Purpose: For the combined 1.5T/6MV MRI‐linac system, the perpendicular magnetic field to the radiation beam results in altered radiation dose distributions. This Monte Carlo study investigates the change in dose at interfaces for common organs neighboring soft tissue. Methods: MCNP6 was used to simulate the effects of a 1.5T magnetic field when irradiating tissues with a 6 MV beam. The geometries used in this study were not necessarily anatomically representative in size in order to directly compare quantitative dose effects for each tissue at the same depths. For this purpose, a 512 cm3 cubic material was positioned at the center of a 2744 cm3 cubic soft tissue material phantom. The following tissue materials and their densities were used in this study: lung (0.296 g/cm3), fat (0.95), spinal cord (1.038), soft tissue (1.04), muscle (1.05), eye (1.076), trabecular bone (1.40), and cortical bone (1.85). Results: The addition of a 1.5T magnetic field caused dose changes of +46.5%, +2.4%, −0.9%, −0.8%, −1.5%, −6.5%, and −8.8% at the entrance interface between soft tissue and lung, fat, spinal cord, muscle, eye, trabecular bone, and cortical bone tissues respectively. Dose changes of −39.4%, −4.1%, −0.8%, −0.8%, +0.5%, +6.7%, and +10.9% were observed at the second interface between the same tissues respectively and soft tissue. On average, the build‐up distance was reduced by 0.6 cm, and a dose increase of 62.7% was observed at the exit interface between soft tissue and air of the entire phantom. Conclusion: The greatest changes in dose were observed at interfaces containing lung and bone tissues. Due to the prevalence and proximity of bony anatomy to soft tissues throughout the human body, these results encourage further examination of these tissues with anatomically representative geometries using multiple beam configurations for safe treatment using the MRI‐linac system. NSF GRFP Grant Award #LH‐102SPS

Purpose: To assess MR signal contrast for different ferrous ion compounds used in Fricke‐type gel dosimeters for real‐time dose measurements for MR‐guided radiation therapy applications. Methods: Fricke‐type gel dosimeters were prepared in 4% w/w gelatin prior to irradiation in an integrated 1.5 T MRI and 7 MV linear accelerator system (MR‐Linac). 4 different ferrous ion (Fe2?) compounds (referred to as A, B, C, and D) were investigated for this study. Dosimeter D consisted of ferrous ammonium sulfate (FAS), which is conventionally used for Fricke dosimeters. Approximately half of each cylindrical dosimeter (45 mm diameter, 80 mm length) was irradiated to ∼17 Gy. MR imaging during irradiation was performed with the MR‐Linac using a balanced‐FFE sequence of TR/TE = 5/2.4 ms. An approximate uncertainty of 5% in our dose delivery was anticipated since the MR‐Linac had not yet been fully commissioned. Results: The signal intensities (SI) increased between the un‐irradiated and irradiated regions by approximately 8.6%, 4.4%, 3.2%, and 4.3% after delivery of ∼2.8 Gy for dosimeters A, B, C, and D, respectively. After delivery of ∼17 Gy, the SI had increased by 24.4%, 21.0%, 3.1%, and 22.2% compared to the un‐irradiated regions. The increase in SI with respect to dose was linear for dosimeters A, B, and D with slopes of 0.0164, 0.0251, and 0.0236 Gy−1 (R2 = 0.92, 0.97, and 0.96), respectively. Visually, dosimeter A had the greatest optical contrast from yellow to purple in the irradiated region. Conclusion: This study demonstrated the feasibility of using Fricke‐type dosimeters for real‐time dose measurements with the greatest optical and MR contrast for dosimeter A. We also demonstrated the need to investigate Fe2+ compounds beyond the conventionally utilized FAS compound in order to improve the MR signal contrast in 3D dosimeters used for MR‐guided radiation therapy. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. LH‐ 102SPS.

Purpose: To develop and evaluate novel radiochromic films for quality assurance in radiotherapy dosimetry. Materials and Methods: Novel radiochromic film compositions were formulated using leuco crystal violet (LCV) as a reporting system and tetrabromoethane as a free radical source. The film matrix used consisted of polyurethane polymer mixed with dibutyl phthalate plasticizer (20 wt%). The concentration of the radical initiator was kept constant at 10 wt% and the concentration of the LCV dye varied (1 and 2 wt%). To ensure uniform thickness of the film, its precursors were sandwiched between two pieces of glass separated by a 1 mm gap between during the curing process. The films were cut into pieces and were irradiated with a 6 MV X‐ray beam to selected doses. The change in optical density was measured using a flatbed scanner and a spectrophotometer. Results: The results showed that all film formulations exhibited a linear response with dose and an absorption maximum at ∼ 590 nm. The formulation with 2 wt% LCV was ∼ 30% more sensitive to dose than the formulation with 1 wt% LCV. Both films were very deformable. In addition, the radiochromic response of the film was found to bleach over a short period of time (few weeks) allowing the film to be reused for dose verification measurements. Conclusion: Both film formulations displayed excellent sensitivity and linearity to radiation dose and thus can be used for the 2D dosimetry of clinical megavoltage and kilovoltage X‐ray beams. In addition, the thickness of the film could easily be increased allowing for their potential use as a deformable bolus material. However, thicker films would need more optimization of the manufacturing procedure to ensure consistent material uniformity and sensitivity are recommended.

