In recent years, small animal image-guided irradiators have been widely utilized in preclinical studies involving rodent models. However, the dosimetry commissioning of such equipment involving kilovoltage small-field dosimetry has not received as much interest as the megavoltage small-field dosimetry used clinically. To date, a paucity of measured kilovoltage beam data, especially for field sizes less than 3 mm, can be found in the literature. For improvement of rodent treatments in the future, this work aims to provide comprehensive and accurate beam data for the small animal radiation research platform (SARRP, Xstrahl) using EBT3 Gafchromic films and Monte Carlo calculation, with submillimeter resolution and accuracy. This work includes three primary tasks: (1) establish an optimized film measurement protocol for small field dosimetry of kilovoltage photon beam. (2) Acquire dosimetric data including (a) depth dose curves from the surface to 6 cm depth (b) beam profiles, (c) penumbra, (d) cone factors and (e) 2D dose distribution. These tasks were undertaken for a 220 kVp photon beam with five different small field widths and 33 cm source to surface distance (0.5 mm and 1 mm circular fields, 3  ×  3 mm, 5  ×  5 mm, 10  ×  10 mm square fields). Beam data was measured with EBT3 films. (3) Provide comparative dosimetry for film measurements, Monte Carlo calculations, and the dose calculations performed with the SARRP treatment planning system, Muriplan. For the majority of parameters, film measurement agreed with Monte Carlo simulation within 1%. There were, however, discrepancies between measured beam data and Muriplan treatment planning data. Specifically, for PDD, Muriplan underestimates the dose for field sizes of 0.5 mm and 1 mm. For beam profiles comparisons, the calculation from Muriplan predicts a smaller lateral distance between the 50% isodose lines compared to film measurement. There is a difference of 0.18, 0.72, 0.6 mm between Muriplan and film for field sizes of 3, 5, 10 mm, respectively. This work demonstrates that accurate and precise kilovoltage small-field dosimetry can be conducted using EBT3 Gafchromic film with an optimized protocol. In addition, discrepancies between measured beam data and Muriplan were identified.

Purpose: Cherenkov light emission has been shown to correlate with ionizing radiation dose delivery in solid tissue. An important clinical application of Cherenkov light is the real-time verification of radiation treatment delivery in vivo. To test the feasibility of treatment field verification, Cherenkov light images were acquired concurrent with radiation beam delivery to standard and anthropomorphic phantoms. Specifically, we tested two clinical treatment scenarios: (a) Observation of field overlaps or gaps in matched 3D fields and (b) Patient positioning shifts during intensity modulated radiation therapy (IMRT) field delivery. Further development of this technique would allow real-time detection of treatment delivery errors on the order of millimeters so that patient safety and treatment quality can be improved. Methods: Cherenkov light emission was captured using a PI-MAX4 intensified charge coupled device (ICCD) system (Princeton Instruments). All radiation delivery was performed using a Varian Trilogy linear accelerator (linac) operated at 6 MV or 18 MV for photon and 6 MeV or 16 MeV for electron studies. Field matching studies were conducted with photon and electron beams at gantry angles of 0°, 15°, and 45°. For each modality and gantry angle, a total of three data sets were acquired. Overlap and gap distances of 0, 2, 5, and 10 mm were tested and delivered to solid phantom material of 30 9 30 9 5 cm 3 . Phantom materials used were white plastic water and brown solid water. Tests were additionally performed on an anthropomorphic phantom with an irregular surface. Positioning shift studies were performed using IMRT fields delivered to a thoracic anthropomorphic phantom. For thoracic phantom measurements, the camera was placed laterally to observe the entire right side of the phantom. Fields were delivered with known translational patient positioning shifts in four directions. Changes in the Cherenkov fluence were evaluated through the generation of difference maps from unshifted Cherenkov images. All images were evaluated using ImageJ, Python, and MATLAB software packages. Results: For matched fields, Cherenkov images were able to quantitate matched field separations with discrepancies between 2 and 4 mm, depending on gantry angle and beam energy or modality. For all photon and electron beams delivered at a gantry angle of 0°, image analysis indicated average discrepancies of less than 2 mm for all field gaps and overlaps, with 83% of matched fields exhibiting discrepancies less than 1 mm. Beams delivered obliquely to the phantom surface exhibited average discrepancies as high as 4 mm for electron beams delivered at large oblique angles. Finally, for IMRT field delivery, vertical and lateral patient positioning shifts of 2 mm were detected in some cases, indicating the potential detectability threshold of using this technique alone. Conclusions: Our study indicates that Cherenkov imaging can be used to support and bolster current treatment delivery verification techniques, improving our ability to recog...

Purpose/Objective To establish a novel preclinical model for stereotactic radiosurgery with combined mouse-like phantom quality assurance in the setting of brain metastases. Material Methods C57B6 mice underwent intracranial injection of B16-F10 melanoma cells. T1-post contrast MRI was performed on Day 11 after injection. The MRI images were fused with cone beam computed tomography (CBCT) images using the SARRP. Gross tumor volume (GTV) was contoured using the MRI. A single sagittal arc utilizing the 3×3 mm2 collimator was used to deliver 18 Gy prescribed to the isocenter. MRI was performed 7 days after radiation treatment and the dose delivered to the mice was confirmed using two mouse-like anthropomorphic phantoms: one in the axial and the other in the sagittal orientation. SARRP output was measured using a PTW Farmer type ionization chamber as per AAPM TG-61 and the H-D curve was generated up to a max dose of 30 Gy. Irradiated films were analyzed based on optical density distribution and H-D curve. Results The tumor volume at Day 11, before intervention, was 2.48±1.37 mm3 in the no SRS arm versus 3.75±1.19 mm3 in the SRS arm (NS). In the SRS arm, GTV Dose max (Dmax) and mean dose were 2048±207 and 1785±14 cGy. Using the mouse-like phantoms, the radiochromic film showed close precision as compared with projected isodose lines with a Dmax of 1903.4 and 1972.7 cGy, the axial and sagittal phantom respectively. Tumor volume 7 days post-treatment was 7.34±8.24 mm3 in the SRS arm and 60.20±40.4 mm3 in the no SRS arm (p=0.009). No mice in the control group survived more than 22 days after implantation with a median overall survival (mOS) of 19 days. mOS was not reached in the SRS group with one death noted. Conclusion Single fraction SRS of 18 Gy delivered in a single arc can be delivered accurately with MRI T1-post contrast based treatment planning. The mouse-like phantom allows for verification of dose delivery and accuracy.

This study demonstrated that 3D PRESAGE dosimeters can be used for verification of precise targeting for the SARRP. This 3D dosimetry system can be utilized to obtain information on both geometric and dosimetric properties, as well as acquire beam data parameters for the purpose of commissioning image-guided small animal irradiator systems.