A high-throughput focused collimator for OAR-sparing preclinical proton FLASH studies: commissioning and validation.

Objective. To fabricate and validate a novel focused collimator designed to spare normal tissue in a murine hemithoracic irradiation model using 250 MeV protons delivered at ultra-high dose rates (UHDRs) for preclinical FLASH radiation therapy (FLASH-RT) studies.Approach. A brass collimator was developed to shape 250 MeV UHDR protons from our Varian ProBeam. Six 13 mm apertures, of equivalent size to kV x-ray fields historically used to perform hemithorax irradiations, were precisely machined to match beam divergence, allowing concurrent hemithoracic irradiation of six mice while sparing the contralateral lung and abdominal organs. The collimated field profiles were characterized by film dosimetry, and a radiation survey of neutron activation was performed to ensure the safety of staff positioning animals.Main results. The brass collimator produced 1.2 mm penumbrae radiation fields comparable to kV x-rays used in preclinical studies. The penumbrae in the six apertures are similar, with full-width half-maxima of 13.3 mm and 13.5 mm for the central and peripheral apertures, respectively. The collimator delivered a similar dose at an average rate of 52 Gy s-1for all apertures. While neutron activation produces a high (0.2 mSv h-1) initial ambient equivalent dose rate, a parallel work-flow in which imaging and setup are performed without the collimator ensures safety to staff.Significance. Scanned protons have the greatest potential for future translation of FLASH-RT in clinical treatments due to their ability to treat deep-seated tumors with high conformality. However, the Gaussian distribution of dose in proton spots produces wider lateral penumbrae compared to other modalities. This presents a challenge in small animal pre-clinical studies, where millimeter-scale penumbrae are required to precisely target the intended volume. Offering high-throughput irradiation of mice with sharp penumbrae, our novel collimator-based platform serves as an important benchmark for enabling large-scale, cost-effective radiobiological studies of the FLASH effect in murine models.

Clinical Implementation and Dosimetric Evaluation of a Robust Proton Lattice Planning Strategy Using Primary and Robust Complementary Beams.

PURPOSE: Pencil-beam scanning proton therapy has been considered a potential modality for the 3D form of spatially fractionated radiation therapy called lattice therapy. However, few practical solutions have been introduced in the clinic. Existing limitations include degradation in plan quality and robustness when using single-field versus multifield lattice plans, respectively. We propose a practical and robust proton lattice (RPL) planning method using multifield and evaluate its dosimetric characteristics compared to clinically acceptable photon lattice plans. METHODS AND MATERIALS: Seven cases previously treated with photon lattice therapy were used to evaluate a novel RPL planning technique using 2-orthogonal beams: a primary beam (PB) and a robust complementary beam (RCB) that deliver 67% and 33%, respectively, of the prescribed dose to vertices inside the gross target volume (GTV). Only RCB is robustly optimized for setup and range uncertainties. The number and volume of vertices, peak-to-valley dose ratios (PVDRs), and volume of low dose to GTV of proton and photon plans were compared. The RPL technique was then used in the treatment of 2 patients and their dosimetric parameters were reported. RESULTS: The RPL strategy was able to achieve the clinical planning goals. Compared to previously treated photon plans, the average number of vertices increased by 30%, the average vertex volume by 49% (18.2 ± 25.9 cc vs 12.2 ± 14.5 cc, P = .21), and higher PVDR (10.5 ± 4.8 vs 2.5 ± 0.9, P < .005) was achieved. In addition, RPL plans show more conformal dose with less low dose to GTV (V30%, 60.9% ± 7.2% vs 81.6% ± 13.9% and V10%, 88.3% ± 4.5% vs 98.6% ± 3.6% [P < .01]). The RPL plan for 2 treated patients showed PVDRs of 4.61 and 14.85 with vertices-to-GTV ratios of 1.52% and 1.30%, respectively. CONCLUSIONS: A novel RPL planning strategy using a pair of orthogonal beams was developed and successfully translated to the clinic. The proposed method can generate better quality plans, a higher number of vertices, and higher PVDRs than currently used photon lattice plans.

Technical note: A small animal irradiation platform for investigating the dependence of the FLASH effect on electron beam parameters.