Purpose: To compare novel radiation reporting systems utilizing ferric ion (Fe3+) reduction versus ferrous ion (Fe2+) oxidation in gelatin matrixes for 3D and 4D (3D+time) MR‐guided radiation therapy dosimetry. Methods: Dosimeters were irradiated using an integrated 1.5T MRI and 7MV linear accelerator (MR‐Linac). Dosimeters were read‐out with both a spectrophotometer and the MRI component of the MR‐Linac immediately after irradiation. Changes in optical density (OD) were measured using a spectrophotometer; changes in MR signal intensity due to the paramagnetic differences in the iron ions were measured using the MR‐Linac in real‐time during irradiation (balanced‐FFE sequences) and immediately after irradiation (T1‐weighted and inversion recovery sequences). Results: Irradiation of Fe3+ reduction dosimeters resulted in a stable red color with an absorbance peak at 512 nm. The change in OD relative to dose exhibited a linear response up to 100 Gy (R2=1.00). T1‐weighted‐MR signal intensity (SI) changed minimally after irradiation with increases of 8.0% for 17 Gy and 9.7% after escalation to 35 Gy compared to the un‐irradiated region. Irradiation of Fe2+ oxidation dosimeters resulted in a stable purple color with absorbance peaks at 440 and 585 nm. The changes in OD, T1‐weighted‐MR SI, and R1 relative to dose exhibited a linear response up to at least 8 Gy (R2=1.00, 0.98, and 0.99) with OD saturation above 40 Gy. The T1‐weighted‐MR SI increased 50.3% for 17 Gy compared to the un‐irradiated region. The change in SI was observed in both 2D+time and 4D (3D+time) acquisitions post‐irradiation and in real‐time during irradiation with a linear increase with respect to dose (R2>0.93). Conclusion: The Fe2+ oxidation‐based system was superior as 4D dosimeters for MR‐guided radiation therapy due to its higher sensitivity in both optical and MR signal readout and feasibility for real‐time 4D dose readout. The Fe3+ reduction system is recommended for high dose applications. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. LH‐102SPS.

Purpose: To demonstrate the capability of 3D radiochromic PRESAGE and Fricke‐type dosimeters to measure the influence of magnetic fields on dose distribution, including the electron return effect (ERE), for MR‐guided radiation therapy applications. Methods: Short cylindrical 3D dosimeters with PRESAGE and Fricke‐type formulations were created in‐house prior to irradiations in a 1.5T/7MV MR‐linac. Each dosimeter was prepared with a concentric cylindrical air cavity with diameters of 1.5 cm and 2.5 cm, and the diameters of the dosimeters were 7.2 cm and 8.8 cm for PRESAGE and Fricke‐type respectively. The dosimeters were irradiated within the bore of the MR‐linac with the flat face of the dosimeters perpendicular to the magnetic field. Dosimeters were irradiated to approximately 9 Gy and 29 Gy to the center of dosimeters with a 15×15 cm2 field. The PRESAGE dosimeter was scanned using an optical‐CT 2 hours post‐irradiation; the Fricke‐type dosimeter was immediately imaged with the MR component of the MR‐linac post‐irradiation. Results: Axial slices of the dose distributions show a clear demonstration of the dose enhancement due to the ERE above the cavity and the region of reduced dose below the cavity. The regions of increased and reduced dose are rotated with respect to the radiation beam axis due to the average directional change of the electrons. Measurements from line profiles show the dose enhanced up to ∼0.5 cm around the cavity by up to a factor of 1.3 and 1.4 for PRESAGE and Fricke‐type dosimeters respectively. Conclusion: PRESAGE and Fricke‐type dosimeters are able to qualitatively measure the ERE with good agreement with previously published simulation and 2D dosimetry demonstrations of the ERE. Further investigation of these 3D dosimeters as promising candidates for quality assurance of MR‐guided radiation therapy systems is encouraged to assess changes in response and measurement accuracy due to the magnetic field.