Half brain irradiation in a murine model of breast cancer brain metastasis: magnetic resonance imaging and histological assessments of dose-response

For this study, the brain tropic clone of human triple-negative breast cancer cell line, MDA-MB-231-BR, stably transfected with enhanced green fluorescent protein (EGFP) was used [18]. Cells were cultured and maintained in Dulbecco’s modified Eagle’s medium (DMEM) containing 10% fetal bovine serum and 1% penicillin/streptomycin. Cultured cells were kept in 5% CO₂ at 37 °C. Trypan blue exclusion assay was done to determine cell viability.

To deliver MDA-MB-231-BR cells into the brain, the intra-cardiac injection method was used to distribute cells through arterial circulation. Female nu/nu mice (N = 19, 6–8 weeks old; Charles River Laboratories) were anesthetized with 1.5 to 2% vaporized inhaled isoflurane in O₂. A suspension containing 1.5 × 10⁵ MDA-MB-231-BR cells in 0.1 ml of Hanks balanced salt solution was slowly injected into the left ventricle of the beating heart of the mouse [19]. Animals were housed in ventilated cages with a 12-h light/dark cycle and controlled temperature (20-22 °C), fed normal chow and given water ad libitum. Animal’s appearance and behavior was scored daily through the experiment and no profound effect of pain and distress on behavior was observed. This study followed animal care protocols approved by the Animal Use Subcommittee of The University of Western Ontario and were consistent with the policies of the Canadian Council on Animal Care. Mice received half brain radiation 26 days after cell injection.

Mice received half brain radiation therapy on the modified GE eXplore CT 120 (GE Healthcare, Milwaukee, WI) preclinical imaging system [20, 21]. They were anesthetized using 1.5 to 2% vaporized inhaled isoflurane and were immobilized using the customized 3D-printed mouse head holder with a targeting accuracy of < 0.15 mm [6]. Mice were set-up in a feet first prone position. The longitudinal fissure (LF) was visually set as the anatomical target for the radiation field. Setup lasers and CT images were used to verify the alignment of the animal’s head in the head holder. Once the mouse was immobilized for treatment, online dorsal-ventral fluoroscopy was acquired to identify the rim of the skull and to position the collimators. A small CT localization marker was placed on the right side of the head holder to help with animal orientation on CT and fluoroscopy. The right half of the brain was irradiated with a single field (14 × 20 mm²) from the dorsal direction. Mice received doses of 8, 16 or 24 Gy in a single fraction. These dose levels were chosen because the biological effective dose (BED, assuming α/β = 10 Gy) of 16 Gy and 24 Gy in a single fraction are meant to represent doses prescribed for whole brain radiation therapy (30 Gy in 10 fractions) [22, 23] and stereotactic radiosurgery respectively (18-24 Gy in one fraction) [24]. Figure 1 shows a representative dose distribution in the mouse brain for 16 Gy. The 16 Gy iso-dose line (magenta color) in Fig. 1 shows homogenous radiation dose for the hemisphere away from the field edge near the midline of the brain. We have measured the dose drop off to be 7.5% per 5 mm [20]. We prescribed the dose to the midplane of the brain, and expected then the variation to be +/− 3.75%. That is, when we prescribed 16 Gy to the midplane, the variation across the brain will be 16 Gy +/− 0.6 Gy. This dose variation is minimal compared to the dose levels of 8, 16 and 24 Gy. The dose received by the un-irradiated side of the brain and tumors were denoted as 0* and will be employed as the control of the irradiated side in the same mouse. After recovery from radiotherapy, mice were selected either for acute or longitudinal dose-response study.

All mice were imaged on a 3 T GE clinical MR scanner (General Electric, Mississauga, Canada) with a custom-built gradient insert coil on day 26 after tumor injection and before receiving radiation. MRI was performed to verify the presence of the tumors in the mouse brain, particularly in both brain hemispheres. Mice that had no identifiable brain metastases on MR did not proceed to RT and excluded from this study. Images were acquired using 3D balanced steady-state free precession (bSSFP) protocol (acquisition resolution = 100 × 100 × 200 μm, repetition time = 8 ms, echo time = 4 ms, flip angle = 35°, receive bandwidth = 19.23 kHz, signal averages = 2, radiofrequency phase cycles = 8, scan time = 29 min, along with ZIP2 and ZIP512 upscaling), a well-established imaging technique for this model [25‐27]. To evaluate the response of breast cancer brain metastases to different radiation doses in-vivo, the longitudinal group was imaged again 11 days after receiving half brain radiotherapy (37 days after tumor injection) with the same imaging protocol.