BACKGROUND: The recent rediscovery of the FLASH effect, a normal tissue sparing phenomenon observed in ultra-high dose rate (UHDR) irradiations, has instigated a surge of research endeavors aiming to close the gap between experimental observation and clinical treatment. However, the dependences of the FLASH effect and its underpinning mechanisms on beam parameters are not well known, and large-scale in vivo studies using murine models of human cancer are needed for these investigations. PURPOSE: To commission a high-throughput, variable dose rate platform providing uniform electron fields (≥15 cm diameter) at conventional (CONV) and UHDRs for in vivo investigations of the FLASH effect and its dependences on pulsed electron beam parameters. METHODS: A murine whole-thoracic lung irradiation (WTLI) platform was constructed using a 1.3 cm thick Cerrobend collimator forming a 15 × 1.6 cm2 slit. Control of dose and dose rate were realized by adjusting the number of monitor units and couch vertical position, respectively. Achievable doses and dose rates were investigated using Gafchromic EBT-XD film at 1 cm depth in solid water and lung-density phantoms. Percent depth dose (PDD) and dose profiles at CONV and various UHDRs were also measured at depths from 0 to 2 cm. A radiation survey was performed to assess radioactivation of the Cerrobend collimator by the UHDR electron beam in comparison to a precision-machined copper alternative. RESULTS: This platform allows for the simultaneous thoracic irradiation of at least three mice. A linear relationship between dose and number of monitor units at a given UHDR was established to guide the selection of dose, and an inverse-square relationship between dose rate and source distance was established to guide the selection of dose rate between 20 and 120 Gy·s-1 . At depths of 0.5 to 1.5 cm, the depth range relevant to murine lung irradiation, measured PDDs varied within ±1.5%. Similar lateral dose profiles were observed at CONV and UHDRs with the dose penumbrae widening from 0.3 mm at 0 cm depth to 5.1 mm at 2.0 cm. The presence of lung-density plastic slabs had minimal effect on dose distributions as compared to measurements made with only solid water slabs. Instantaneous dose rate measurements of the activated copper collimator were up to two orders of magnitude higher than that of the Cerrobend collimator. CONCLUSIONS: A high-throughput, variable dose rate platform has been developed and commissioned for murine WTLI electron FLASH radiotherapy. The wide field of our UHDR-enabled linac allows for the simultaneous WTLI of at least three mice, and for the average dose rate to be modified by changing the source distance, without affecting dose distribution. The platform exhibits uniform, and comparable dose distributions at CONV and UHDRs up to 120 Gy·s-1 , owing to matched and flattened 16 MeV CONV and UHDR electron beams. Considering radioactivation and exposure to staff, Cerrobend collimators are recommended above copper alternatives for electron FLASH research. This platform enables high-throughput animal irradiation, which is preferred for experiments using a large number of animals, which are required to effectively determine UHDR treatment efficacies.

Minimum reporting standards should be expected for preclinical radiobiology irradiators and dosimetry in the published literature.

The cornerstones of science advancement are rigor in performing scientific research, reproducibility of research findings and unbiased reporting of design and results of the experiments. For radiation research, this requires rigor in describing experimental details as well as the irradiation protocols for accurate, precise and reproducible dosimetry. Most institutions conducting radiation biology research in in vitro or animal models do not have describe experimental irradiation protocols in sufficient details to allow for balanced review of their publication nor for other investigators to replicate published experiments. The need to increase and improve dosimetry standards, traceability to National Institute of Standards and Technology (NIST) standard beamlines, and to provide dosimetry harmonization within the radiation biology community has been noted for over a decade both within the United States and France. To address this requirement subject matter experts have outlined minimum reporting standards that should be included in published literature for preclinical irradiators and dosimetry.

Is noncoplanar plan more robust to inter-fractional variations than coplanar plan in treating bilateral HN tumors with pencil-beam scanning proton beams?

PURPOSE: Noncoplanar plans (NCPs) are commonly used for proton treatment of bilateral head and neck (HN) malignancies. NCP requires additional verification setup imaging between beams to correct residual errors of robotic couch motion, which increases imaging dose and total treatment time. This study compared the quality and robustness of NCPs with those of coplanar plans (CPs). METHODS AND MATERIALS: Under an IRB-approved study, CPs were created retrospectively for 10 bilateral HN patients previously treated with NCPs maintaining identical beam geometry of the original plan but excluding couch rotations. Plan robustness to the inter-fractional variation (IV) of both plans was evaluated through the Dose Volume Histograms (DVH) of weekly quality assurance CT (QACT) sets (39 total). In addition, delivery efficiency for both plans was compared using total treatment time (TTT) and beam-on time (BOT). RESULTS: No significant differences in plan quality were observed in terms of clinical target volume (CTV) coverage (D95) or organ-at-risk (OAR) doses (p > 0.4 for all CTVs and OARs). No significant advantage of NCPs in the robustness to IV was found over CP, either. Changes in D95 of QA plans showed a linear correlation (slope = 1.006, R2  > 0.99) between NCP and CP for three CTV data points (CTV1, CTV2, and CTV3) in each QA plan (117 data points for 39 QA plans). NCPs showed significantly higher beam delivery time than CPs for TTT (539 ± 50 vs. 897 ± 142 s; p < 0.001); however, no significant differences were observed for BOT. CONCLUSION: NCPs are not more robust to IV than CPs when treating bilateral HN tumors with pencil-beam scanning proton beams. CPs showed plan quality and robustness similar to NCPs while reduced treatment time (∼6 min). This suggests that CPs may be a more efficient planning technique for bilateral HN cancer proton therapy.

