Dosimetric investigations of Kilovoltage Cone Beam Computed Tomography (kV-CBCT) utilized in Image Guided Radiation Therapy (IGRT) using Monte Carlo simulations

Many studies have shown that the computed tomography dose index (CTDI100) which is considered to be the main dose descriptor for CT dosimetry fails to provide a realistic reflection of the dose involved in cone beam CT (CBCT) scans. The main reason for this failure is that CTDI100 measurements are performed within standard head and body phantoms made of polymethyl methacrylate (PMMA) that are only 150 cm long, which is less than or similar to beam widths used for CBCT scans. Therefore, much of the scatter that would contribute to the dose received by a patient is not recorded. Several practical approaches have been proposed to overcome drawbacks of the CTDI100. The aim of this project was to investigate the various dose indices based on the approaches proposed. The dose indices studied were: (1) CTDIIEC proposed by the International Electrotechnical Commission (IEC) and based on measuring CTDI100 using a reference beam and the application of a correction factor based on free-in-air CTDI measurements, (2) f(0,150) the cumulative dose measured with a small ionization chamber within the standard PMMA phantoms, (3) f100 (150) the cumulative dose measured in the standard PMMA phantoms with a 100 mm pencil ionization chamber, (4) f(0,∞) proposed by the American Association of Physicists in Medicine (AAPM) TG - 111 and similar to f(0,150), but measured in infinitely long phantoms made of PMMA, polyethylene, and water, (5) f100 (∞) similar to f100 (150), but measured in infinitely long phantoms. The project also aimed to facilitate the use of indices defined in long phantoms through the generation of correction factors that could be applied to measurements in standard phantoms. This project was based on the use of the Monte Carlo (MC) technique. MC EGSnrc-based user codes namely BEAMnrc and DOSXYZnrc were used to simulate the On-Board-Imager (OBI) imaging system mounted on a Varian TrueBeam linear accelerator. The MC model was benchmarked against experimental measurements and good agreement shown. PMMA, polyethylene, and water head and body phantoms of various lengths and diameters were simulated including a new polyethylene phantom named ICRU/AAPM phantom made by the International Commission on Radiation Units and Measurements (ICRU) and AAPM. A wide range of beam widths with different beam qualities were employed. Four scanning protocols using two acquisition modes (full and half), employed in routine clinical practice, were utilized. In addition, organ doses resulting from three CBCT scans (head, thorax, and pelvis) were evaluated in terms of absorbed dose to organs and tissues using MC simulations on the International Commission on Radiological Protection (ICRP) 110 adult male and female reference computational phantoms. The suitability of the dose indices for CBCT dosimetry was investigated by taking three factors into consideration: (1) the efficiency of the approach as a dose descriptor to report CTDI∞, which is close to the dose received by body tissues near to the middle of a CBCT scan of a patient, (2) the simplicity of the application of the approach in the clinical environment in terms of availability of the measuring instruments, simplicity of the technique, and the number of the scans required to accomplish a quality assurance (QA) assessment, i.e. the QA time, and (3) the ability of the approach in providing an evaluation of organ doses resulting from CBCT scans. To facilitate the use of long phantoms, the relationship between f(0,150) and f100 (150) measurements obtained within the standard PMMA phantoms and those for f(0,∞) obtained within longer phantoms of different compositions were studied. Considering the three factors for the dose indices investigated, all the dose indices were found to be comparable, but each index has advantages and disadvantages. Overall, f(0,150) was considered to be the most suitable with f100 (150) providing an alternative for wider beams. Therefore, the dose indices f(0,150) followed by f100 (150) are recommended for practical CBCT dosimetry. In addition, a function called Gx(W)100 was proposed for evaluating the cumulative dose in long phantoms, and correction factors were also provided to avoid the use of long phantoms. The Gx(W)100 function did not vary significantly with tube potential, but the tube potential did influence the correction factors. The use of the Gx(W)100 function is recommended for estimation of f(0,∞) values from f100 (150) measurements taken in the standard PMMA phantoms.