In Vivo Microbeam Radiation Therapy at a Conventional Small Animal Irradiator

Microbeam radiation therapy (MRT) is a still pre-clinical form of spatially fractionated radiotherapy, which uses an array of micrometer-wide, planar beams of X-ray radiation. The dose modulation in MRT has proven effective in the treatment of tumors while being well tolerated by normal tissue. Research on understanding the underlying biological mechanisms mostly requires large third-generation synchrotrons. In this study, we aimed to develop a preclinical treatment environment that would allow MRT independent of synchrotrons. We built a compact microbeam setup for pre-clinical experiments within a small animal irradiator and present in vivo MRT application, including treatment planning, dosimetry, and animal positioning. The brain of an immobilized mouse was treated with MRT, excised, and immunohistochemically stained against γH2AX for DNA double-strand breaks. We developed a comprehensive treatment planning system by adjusting an existing dose calculation algorithm to our setup and attaching it to the open-source software 3D-Slicer. Predicted doses in treatment planning agreed within 10% with film dosimetry readings. We demonstrated the feasibility of MRT exposures in vivo at a compact source and showed that the microbeam pattern is observable in histological sections of a mouse brain. The platform developed in this study will be used for pre-clinical research of MRT.

Equivalent Uniform Dose (EUD) and the Evaluation of Cell Survival in Spatially Fractionated Radiotherapy (SFRT)

PURPOSE/OBJECTIVE(S)

SFRT has shown promise as a treatment modality to decrease normal tissue sparing without compromising tumor coverage, i.e., an increase in the therapeutic window compared to more conventional uniform radiation therapy (RT). The aim of this work is to examine and test several alternative bio-dosimetric parameters for the prediction of cell survival for normal-tissue and tumor cell lines irradiated in vitro with uniform and microbeam radiotherapy (MRT).

MATERIALS/METHODS

A bespoke tungsten collimator with 50 parallel, 50 µm wide slits and 400 µm slit spacing was mounted into an x-ray cabinet. Human fibroblast (MRC5) and two human tumor cell lines (LN18 and A549) were irradiated with a range of doses (< 10 Gy) for uniform and MRT (50um slits, 400um center spacing) using kV X-rays. Average, mean and valley dose as useful predictive metrics of cell survival are compared to the equivalent uniform dose (EUD) with biological parameters estimated from uniform-dose experiments.

RESULTS

We find that EUD, with linear-quadratic (LQ) model parameters, is more predictive for survival after SFRT than maximum, minimum or average dose. The maximum and average doses are correlated very poorly with in vitro cell survival. The difference in cell survival between uniform and MRT irradiation as a function of EUD is cell-type and dose dependent. The report results suggest that MRT is more effective at cell killing of tumor-cell lines than uniform irradiation for both tumor cell lines. However, MRT is less effective at killing normal tissue cells than uniform irradiation.

CONCLUSION

EUD is a superior predictor of in vitro cell survival than other metrics sometimes used in the SFRT literature, including mean dose, maximum dose, and valley dose. The reported studies provide some evidence that SFRT may increase the therapeutic ratio by producing spatial dose distributions that effectively reduce normal-tissue damage with little or no change in biological damage to tumor cells. Additional studies are needed to further extend and generalize our results and to test our conclusions against a larger dose range, low and high linear energy transfer (LET) radiations and additional cell lines.

Curcumin Nanodiscs Improve Solubility and Serve as Radiological Protectants against Ionizing Radiation Exposures in a Cell-Cycle Dependent Manner

Curcumin, a natural polyphenol derived from the spice turmeric (Curcuma longa), contains antioxidant, anti-inflammatory, and anti-cancer properties. However, curcumin bioavailability is inherently low due to poor water solubility and rapid metabolism. Here, we further refined for use curcumin incorporated into "biomimetic" nanolipoprotein particles (cNLPs) consisting of a phospholipid bilayer surrounded by apolipoprotein A1 and amphipathic polymer scaffolding moieties. Our cNLP formulation improves the water solubility of curcumin over 30-fold and produces nanoparticles with ~350 µg/mL total loading capacity for downstream in vitro and in vivo applications. We found that cNLPs were well tolerated in AG05965/MRC-5 human primary lung fibroblasts compared to cultures treated with curcumin solubilized in DMSO (curDMSO). Pre-treatment with cNLPs of quiescent G0/G1-phase MRC-5 cultures improved cell survival following ¹³⁷Cs gamma ray irradiations, although this finding was reversed in asynchronously cycling log-phase cell cultures. These findings may be useful for establishing cNLPs as a method to improve curcumin bioavailability for administration as a radioprotective and/or radiomitigative agent against ionizing radiation (IR) exposures in non-cycling cells or as a radiosensitizing agent for actively dividing cell populations, such as tumors.