A C57L/J Mouse Model of the Delayed Effects of Acute Radiation Exposure in the Context of Evolving Multi-Organ Dysfunction and Failure after Total-Body Irradiation with 2.5% Bone Marrow Sparing.

The objective of the current study was to establish a mouse model of acute radiation syndrome (ARS) after total-body irradiation with 2.5% bone marrow sparing (TBI/BM2.5) that progressed to the delayed effects of acute radiation exposure, specifically pneumonitis and/or pulmonary fibrosis (DEARE-lung), in animals surviving longer than 60 days. Two hundred age and sex matched C57L/J mice were assigned to one of six arms to receive a dose of 9.5 to 13.25 Gy of 320 kV X-ray TBI/BM2.5. A sham-irradiated cohort was included as an age- and sex-matched control. Blood was sampled from the facial vein prior to irradiation and on days 5, 10, 15, 20, 25, and 30 postirradiation for hematology. Respiratory function was monitored at regular intervals throughout the in-life phase. Animals with respiratory dysfunction were administered a single 12-day tapered regimen of dexamethasone, allometrically scaled from a similar regimen in the non-human primate. All animals were monitored daily for up to 224 days postirradiation for signs of organ dysfunction and morbidity/mortality. At euthanasia due to criteria or at the study endpoint, wet lung weights were recorded, and blood sampled for hematology and serum chemistry. The left lung, heart, spleen, small and large intestine, and kidneys were processed for histopathology. A dose-response curve with the estimated lethal dose for 10-99% of animals with 95% confidence intervals was established. The median survival time was significantly prolonged in males as compared to females across the 10.25 to 12.5 Gy dose range. Animal sex played a significant role in overall survival, with males 50% less likely to expire prior to the study endpoint compared to females. All animals developed pancytopenia within the first one- to two-weeks after TBI/BM2.5 followed by a progressive recovery through day 30. Fourteen percent of animals expired during the first 30-days postirradiation due to ARS (e.g., myelosuppression, gastrointestinal tissue abnormalities), with most deaths occurring prior to day 15. Microscopic findings show the presence of radiation pneumonitis as early as day 57. At time points later than day 70, pneumonitis was consistently present in the lungs of mice and the severity was comparable across radiation dose arms. Pulmonary fibrosis was first noted at day 64 but was not consistently present and stable in severity until after day 70. Fibrosis was comparable across radiation dose arms. In conclusion, this study established a multiple organ injury mouse model that progresses through the ARS phase to DEARE-lung, characterized by respiratory dysfunction, and microscopic abnormalities consistent with radiation pneumonitis/fibrosis. The model provides a platform for future development of medical countermeasures for approval and licensure by the U.S. Food and Drug Administration under the animal rule regulatory pathway.

Technical note: Characterization and practical applications of a novel plastic scintillator for online dosimetry for an ultrahigh dose rate (FLASH).

PURPOSE: Although flash radiation therapy (FLASH-RT) is a promising novel technique that has the potential to achieve a better therapeutic ratio between tumor control and normal tissue complications, the ultrahigh pulsed dose rates (UHPDR) mean that experimental dosimetry is very challenging. There is a need for real-time dosimeters in the development and implementation of FLASH-RT. In this work, we characterize a novel plastic scintillator capable of temporal resolution short enough (2.5 ms) to resolve individual pulses. METHODS: We characterized a novel plastic dosimeter for use in a linac converter to deliver 16-MeV electrons at 100-Gy/s UHPDR average dose rates. The linearity and reproducibility were established by comparing relative measurements with a pinpoint ionization chamber placed at 10-cm water-equivalent depth where the electrometer is not saturated by the high dose per pulse. The accuracy was established by comparing the plastic scintillator dose measurements with EBT-XD Gafchromic radiochromic films, the current reference dosimeter for UHPDR. Finally, the plastic scintillator was compared against EBT-XD films for online dosimetry of two in vitro experiments performed at UHPDR. RESULTS: Relative ion chamber measurements were linear with plastic scintillator response within ≤1% over 4-20 Gy and pulse frequencies (18-180 Hz). When characterized under reference conditions with NIST-traceability, the plastic scintillator maintained its dose response under UHPDR conditions and agreed with EBT-XD film dose measurements within 4% under reference conditions and 6% for experimental online dosimetry. CONCLUSION: The plastic scintillator shows a linear and reproducible response and is able to accurately measure the radiation absorbed dose delivered by 16-MeV electrons at UHPDR. The dose is measured accurately in real time with a greater level of precision than that achieved with a radiochromic film.

