A comprehensive Monte Carlo study of CT dose metrics proposed by the AAPM Reports 111 and 200

Abstract

Purpose: A Monte Carlo (MC) modeling of single axial and helical CT scan modes has been developed to compute single and accumulated dose distributions. The radiation emission characteristics of an MDCT scanner has been modeled and used to evaluate the dose deposition in infinitely long head and body PMMA phantoms. The simulated accumulated dose distributions determined the approach to equilibrium function, H(L). From these H(L) curves, doserelated information was calculated for different head and body clinical protocols. Methods: The PENELOPE/penEasy package has been used to model the single axial and helical procedures and the radiation transport of photons and electrons in the phantoms. The bowtie filters, heel effect, focal-spot angle, and fan-beam geometry were incorporated. Head and body protocols with different pitch values were modeled for x-ray spectra corresponding to 80, 100, 120, and 140 kV. The analytical formulation for the single dose distributions and experimental measurements of single and accumulated dose distributions were employed to validate the MC results. The experimental dose distributions were measured with OSLDs and a thimble ion chamber inserted into PMMA phantoms. Also, the experimental values of the CTDI 100 along the center and peripheral axes of the CTDI phantom served to calibrate the simulated single and accumulated dose distributions. Results: The match of the simulated dose distributions with the reference data supports the correct modeling of the heel effect and the radiation transport in the phantom material reflected in the tails of the dose distributions. The validation of the x-ray source model was done comparing the CTDI ratios between simulated, measured and CTDosimetry data. The average difference of these ratios for head and body protocols between the simulated and measured data was in the range of 13-17% and between simulated and CTDosimetry data varied 10-13%. The distributions of simulated doses and those measured with the thimble ion chamber are compatible within 3%. In this study, it was demonstrated that the efficiencies of the CTDI 100 measurements in head phantoms with nT = 20 mm and 120 kV are 80.6% and 87.8% at central and peripheral axes, respectively. In the body phantoms with nT = 40 mm and 120 kV, the efficiencies are 56.5% and 86.2% at central and peripheral axes, respectively. In general terms, the clinical parameters such as pitch, beam intensity, and voltage affect the D eq values with the increase of the pitch decreasing the D eq and the beam intensity and the voltage increasing its value. The H(L) function does not change with the pitch values, but depends on the phantom axis (central or peripheral). Conclusions: The computation of the pitch-equilibrium dose product, Deq , evidenced the limitations of the CTDI 100 method to determine the dose delivered

Comparison between two methods for estimation of Equilibrium Dose in MSCT procedures

“…On Figure 1, "c: Simulation" and "p: Simulation" are referred to the simulation on the central hole and peripheral holes, respectively; "c: AAPM" and "p: AAPM" are referred to the estimation of the curve by the AAPM method on the central hole and peripheral holes, respectively; "c: L-M" and "p: L-M" are referred to the estimation of the curve by the L-M method on the central hole and peripheral holes, respectively. [7].…”

“…Table 3 shows the results of the ratios for all combination of protocols evaluated applying the equations (7), ( 8) and (9). The method proposed by the AAPM Report 200 results in different values of 𝐷 * according to the initial guess.…”

“…The parameters 𝐷 #$ , 𝛼 and 𝐿 #$ were obtained by fitting eq.2 to the simulated data. Details of the applied methodology and the results are presented in Costa et al [7].…”

“…Previously simulated [7] cumulative dose data, D(L) were used to evaluate the L-M and AAPM methods. The simulations were performed on an infinitely long phantom of PMMA for the center hole and on the peripheral holes.…”

Detector Simulation Challenges for Future Accelerator Experiments

“…MC has long been used in the design of imaging systems such as positron emission tomography (PET) or computed tomography (CT) [88]. Like in therapy, HEP codes are being applied either directly or tailored to imaging applications, i.e.…”