measured at these locations.
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CT TABLE
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Figure1: The phantoms in the middle of the CT scan and 1 is the AP location, 2 is the LAT location and 3 is the PA location.
Experimental set-up and procedure:
The CTDIvol displayed by the scanner was validated to the true CTDIvol following the ACR testing guidelines. A correction factor was used to correct the inaccuracies in the displayed value. This correction was applied to the DLP displayed by the scanner.
Peak skin dose and its relation were measured by the 2 phantoms, and the phantoms were aligned at the isocenter of the scanner and a single axial CT scan was made. After placing the Nanodot dosimeters on the AP, LAT and PA locations, the phantoms were scanned over the scan length for a fixed value of the tube current. The measurement was repeated several times using various scanning techniques (with varying energy, current) as shown in table 1. Size conversion factors used were based on the dimension of the phantom being scanned. These K-factors with the CTDIvol produced the size-specific dose estimates (SSDEs), and since the CT dose index was provided at the CT scanner too, the size-specific dose estimate for the phantoms was calculated. Also testing if the correlation between the size-specific dose estimate and the measurement of the peak skin dose match was done, and since such a relationship exists, finding that factor was achieved.
Results:
After measuring the Peak Skin Dose and Size Specific Dose Estimates (SSDE), a comparison was done. The SSDE was calculated using the corresponding k-factor based on the AP and lateral dimension from TG204 and the CTDIvol value which was displayed on the console (SSDE = CTDIvol x K factor).
The conversion factor based on the use of the 32 cm diameter NEMA phantom for CTDIvol was 1.35 for the AP and PA locations, and the conversion factor for the Lat location was 1.55. Also, the AAPM Report 204 stated that the conversion factor based on the use of the 16-cm diameter ACR phantom was 0.89 for the three locations.
Figure 1: The graph illustrates the relationship between Peak Skin Dose in AP location and the Size Specific Dose Estimates in AP location in 32 cm NEMA phantom and 16 cm ACR phantom.
The figure above illustrates the measured PSD in AP location against the SSDE in AP location with using 2 different phantoms (32-cm NEMA phantom and 16-cm ACR phantom). For both phantoms, there was linear relationship between the size specific dose estimates and the peak skin dose. In this study an R-squared value was used to value the data in the graphs and to tell how accurate the line is. In this study, the R-squared value was 0.21 which indicate that 21% of the variance of the dependent variable being studied is explained by the variance of the independent variable. Therefore, the relationship between the PSD in AP location and the SSDE in AP location has a weak correlation.
Figure 2: The graph demonstrates the relationship between Peak Skin Dose in PA location and the Size Specific Dose Estimates in PA location in 32 cm NEMA phantom and 16 cm ACR phantom.
The second figure demonstrates the measured PSD in PA location against the SSDE in PA location. For both phantoms, there was linear relationship between the size specific dose estimates and the peak skin dose. In this graph, the R-squared value was 0.66. Therefore, the relationship between the PSD in PA location and the SSDE has a moderate positive relationship, so a correlation might occur.
Figure 3: The graph illustrates the relationship between Peak Skin Dose in Lateral location and the Size Specific Dose Estimates in Lateral location in 32 cm NEMA phantom and 20 cm ACR phantom.
The third figure illustrates the measured PSD in the lateral location against the SSDE in Lateral location. For both phantoms, there was linear relationship between the size specific dose estimates and the peak skin dose. In this graph the R-squared value was 0.61 which indicated that there was a moderate positive relationship between the PSD in lateral location and the SSDE in lateral location.
In all the plots, linear relationship between the PSD and SSDE was found, and the linear fitting equation was calculated by Excel. (SSDE = 3.4827 x (PSD) + 5.522), this was the fitting equation for the AP location graph (1st graph). However, since there was a weak correlation between the PSD and SSDE in the AP location, calculating the SSDE will not be accurate.
(SSDE = 6.7198 x (PSD) + 2.1234) and (SSDE = 8.2489 x (PSD) + 2.3624), Those two linear fitting equations were for the PA location graph (2nd graph) and lateral location graph (3rd graph) respectively. Both equations have a moderate positive relationship. Therefore, predicting the value of SSDE or PSD will be possible but not 100% accurate. With using these data and fitting equations, a physicist can estimate the PSD, but with some limitations. The physicist would be within 30% the true dose estimates and a large error would be there as well. The regression was almost 65% in both locations, so roughly 65% of the data points will fall close to the linear line.