Interspecies Comparison and Radiation Effect on Pharmacokinetics of BIO 300, a Nanosuspension of Genistein, after Different Routes of Administration in Mice and Non-Human Primates.

BIO 300, a suspension of synthetic genistein nanoparticles, is being developed for mitigating the delayed effects of acute radiation exposure (DEARE). The purpose of the current study was to characterize the pharmacokinetic (PK) profile of BIO 300 administered as an oral or parenteral formulation 24 h after sham-irradiation, total-body irradiation (TBI) with 2.5-5.0% bone marrow sparing (TBI/BMx), or in nonirradiated sex-matched C57BL/6J mice and non-human primates (NHP). C57BL/6J mice were randomized to the following arms in two consecutive studies: sham-TBI [400 mg/kg, oral gavage (OG)], TBI/BM2.5 (400 mg/kg, OG), sham-TBI [200 mg/kg, subcutaneous (SC) injection], TBI/BM2.5 (200 mg/kg, SC), sham-TBI (100 mg/kg, SC), or nonirradiated [200 mg/kg, intramuscular (IM) injection]. The PK profile was also established in NHP exposed to TBI/BM5.0 (100 mg/kg, BID, OG). Genistein-aglycone serum concentrations were measured in all groups using a validated liquid chromatography-tandem mass spectrometry (LC-MS/MS) assay. The PK profile demonstrates 11% and 19% reductions in Cmax and AUC0-inf, respectively, among mice administered 400 mg/kg, OG, after TBI/BM2.5 compared to the sham-TBI control arm. Administration of 200 mg/kg SC in mice exposed to TBI/BM2.5 showed a 53% increase in AUC0-inf but a 28% reduction in Cmax compared to the sham-TBI mice. The relative bioavailability of the OG route compared to the SC and IM routes in mice was 9% and 7%, respectively. After the OG route, the dose-normalized AUC0-inf was 13.37 (ng.h/mL)/(mg/kg) in TBI/BM2.5 mice compared to 6.95 (ng.h/mL)/(mg/kg) in TBI/BM5.0 NHPs. Linear regression of apparent clearances and weights of mice and NHPs yielded an allometric coefficient of 1.06. Based on these data, the effect of TBI/BMx on BIO 300 PK is considered minimal. Future studies should use SC and IM routes to maximize drug exposure when administered postirradiation. The allometric coefficient is useful in predicting therapeutic drug dose regimens across species for drug approval under the FDA animal rule.

Quantifying the DNA-damaging Effects of FLASH Irradiation With Plasmid DNA.

PURPOSE: To investigate a plasmid DNA nicking assay approach for isolating and quantifying the DNA-damaging effects of ultrahigh-dose-rate (ie, FLASH) irradiation relative to conventional dose-rate irradiation. METHODS AND MATERIALS: We constructed and irradiated phantoms containing plasmid DNA to nominal doses of 20 Gy and 30 Gy using 16 MeV electrons at conventional (0.167 Gy/s) and FLASH (46.6 Gy/s and 93.2 Gy/s) dose rates. We delivered conventional dose rates using a standard clinical Varian iX linear accelerator and FLASH dose rates (FDRs) using a modified Varian 21EX C-series linear accelerator. We ran the irradiated DNA and controls (0 Gy) through an agarose gel electrophoresis procedure that sorted and localized the DNA into bands associated with single strand breaks (SSBs), double strand breaks (DSBs), and undamaged DNA. We quantitatively analyzed the gel images to compute the relative yields of SSBs and DSBs and applied a mathematical model of plasmid DNA damage as a function of dose to compute the relative biological effectiveness (RBE) of SSB and DSB (RBESSB and RBEDSB) damage for a given endpoint and FDR. RESULTS: Both RBESSB and RBEDSB were less than unity with the FDR irradiations, indicating FLASH sparing. With regard to the more deleterious DNA DSB damage, the DSB RBEs of FLASH beams at dose rates of 46.6 Gy/s and 93.2 Gy/s relative to the conventional 16 MeV beam dose rate were 0.54 ± 0.15 and 0.55 ± 0.17, respectively. CONCLUSIONS: This study demonstrated the feasibility of using a DNA-based phantom to isolate and assess the FLASH sparing effect on DNA. We also found that FLASH irradiation causes less damage to DNA compared with a conventional dose rate. This result supports the notion that the protective effect of FLASH irradiation occurs at least partially via fundamental biochemical processes.