Brain metastases were segmented manually on pre and post-radiotherapy images by a single observer using open-source OsiriX image software version 6.0. Tumors in the midline of the brain (±200 μm of the longitudinal fissure) were excluded from the study as only part of these tumors may have been irradiated. Figure 2a showed an example of the manual segmentation of the tumors performed on an MR acquired on day 11 after RT. Mean fractional volume changes of the tumors were calculated by dividing the post-treatment tumor volume by the volume of the same tumor before treatment and averaged for all brain metastasis for mice in each group. One mouse in the 24 Gy longitudinal cohort had to be sacrificed at 7 days due to its deteriorating condition.

At the two post-irradiation time-points (30 min or 11 days) mouse brain samples were collected and processed for immunohistochemistry staining. Mice were perfused with 0.9% saline followed by 4% paraformaldehyde (PFA). Brains were harvested and post-fixed in 4% PFA and transferred to 30% sucrose solution until the specimen sank to the bottom. Brain samples were embedded in Tissue-Tek OCT Compound (Sakura, Torrance, CA) and frozen. Cyrosectioning of coronal slices was performed with 10-μm slice thickness. Tissue sections were stained with hematoxylin and eosin (H&E) to assess the morphology of the tumors.

Immunostaining was performed with the primary monoclonal antibody against γ-H2AX using a protocol published by Ford et al. [28]. Staining of sections consisted of antigen retrieval with sodium citrate, 1 h incubation in blocking serum (10% goat serum with 0.1% Triton X-100 for membrane permeabilization), overnight incubation at 4 °C in mouse anti-γ-H2AX antibody (anti-phospho-histone H2AX, Ser139, clone JBW301; Millipore, Billerica, MA, USA) at the dilution of 1:700, 1 h incubation in secondary antibody (1:500 goat anti-mouse Alexa Fluor 594 conjugated, Life Technologies, Carlsbad, CA, USA.), DAPI counterstain 5 min, and mount with anti-fade mounting medium Vectashield (Vector Laboratories, Inc. Burlington, ON). This protocol was used consistently to stain sections from the two time-points. For quantification, images were acquired with 100X (oil immersion) objective lens on a fluorescence microscope (Carl Zeiss Canada Ltd). Imaging parameters such as intensity, exposure time and gain were kept consistent during the experiment. We collected a total of ten to thirteen images of different tumors for each mouse.

To evaluate the DNA damage response, γ-H2AX stained sections of tumors were analyzed for each radiation dose level. The amount of damage was also quantified in neighboring normal brain tissues under the same conditions as the tumors. Initially, we employed an inverted confocal microscope (Olympus Fluoview FV1000 Confocal Imaging System) for high resolution 3D images of γ-H2AX foci within the nuclei [29]. We observed in the acute setting γ-H2AX foci were over-lapping, which made detection of individual foci impossible. Similarly, foci saturation was observed in the irradiated tumors in the longitudinal experiment. Unable to count individual foci, we quantified γ-H2AX based on the fluorescent stain intensity, which is a more reliable method for high radiation doses [30, 31].

All IHC analyses were performed on images taken from the fluorescence microscope using 100X oil immersion objective. The γ-H2AX intensity was measured for both normal mouse brain and tumor tissues. Tumor nuclei were visually distinguished from mouse nuclei based on the characteristic punctuate pattern of mouse DAPI staining [32]. To quantify γ-H2AX intensity, DAPI-stained nuclei were used to generate nuclear outlines in which the γ-H2AX intensity would be measured. Nuclear segmentations were used to eliminate signal from background fluorescence. Nuclei on DAPI images were manually segmented using Adobe Photoshop CC. For each field of view, total γ-H2AX fluorescence intensity was obtained by summing the intensity values of all pixels within the segmented boundary using an in-house code developed and validated in MATLAB (MathWorks, Natick, MA, USA). The total γ-H2AX fluorescence intensity for each field of view was normalized to the total area of segmented nuclei for the same field (Eq. 1).

Mean γ-H2AX intensity per unit area was determined for each treatment condition in the acute and longitudinal settings. The total number of nuclei analyzed for each dose level varied from 350 to 950.

We observed that MDA-MB-231-BR tumors grew in clusters surrounded by edema. We obtained the number of tumor nuclei per cluster area. This index gave us the density of tumor nuclei/cells in each cluster (Eq. 2).

We quantified both the tumor cell density and size of tumor nucleus for all radiation doses at the two time-points. Figure 3 shows the flow chart of the processes involved in these histological quantifications. IHC staining was repeated three times for the acute study and twice for the longitudinal study.

We also observed an increase in tumor nuclei size and we quantified the size of tumor nuclei by computing the average area of each nucleus from DAPI images (Eq. 3).