Normal Tissue Response of Combined Temporal and Spatial Fractionation in Proton Minibeam Radiation Therapy

PURPOSE

Proton minibeam radiation therapy, a spatial fractionation concept, widens the therapeutic window. By reducing normal tissue toxicities, it allows a temporally fractionated regime with high daily doses. However, an array shift between daily fractions can affect the tissue-sparing effect by decreasing the total peak-to-valley dose ratio. Therefore, combining temporal fractions with spatial fractionation raises questions about the impact of daily applied dose modulations, reirradiation accuracies, and total dose modulations.

METHODS AND MATERIALS

Healthy mouse ear pinnae were irradiated with 4 daily fractions of 30 Gy mean dose, applying proton pencil minibeams (pMB) of Gaussian σ = 222 μm in 3 different schemes: a 16 pMB array with a center-to-center distance of 1.8 mm irradiated the same position in all sessions (FS1) or was shifted by 0.9 mm to never hit the previously irradiated tissue in each session (FS2), or a 64 pMB array with a center-to-center distance of 0.9 mm irradiated the same position in all sessions (FS3), resulting in the same total dose distribution as FS2. Reirradiation positioning and its accuracy were obtained from image guidance using the unique vessel structure of ears. Acute toxicities (swelling, erythema, and desquamation) were evaluated for 153 days after the first fraction. Late toxicities (fibrous tissue, inflammation) were analyzed on day 153.

RESULTS

Reirradiation of highly dose-modulated arrays at a positioning accuracy of 110 ± 52 μm induced the least severe acute and late toxicities. A shift of the same array in FS2 led to significantly inducted acute toxicities, a higher otitis score, and a slight increase in fibrous tissue. FS3 led to the strongest increase in acute and late toxicities.

CONCLUSIONS

The highest normal-tissue sparing is achieved after accurate reirradiation of a highly dose modulated pMB array, although high positioning accuracies are challenging in a clinical environment. Nevertheless, the same integral dose applied in highly dose-modulated fractions is superior to low daily dose-modulated fractions.

FRET-based Tau seeding assay does not represent prion-like templated assembly of Tau filaments

Tau aggregation into amyloid fibers based on the cross-beta structure is a hallmark of several Tauopathies, including Alzheimer Disease (AD). Trans-cellular propagation of Tau with pathological conformation has been suggested as a key disease mechanism. This is thought to cause the spreading of Tau pathology in AD by templated conversion of naive Tau in recipient cells into a pathological state, followed by assembly of pathological Tau fibers, similar to the mechanism of nucleated polymerization proposed for prion pathogenesis. In cell cultures, the process is often monitored by a FRET assay where the recipient cell expresses the Tau repeat domain (Tau^RD) with a pro-aggregant mutation, fused to GFP-based FRET pairs. Since the size of the reporter GFP (barrel of ~ 3 nm × 4 nm) is ~ 7 times larger than the β-strand distance (0.47 nm), this points to a potential steric clash. Hence, we investigated the influence of the GFP tag on Tau^FL or Tau^RD aggregation. Using biophysical methods (light scattering, atomic force microscopy (AFM), and scanning-transmission electron microscopy (STEM)), we found that the assembly of Tau^RD-GFP was severely inhibited and incompatible with that of Alzheimer filaments. These observations argue against the hypothesis that the propagation of Tau pathology in AD is caused by the prion-like templated aggregation of Tau protein, transmitted via cell-to-cell spreading of Tau. Thus, even though the observed local increase of FRET in recipient cells may be a valid hallmark of a pathological reaction, our data argue that it is caused by a process distinct from assembly of Tau^RD filaments.