Dose calculations for preclinical radiobiology experiments conducted with single-field cabinet irradiators.

PURPOSE: To provide percentage depth dose (PDD) data along the central axis for dosimetry calculations in small-animal radiation biology experiments performed in cabinet irradiators. The PDDs are provided as a function of source-to-surface distance (SSD), field size, and animal size. METHODS: The X-ray tube designs for four biological cabinet irradiators, the RS2000, RT250, MultiRad350, and XRAD320, were simulated using the BEAMnrc Monte Carlo code to generate 160, 200, 250, and 320 kVp photon beams, respectively. The 320 kVp beam was simulated with two filtrations: a soft F1 aluminium filter and a hard F2 thoraeus filter made of aluminium, tin, and copper. Beams were collimated into circular fields with diameters of 0.5-10 cm at SSDs of 10-60 cm. Monte Carlo dose calculations in 1-5-cm diameter homogeneous (soft tissue) small-animal phantoms as well as in heterogeneous phantoms with 3-mm diameter cylindrical lung and bone inserts (rib and cortical bone) were performed using DOSXYZnrc. The calculated depth doses in three test-cases were estimated by applying SSD, field size, and animal size correction factors to a reference case (40-cm SSD, 1-cm field, and 5-cm animal size), and these results were compared with the specifically simulated (i.e., expected) doses to assess the accuracy of this method. Dosimetry for two test-case scenarios of 160 and 250 kVp beams (representative of end-user beam qualities) was also performed, whereby the simulated PDDs at two different depths were compared with the results based on the interpolation from reference data. RESULTS: The depth doses for three test-cases calculated at 200, 320 kVp F1, and 320 kVp F2 with half value layers (HVLs) ranging from ∼0.6 to 3.6 mm Cu, agreed well with the expected doses, yielding dose differences of 1.2%, 0.1%, and 1.0%, respectively. The two end-user test-cases for 160 and 250 kVp beams with respective HVLs of ∼0.8 and 1.8 mm Cu yielded dose differences of 1.4% and 3.2% between the simulated and the interpolated PDDs. The dose increase at the bone-tissue proximal interface ranged from 1.2 to 2.5 times the dose in soft tissue for rib and 1.3 to 3.7 times for cortical bone. The dose drop-off at 1-cm depth beyond the bone ranged from 1.3% to 6.0% for rib and 3.2% to 11.7% for cortical bone. No drastic dose perturbations occurred in the presence of lung, with lung-tissue interface dose of >99% of soft tissue dose and <3% dose increase at 1-cm depth beyond lung. CONCLUSIONS: The developed dose estimation method can be used to translate the measured dose at a point to dose at any depth in small-animal phantoms, making it feasible for preclinical calculation of dose distributions in animals irradiated with cabinet-style irradiators. The dosimetric impact of bone must be accurately quantified as dramatic dose perturbations at and beyond the bone interfaces can occur due to the relative importance of the photoelectric effect at kilovoltage energies. These results will help improve dosimetric accuracy in preclinical experiments.

Pancreatic cancer derived 3D organoids as a clinical tool to evaluate the treatment response.