Statistical analyses were performed using SPSS (Armonk, NY: IBM Corp) and confirmed by GraphPad Prism software (La Jolla, CA, USA). The normality of the measured variables was tested using the Shapiro-Wilk test and the p < 0.05 was used as the significance threshold. For normally distributed variables, between-groups analysis of variance (ANOVA) followed by Tukey post-hoc test was conducted to determine whether the response was statistically significant (p < 0.05). Nonparametric Kruskal-Wallis analysis followed by Mann-Whitney U test was used for variables that were not normally distributed.

In the acute radiation dose-response study, mice received half brain radiation of 8, 16 and 24 Gy (minimum N = 3 per dose) and were sacrificed approximately 30 min after treatment. Tissue sections were stained for γ-H2AX to quantify the initial damage induced in both normal mouse brain and tumors. Figure 1b displays a mouse whole brain coronal section, which received half brain radiation of 16 Gy.

Figure 4a shows the tissue sections of tumors and normal mouse brain stained with DAPI and γ-H2AX at the acute time point. Figure 4b shows our quantification of γ-H2AX based on fluorescence intensity density in the nuclei of normal brain and tumor tissues evaluated at the acute time point. In normal brain, the amount of γ-H2AX intensity density increased linearly (R² = 0.78, p < 0.001) with increasing radiation dose. However, in tumors, this trend stopped at 16 Gy; the level of γ-H2AX intensity density dropped at the dose of 24 Gy compared to 16 Gy. The γ-H2AX intensity density in both tumors and normal brain of the irradiated side were significantly increased (p < 0.0001) compared to the respective un-irradiated side (8 versus 0*(8), 16 versus 0*(16) and 24 versus 0*(24) Gy).

To investigate how much of the initial damage is retained in both tumors and normal brain tissues, γ-H2AX intensity density was measured for the longitudinal group 11 days after hemi brain radiation (Figs. 4c, d). We observed that γ-H2AX intensity density in irradiated normal brain nuclei returned to background levels when compared to un-irradiated side of the brain 11 days after radiotherapy. However, irradiated tumors had higher levels of γ-H2AX intensity density compared to tumors in the contralateral un-irradiated sides (0*(16) and 0*(24) Gy). There was no significant difference in the amount of residual γ-H2AX between irradiated tumors (16 Gy vs. 24 Gy).

To assess the changes in the volume of tumors in response to radiation doses in-vivo, MR images were taken before and 11 days after half brain radiation therapy. Representative images of brain metastases at two different time-points for doses of 16 and 24 Gy are shown (Fig. 5a). The mean fractional growth of the tumors was calculated for each group (Fig. 5b). There was a statistically significant difference (Mann-Whitney U p ≤ 0.05) between the growth of un-irradiated and irradiated brain metastases for both doses of 16 and 24 Gy. A second observer segmented tumors on MRI on two animals treated at 24 Gy and confirmed this finding. The fractional reduction in tumor volume growth as assessed by MRI was not statistically different between 16 and 24 Gy in the longitudinal setting. Tumor Cell Density.

We observed on H&E samples from the longitudinal cohort that irradiated tumors are less compacted with cells, and surrounded by a more substantial amount of edema compared to tumors on the un-irradiated side (Fig. 6a). We quantified this by calculating tumor cell density based on DAPI staining for tumors in both the acute and longitudinal settings. The acute setting was employed to provide a baseline verification. As expected, no significant difference was detected in the density between treated and un-treated tumors and for different radiation doses 30 min after radiation.

On the other hand, there was a significant difference in tumor cell density between treated and un-treated tumors in the longitudinal experiment (Fig. 6b). Furthermore, there was a significantly lower density in those treated with 24 Gy compared to 16 Gy.

DAPI is used as a counterstain for the nucleus of the cell and we used this stain to investigate the size of tumor nuclei for both acute and longitudinal studies. We observed that the nuclei of treated tumors were significantly larger than the un-treated nuclei 11 days after radiotherapy. Figure 6c shows the different morphological appearances of irradiated versus un-irradiated tumor nuclei stained with DAPI. The size of tumor nuclei was quantified for both acute and longitudinal studies. The acute setting quantification was employed to establish a baseline and no significant differences was found in the average size of tumor nuclei 30 min after treatment. A second observer repeated this DAPI nuclei segmentation on tumors that were treated at 24 Gy and their contralateral control and confirmed the manual segmentation results. However, in the longitudinal cohort, there was a significant difference in the size of the nuclei between treated and un-treated sides of the same mice. Radiation dose at 24 Gy resulted in a significantly larger nuclei size than 16 Gy in the longitudinal setting (Fig. 6d).