Accurate dose measurement tools are needed to evaluate the radiation dose delivered to patients by using modern and sophisticated radiation therapy techniques. However, the adequate tools which enable us to directly measure the dose distributions in three-dimensional (3D) space are not commonly available. One such 3D dose measurement device is the polymer-based dosimeter, which changes the material property in response to radiation. These are available in the gel form as polymer gel dosimeter (PGD) and ferrous gel dosimeter (FGD) and in the solid form as solid plastic dosimeter (SPD). Those are made of a continuous uniform medium which polymerizes upon irradiation. Hence, the intrinsic spatial resolution of those dosimeters is very high, and it is only limited by the method by which one converts the dose information recorded by the medium to the absorbed dose. The current standard methods of the dose quantification are magnetic resonance imaging, optical computed tomography, and X-ray computed tomography. In particular, magnetic resonance imaging is well established as a method for obtaining clinically relevant dosimetric data by PGD and FGD. Despite the likely possibility of doing 3D dosimetry by PGD, FGD or SPD, the tools are still lacking wider usages for clinical applications. In this review article, we summarize the current status of PGD, FGD, and SPD and discuss the issue faced by these for wider acceptance in radiation oncology clinic and propose some directions for future development.

The authors demonstrated that the nMAG PGD with MRI could be used to measure the 3D dosimetric structures at the mm-scale in the vicinity of the foil. The current study provided more accurate 3D spatial dose distribution than the previous studies. Furthermore, the measurements were validated by the MC simulation.

Purpose: To evaluate the effect of increasing the number of thin high Z foils on the dose enhancement and the overall radiation attenuation with a 24MV photon beam. Methods: DOSXYZnrc was used to perform Monte Carlo simulations of multi‐layer lead foil configurations. The foil size was 7cm x 7cm. and the foil thickness was adjusted to give a combined thickness of 1mm. The number of foils used was 4, 6, 8, and 10. The separation between foils was also varied from 3 to 9 mm. The Mohan 24MV energy spectrum was used as a photon source. The field size was 5cm x 5cm and SSD was 100 cm. The phantom size was 16cm × 16cm × 28cm. The number of histories ranged from 1 to 2 billion. The percentage difference of the dose between the medium with foils and the homogeneous water was computed along the beam axis. The minimum dose enhancement and the change of integrated dose between the foils were determined. Results: Increasing the number of foils resulted in a decrease in the minimum dose enhancement. The highest dose region occurred in the last section for the 4 and 6 foil cases, whereas the 8 and 10 foil configurations showed the maximum dose region towards the center of the foil group. Increasing the number of foils increased the total integrated dose between foils. For example, the total integrated dose increase between the first and the last foils with a 3mm foil separation were 34.2, 43.4, 57.4, and 64.7% for 4, 6, 8 and 10 foils, respectively. Conclusion: This work showed the degree of dose enhancement around multiple thin lead foils. The results suggest that the total attenuation of photon beam can be increased by increasing the number of foils with a fixed total foil thickness.

Purpose:The purpose of this study is to investigate the feasibility of 3D dose measurements near thin high z materials using high energy photon beams.Methods:A MAGAT‐type polymer gel was manufactured in‐house. Lead foils of dimensions 1cm × 1cm × 0.5mm were encapsulated in 0.06mm thick tape and placed in two foil supports with 4cm depth and 0.7cm separation. The foils and supports were placed in a 500ml polystyrene container and the gel was poured into the phantom. The phantom was irradiated with a 18MV photon beam of 5cm × 5cm field size perpendicular to the foil at 100cm SSD. The gel was imaged with a Siemens 3T MRI scanner using the CPMG pulse sequence. DOSXYZnrc was used to calculate the expected dose and the Monte Carlo dose was compared with the gel. Enough histories were simulated so that the average error was less than 1%. The percentage difference of the dose between the heterogeneous gel and the homogeneous water was computed along and perpendicular to the beam axis.Results:Depth dose measurements made on the central axis showed increased dose upstream and between the foils, but had a decreased dose downstream. The range of increased dose was 1.8cm above and 0.7cm below the foils. The minimum dose point between the foils had an enhancement of 7.4%. The dose reduction downstream averaged around 5%. There was very good agreement between the Monte Carlo measurements and the gel doses. The gel and Monte Carlo results were within 2% at the minimum point.Conclusion:It was demonstrated that the MAGAT polymer‐gel can be used to measure the fine structures around small metallic objects. The results suggest that the dose of a 18MV photon beam can be increased in between thin high Z foils while reducing the dose downstream.

Purpose: To design and construct a second generation optical computed tomography (OCT) system using a fan‐beam with a CMOS array detector for the 3D dosimetry with polymer gel and radiochromic solid dosimeters. The system was specifically designed for the small field dosimetry. Methods: The optical scanner used a fan‐beam laser, which was produced from a collimated red laser beam (λ=620 nm) with a 15‐degree laser‐line generating lens. The fan‐beam was sent through an index‐matching bath which holds the sample stage and a sample. The emerging laser light was detected with a 2.54 cm‐long CMOS array detector (512 elements). The sample stage rotated through the full 360 degree projection angles at 0.9‐degree increments. Each projection was normalized to the unirradiated sample at the projection angle to correct for imperfections in the dosimeter. A larger sample could be scanned by using a motorized mirror and linearly translating the CMOS detector. The height of the sample stage was varied for a full 3D scanning. The image acquisition and motor motion was controlled by a computer. The 3D image reconstruction was accomplished by a fan‐beam reconstruction algorithm. All the software was developed inhouse with MATLAB. Results: The scanner was used on both PRESAGE and PAGAT gel dosimeters. Irreconcilable refraction errors were seen with PAGAT because the fan beam laser line refracted away from the detector when the field was highly varying in 3D. With PRESAGE, this type of error was not seen. Conclusion: We could acquire tomographic images of dose distributions by the new OCT system with both polymer gel and radiochromic solid dosimeters. Preliminary results showed that the system was more suited for radiochromic solid dosimeters since the radiochromic dosimeters exhibited minimal refraction and scattering errors. We are currently working on improving the image quality by thorough characterization of the OCT system.