Background and purpose: Pancreatic cancer (PC) is the fourth leading cause of cancer death in both men and women. The standard of care for patients with locally advanced PC of chemotherapy, stereotactic radiotherapy (RT), or chemo-radiation-therapy has shown highly variable and limited success rates. However, three-dimensional (3D) Pancreatic tumor organoids (PTOs) have shown promise to study tumor response to drugs, and emerging treatments under in vitro conditions. We investigated the potential for using 3D organoids to evaluate the precise radiation and drug dose responses of in vivo PC tumors. Methods: PTOs were created from mouse pancreatic tumor tissues, and their microenvironment was compared to that of in vivo tumors using immunohistochemical and immunofluorescence staining. The organoids and in vivo PC tumors were treated with fractionated X-ray RT, 3-bromopyruvate (3BP) anti-tumor drug, and combination of 3BP + fractionated RT. Results: Pancreatic tumor organoids (PTOs) exhibited a similar fibrotic microenvironment and molecular response (as seen by apoptosis biomarker expression) as in vivo tumors. Untreated tumor organoids and in vivo tumor both exhibited proliferative growth of 6 folds the original size after 10 days, whereas no growth was seen for organoids and in vivo tumors treated with 8 (Gray) Gy of fractionated RT. Tumor organoids showed reduced growth rates of 3.2x and 1.8x when treated with 4 and 6 Gy fractionated RT, respectively. Interestingly, combination of 100 µM of 3BP + 4 Gy of RT showed pronounced growth inhibition as compared to 3-BP alone or 4 Gy of radiation alone. Further, positive identification of SOX2, SOX10 and TGFß indicated presence of cancer stem cells in tumor organoids which might have some role in resistance to therapies in pancreatic cancer. Conclusions: PTOs produced a similar microenvironment and exhibited similar growth characteristics as in vivo tumors following treatment, indicating their potential for predicting in vivo tumor sensitivity and response to RT and combined chemo-RT treatments.

DUOX2, a New Biomarker for Disseminated Gastric Cancer's Response to Low Dose Radiation in Mice.

Treatment options are rather limited for gastrointestinal cancer patients whose disease has disseminated into the intra-abdominal cavity. Here, we designed pre-clinical studies to evaluate the potential application of chemopotentiation by Low Dose Fractionated Radiation Therapy (LDFRT) for disseminated gastric cancer and evaluate the role of a likely biomarker, Dual Oxidase 2 (DUOX2). Nude mice were injected orthotopically with human gastric cancer cells expressing endogenous or reduced levels of DUOX2 and randomly assigned to four treatment groups: 1; vehicle alone, 2; modified regimen of docetaxel, cisplatin and 5'-fluorouracil (mDCF) for three consecutive days, 3; Low Dose- Whole Abdomen Radiation Therapy (LD-WART) (5 fractions of 0.15 Gy in three days), 4; mDCF and LD-WART. The combined regimen increased the odds of preventing cancer dissemination (mDCF + LD-WART OR = 4.16; 80% CI = 1.0, 17.29) in the DUOX2 positive tumors, while tumors expressing lower DUOX2 levels were more responsive to mDCF alone with no added benefit from LD-WART. The molecular mechanisms underlying DUOX2 effects in response to the combined regimen include NF-κB upregulation. These data are particularly important since our study indicates that about 33% of human stomach adenocarcinoma do not express DUOX2. DUOX2 thus seems a likely biomarker for potential clinical application of chemopotentiation by LD-WART.

Radiation shielding and safety implications following linac conversion to an electron FLASH-RT unit.

PURPOSE: Due to their finite range, electrons are typically ignored when calculating shielding requirements in megavoltage energy linear accelerator vaults. However, the assumption that 16 MeV electrons need not be considered does not hold when operated at FLASH-RT dose rates (~200× clinical dose rate), where dose rate from bremsstrahlung photons is an order of magnitude higher than that from an 18 MV beam for which shielding was designed. We investigate the shielding and radiation protection impact of converting a Varian 21EX linac to FLASH-RT dose rates. METHODS: We performed a radiation survey in all occupied areas using a Fluke Biomedical Inovision 451P survey meter and a Wide Energy Neutron Detection Instrument (Wendi)-2 FHT 762 neutron detector. The dose rate from activated linac components following a 1.8-min FLASH-RT delivery was also measured. RESULTS: When operated at a gantry angle of 180° such as during biology experiments, the 16 MeV FLASH-RT electrons deliver ~10 µSv/h in the controlled areas and 780 µSv/h in the uncontrolled areas, which is above the 20 µSv in any 1-h USNRC limit. However, to exceed 20 µSv, the unit must be operated continuously for 92 s, which corresponds in this bunker and FLASH-RT beam to a 3180 Gy workload at isocenter, which would be unfeasible to deliver within that timeframe due to experimental logistics. While beam steering and dosimetry activities can require workloads of that magnitude, during these activities, the gantry is positioned at 0° and the dose rate in the uncontrolled area becomes undetectable. Likewise, neutron activation of linac components can reach 25 µSv/h near the isocenter following FLASH-RT delivery, but dissipates within minutes, and total doses within an hour are below 20 µSv. CONCLUSION: Bremsstrahlung photons created by a 16 MeV FLASH-RT electron beam resulted in consequential dose rates in controlled and uncontrolled areas, and from activated linac components in the vault. While our linac vault shielding proved sufficient, other investigators would be prudent to confirm the adequacy of their radiation safety program, particularly if operating in vaults designed for 6 MV.