Other trend line equations such as exponential, logarithmic, polynomial and power were tested to evaluate the measured PSD and SSDE, but the linear fitting equation was the only one that the line fitted with the data.
Discussion:
The anterior Peak Skin Dose was different in the AP and LAT locations comparison with the lateral location which is because the thickness of the phantom. Considering that examination is performed in the lateral location of the body which has the highest x-ray attenuation, thus requiring higher beam energy to penetrate. With increasing the patient average diameter, the peak skin dose was higher. According to the data that was measured, the measured PSD was higher in all the lateral location than the AP and PA locations. The bigger the phantom (more tissue to penetrate), the more dose was required to attenuate and reached the dosimeter.
In the is study the AP and lateral dimensions of the phantom were used to measure the SSDE which is a factor that is used to estimate the absorbed dose. This could’ve been an error in measuring the peak skin dose since the SSDE was not measured at that time. Also, there was a linear relationship between the PSD and the SSDE because the Size Specific Dose Estimates dictate the patient’s dose and this could be one of the reasons that the linear relationship occurred. Also, there could be better modifications to the K-factors in order to dictate the patient’s more accurately.
When calculating how much radiation dose a patient is actually receiving, it’s best to consider their actual size. CTDIvol and DLP are common methods to estimate a patient’s radiation dose from a CT procedure. The dose is the same regardless of patient size, but the size of the patients is a factor in the overall patient’s absorbed dose. Therefore, SSDE measured in mGy, would allow the physicists to use the patient’s size as a factor in order to estimate the radiation dose. In the other hand the PSD is the maximum absorbed dose in mGy to the most heavily exposed region of the skin in specific location. In this study, the measured values of the PSD and SSDE had a linear relationship in most projections (C-spine, thoracic and pelvis). The higher the PSD was, the higher the SSDE which was due to the measured CTDIvol which displayed in the console (the higher the CTDIvol was, the higher SSDE was calculated).
There is different between the CTDIvol that was shown on the console and the actual CTDIvol. The CTDIvol or its derivative the DLP, as seen on consoles and outputted, do not represent the actual absorbed or effective dose for the patient. They should be taken as an index of radiation output by the system for comparison purposes. In this study, it is not possible to compare the true CTDIvol to the displayed because the phantoms that were used were not CTDI phantoms, so it is not possible to place a CTDI probe.
However, nowadays many modifications to original CTDI concept have attempted to make it more accurate patient dosimetry method, with mixed results. Body CTDIvol reported by the CT scanner, or measured on a CT scanner, is a dose index that results from air kerma measurements at two locations, to a very cylinder of plastic phantom with a density of 1.19 g/cm3 (Morgan, M. 2021).
According to the measured data, some scan projections such as abdomen had high PSD and high SSDE due to the high measured CTDIvol and DLP caused out wire and low regression. Taking out the abdomen PSD and SSDE from the graphs make the regression higher (more positive) which means correlation could exist. Therefore, some projections such as an abdomen and head might make the data points and graphs not clear and hard to be read.
When graphing the measured PSD and SSDE in each phantom separately, a higher regression (more positive correlation) was found (close to 90%) in all the three locations. This means that the closer the patient to become cylindrical, the better relationship between PSD and SSDE will be and more accurate doses will be measured. It fails at very large effective circumferences with perfectly cylindrical patients.
Conclusion:
The results showed there is a moderate positive relationship in both PA and lateral locations, so there might be a correlation between the PSD and SSDE. There is some promises in Posterior and Lateral angles because the higher the PSD was, the higher the SSDE was in most projections. The measured PSD and SSDE showed that a physicist can estimate the PSD within 30% the true dose estimates with a large error due to the moderate positive relationship.
Further studies with more data should be done to prove or decline the hypothesis. In this study, only two phantoms were used (NEMA and ACR phantoms) with 32 cm and 16 cm thicknesses, so other phantoms such as anthropomorphic phantoms and fake human phantoms with different thickness styles could be used to get better data and correlation.
In this study, only 8 measurements were taken in the three different location due to the limitation of the Nanodats. More measurements could have been taken and a better data points would have been measured. With more date testing if the SSDE has a sufficiently strong linear relationship with PSD could be done.