Assessing the Need for Adjusted Organ-at-Risk Planning Goals for Patients Undergoing Adjuvant Radiation Therapy for Locally Advanced Breast Cancer with Proton Radiation.

PURPOSE: Locally advanced breast cancer requires surgical management via lumpectomy or mastectomy with or without systemic therapy followed by chest wall or breast (CW) and comprehensive nodal irradiation (CNI). Radiation (RT) dose constraints for the heart and ipsilateral lung have been developed based on photon RT. Proton therapy (PBT) can deliver significantly lower doses of RT to these organs-at-risk (OARs) and may warrant adjustments to OAR planning goals. METHODS AND MATERIALS: The RT plans of consecutive patients undergoing adjuvant CW-CNI RT with PBT within a single center were reviewed. A inital treatment volume, comprised of CW/intact breast + CNI (CTV_init) structure, including the CW and CNI but excluding any boost plans was analyzed. Frequency distributions were generated based on doses received by the heart, lungs, and esophagus for validated dosimetric parameters. Frequency distributions were generated and then stratified by laterality and compared using the Kruskal-Wallis H test. The 75th, 85th, and 95th percentiles for each dosimetric parameter were calculated, overall and by laterality. The 75th percentile (Q3), was used as a suggested primary goal, and the 95th percentile was used as a suggested secondary goal. RESULTS: One hundred and seventy-two plans were analyzed. Forty-nine plans were right-sided, 107 were left-sided, and 16 were bilateral. The overall Q3 of the mean and V25 of the heart were 1.5 Gy and 1.7%, respectively. The mean and V25 to the heart differed significantly by laterality. Pulmonary values were similar to current recommendations. For all lateralites, the median volume of the esophagus receiving 70% prescription dose was ≤1 cm3. CONCLUSIONS: We present the first dosimetric study providing complete OAR dose-volume histograms data for patients undergoing adjuvant pencil-beam scanning-PBT for locally advanced breast cancer, with detailed information on central tendencies, ranges and distributions of data. We have provided suggested planning goals and metrics for the lungs, heart, and esophagus; the latter 2 differing significantly from current Quantitative Analysis of Normal Tissue Effects in the Clinic (QUANTEC) constraints and classical photon goals.

Characterization of the hemorrhagic syndrome in the New Zealand white rabbit model following total body irradiation.

PURPOSE: The hemorrhagic syndrome is a major cause of morbidity and mortality associated with the acute radiation syndrome (ARS). We previously characterized the dose-response relationship for total body irradiation (TBI)-induced ARS in the New Zealand White (NZW) rabbit. Thrombocytopenia, hemorrhage, and anemia were strongly associated with morbidity/mortality during the first three weeks post-TBI. The objective of the current study was to further characterize the natural history of thrombocytopenia, hemostatic dysfunction and hemorrhage in the rabbit model at a TBI dose range to induce ARS. METHODS: Fifty male NZW rabbits were randomized to receive 7.0 or 7.5 Gy of 6 MV-derived TBI. Sham-irradiated controls (n = 6) were included as a comparator. Animals were treated with minimal supportive care including pain medication, antibiotics, antipyretics for temperature >104.8 °F, and fluids for signs of dehydration. Animals were culled at pre-determined timepoints post-TBI, or for signs of imminent mortality based on pre-defined euthanasia criteria. Hematology parameters, serum chemistry, viscoelasticity of whole blood, coagulation tests, and coagulation factor activities were measured. A gross exam of vital organs was performed at necropsy. RESULTS: Findings in this study include severe neutropenia during the first week post-TBI followed by thrombocytopenia and severe acute anemia with petechial hemorrhages of the skin and hemorrhage of the vital organs during the second to third weeks post-TBI. Abnormalities in whole blood viscoelastometry were observed concurrent with thrombocytopenia and hemorrhage. Antithrombin activity was significantly elevated in animals after exposure to 7.5 Gy, but not 7.0 Gy TBI. CONCLUSIONS: The hemorrhagic syndrome in the rabbit model of TBI recapitulates the pathogenesis described in humans following accidental or deliberate exposures. The rabbit may present an alternative to the rodent model as a small animal species for characterization of the full spectrum of multiorgan injury following TBI and early testing of promising medical countermeasures.

Use of CT simulation and 3-D radiation therapy treatment planning system to develop and validate a total-body irradiation technique for the New Zealand White rabbit.

PURPOSE: Well-controlled ionizing radiation injury animal models for testing medical countermeasure efficacy require robust radiation physics and dosimetry to ensure accuracy of dose-delivery and reproducibility of the radiation dose-response relationship. The objective of this study was to establish a simple, convenient, robust and accurate technique for validating total body irradiation (TBI) exposure of the New Zealand White rabbit. METHODS: We use radiotherapy techniques such as computed tomography simulation and a 3 D-conformal radiation therapy treatment planning system (TPS) on three animals to comprehensively design and preplan a TBI technique for rabbits. We evaluate the requirement for bolus, treatment geometry, bilateral vs anterior-posterior treatment delivery, the agreement between monitor units calculated using the TPS vs a traditional hand calculation to the mid-plane, and resulting individual organ doses. RESULTS: The optimal technique irradiates animals on the left-decubitus position using two isocentric bilateral parallel-opposed 6 MV x-ray beams. Placement of a 5 mm bolus and 8.5 mm beam spoiler was shown to increase the dose to within ≤5 mm of the surface, improving dose homogeneity throughout the body of the rabbit. A simple hand calculation formalism, dependent only on mid-abdominal separation, could be used to calculate the number of monitor units (MUs) required to accurately deliver the prescribed dose to the animal. For the representative animal, the total body volume receiving > 95% of the dose, V95% > 99%, V100% > 95%, and V107% < 20%. The area of the body receiving >107% of the prescribed dose was mainly within the limbs, head, and around the lungs of the animal, where the smaller animal width reduces the x-ray attenuation. Individual organs were contoured by an experienced dosimetrist, and each received doses within 95-107% of the intended dose, with mean values ∼104%. Only the bronchus showed a maximal dose >107% (113%) due to the decreased attenuation of the lungs. To validate the technique, twenty animals were irradiated with four optically-stimulated luminescence dosimeters (OSLDs) placed on the surface of each animal (two on each side in the center of the radiation beam). The average dose over all animals was within <0.1% from intended values, with no animal receiving an average dose more than ±3.1% from prescription. CONCLUSION: The TBI technique developed in this pilot study was successfully used to establish the dose-response relationship for 45-day lethality across the dose-range to induce the hematopoietic-subsyndrome of the acute radiation syndrome (ARS).

A New Zealand White rabbit model of thrombocytopenia and coagulopathy following total body irradiation across the dose range to induce the hematopoietic-subsyndrome of acute radiation syndrome.

PURPOSE: The purpose if this study was to develop a rabbit model of total body irradiation (TBI) -induced thrombocytopenia and coagulopathy across the dose-range which induces the hematopoietic subsyndrome of the acute radiation syndrome (H-ARS). METHODS: Twenty male New Zealand White rabbits were assigned to arms to receive 6-MV of TBI at a dose of 6.5, 7.5, 8.5 or 9.5 Gy. Animals were treated with moderate levels of supportive care including buprenorphine for pain management, antibiotics, antipyretics for rectal body temperature >104.8 °F, and fluids for signs of dehydration. Animals were closelyfollowed for up to 45 days after TBI for signs of major morbidity/mortality. Hematology and serum chemistry parameters were routinely monitored. Hemostasis parameters were analyzed prior to TBI, 2 and 6 hours post-TBI, and at the time of euthanasia. RESULTS: Animals developed the characteristic signs and symptoms of H-ARS during the first-week post TBI. Animals became thrombocytopenic with signs of severe acute anemia during the second week post TBI. Moribund animals presented with petechia and ecchymosis of the skin and generalized internal hemorrhage. Multiorgan dysfunction characterized by bone marrow failure, gastric ileus, acute renal toxicity, and liver abnormalities were common. Severe abnormalities in coagulation parameters were observed. CONCLUSIONS: The presentation of bone marrow failure and multiorogan injury associated with ARS in the New Zealand White rabbit model is consistent with that described in the canine, swine, non-human primate, and in humans. The hemorrhagic syndrome associated with the ARS in rabbits is characterized by thrombocytopenia and hemostasis dysfunction, which appear to underlie the development of multiorgan dysfunction following TBI to rabbits. Taken together, the rabbit recapitulates the pathogenesis of ARS in humans, and may present an alternative small animal model for medical countermeasure pilot efficacy screening, dose-finding and schedule optimization studies prior to moving into large animal models of TBI-induced ARS.