Experimental iodine-125 seed irradiation of intracerebral brain tumors in nude mice
© Verhoeff et al; licensee BioMed Central Ltd. 2007
Received: 28 June 2007
Accepted: 26 September 2007
Published: 26 September 2007
High-dose radiotherapy is standard treatment for patients with brain cancer. However, in preclinical research external beam radiotherapy is limited to heterotopic murine models– high-dose radiotherapy to the murine head is fatal due to radiation toxicity. Therefore, we developed a stereotactic brachytherapy mouse model for high-dose focal irradiation of experimental intracerebral (orthotopic) brain tumors.
Twenty-one nude mice received a hollow guide-screw implanted in the skull. After three weeks, 5 × 105 U251-NG2 human glioblastoma cells were injected. Five days later, a 2 mCi iodine-125 brachytherapy seed was inserted through the guide-screw in 11 randomly selected mice; 10 mice received a sham seed. Mice were euthanized when severe neurological or physical symptoms occurred. The cumulative irradiation dose 5 mm below the active iodine-125 seeds was 23.0 Gy after 13 weeks (BEDtumor = 30.6 Gy).
In the sham group, 9/10 animals (90%) showed signs of lethal tumor progression within 6 weeks. In the experimental group, 2/11 mice (18%) died of tumor progression within 13 weeks. Acute side effects in terms of weight loss or neurological symptoms were not observed in the irradiated animals.
The intracerebral implantation of an iodine-125 brachytherapy seed through a stereotactic guide-screw in the skull of mice with implanted brain tumors resulted in a significantly prolonged survival, caused by high-dose irradiation of the brain tumor that is biologically comparable to high-dose fractionated radiotherapy– without fatal irradiation toxicity. This is an excellent mouse model for testing orthotopic brain tumor therapies in combination with radiation therapy.
Postoperative radiotherapy is standard treatment for most patients with malignant brain tumors, including gliomas, malignant meningiomas, brain metastases, and medulloblastomas . High doses of radiotherapy, i.e. 50–80 Gy or more given during 5–8 weeks, are required to control tumor growth. Despite these aggressive radiotherapy regimens, the five-year survival of patients with a brain tumor remains poor, ranging from a few percent for brain metastases, 10% for glioblastoma multiforme , 50% for low-grade glioma , 70% for childhood medulloblastoma , to 88% for meningioma . Brachytherapy with radioactive iodine-125 (125I) seeds, which is effective against brain tumors, is used mostly for re-irradiation of recurrent brain tumors [6–9].
Although radiotherapy has been proven to be effective in prolonging survival and improving the quality-of-life of brain tumor patients, further research is warranted to increase its effectiveness. In glioma patients, success is expected of combination therapy using radiotherapy and either cytostatic drugs [10–12], molecular pathway modifiers , or hyperthermia [14, 15], although none of these strategies has resulted in major improvements in prognosis.
Experimental treatment requires pre-clinical animal models. Experimental radiotherapeutic studies are usually performed with xenotransplantation of tumors in the hind leg of immunocompromised mice (heterotopic implantation) instead of implantation in the brain (orthotopic implantation) because high-dose external beam radiotherapy in the latter case is fatal for small animals [16–18]. The minimal reliable field size for external irradiation is approximately 1 cm2 with the use of appropriate shielding. This is feasible for low dose treatment of e.g. rat brains, which are much larger than murine brains . Due to the dimensions of smaller rodents such as mice, it is not possible to deliver a high external beam radiation dose to tumors in the brain without irradiating surrounding tissues such as the normal brain and esophagus.
In the present study, we have investigated whether fatal irradiation toxicity can be prevented by implantation of an 125I brachytherapy seed to enable local high-dose irradiation of an orthotopic xenotransplanted glioblastoma in the mouse brain.
Implantation of a hollow guide-screw in the mouse skull (day -26)
Culture of glioblastoma cells
The glioblastoma cell line U251-NG2, a transfected human glioblastoma cell line with an overexpression of NG2 (provided by dr. Chekenya and prof. Bjerkvig, University of Bergen, Norway), was used for orthotopic xenotransplantation in the nude mouse brain. We selected the U251-NG2 cell line to investigate combination therapy consisting of radiotherapy and antiangiogenic drugs in future experiments. The glial precursor proteoglycan, neuron glia 2 (NG2), promotes angiogenesis-dependent tumor growth in the brain [21, 22].
The cells were grown in Dulbecco's Modified Eagle's Medium (DMEM) with the addition of 10% heat inactivated newborn calf serum, four times the prescribed amount of amino acids, 800 μg/ml neomycin and 100 IU/ml penicillin/streptomycin (Gibco, Paisley, UK) . Neomycin was added to select for neomycin-resistant clones that overexpress NG2. Cells were grown in culture flasks (Costar, Corning, NY, USA) in a humidified incubator at 37°C, in air mixed with 10% CO2. Two hours before cell transplantation, cells were harvested from the flasks by adding 2 ml trypsin. After washing, suspensions of 5 × 107 cancer cells in 300 μl PBS were prepared and kept on ice.
Administration of U251-NG2 cells (day -5)
Iodine-125 brachytherapy seed implant (day 0)
Follow-up and euthanasia (day 1–91)
All mice were kept in a specific pathogen-free environment with optimal feeding, temperature and hygienic conditions and ample space, according to stringent experimental animal facility regulations. Mice were observed and weighed daily. Euthanasia was performed in case of extreme weight loss (more than 20 percent of body weight at the start of the experiment), the development of a domehead, occurrence of rotational behavior or other severe neurological signs, or when there were signs of dehydration. Euthanasia was performed on all surviving animals after 13 weeks by introducing CO2 into a sealed cage. After death, the skull was opened and the brains removed for histological analysis. All mouse experiments were approved, monitored and reviewed by the university committee for animal experiments.
After hematoxylin-eosin staining, histological analysis was performed on every twentieth 8 μm section across the entire formalin-fixed brain. Magnetic resonance imaging with gadolinium enhancement was performed in a 1.5 T MRI scanner (Signa LX, General Electric Co, Waukesha, USA). Two tumor-bearing mice without implanted seeds were scanned 61 days after cancer cell injection. Mouse survival was calculated by the Kaplan & Meier method and compared by the log rank-test in SPSS©.
Our present study describes for the first time a mouse model for high-dose radiotherapy of orthotopic brain tumors. Irradiation of tumors was performed using a guide-screw with a 2 mCi 125I brachytherapy seed. Irradiation led to a significantly improved survival and higher tumor control probability in U251-NG2 glioblastoma-bearing nude mice. This mouse model may serve as an experimental animal model to study the effects of combination therapy consisting of local high-dose irradiation and chemical compounds that may have an additional therapeutic effect, such as new radio-sensitizing and anti-angiogenic compounds.
The major advantage of this brachytherapy model compared to external beam (orthovoltage) irradiation is that a high radiation dose can be delivered to the murine orthotopic brain tumor without causing fatal toxicity to the normal brain and pharynx, as observed when comparable doses of external beam irradiation are delivered. Apart from a lower applicable radiation dose, external beam irradiation is more labor-intensive because of the multiple fractions of anesthetized treatment and is only applicable to much larger rodents such as rats .
A few points of consideration about the model include the steep dose and dose-rate-gradient around the 125I seed, the relatively fast 125I radioactive decay (half-life of 59.4 days) and the subsequent radiobiological uncertainties. However, similar or worse dosimetric problems are encountered when very small external photon beam fields are used.
Dosimetric studies have shown that the measured dose rates and the anisotropically 'apple' shaped dose rate distribution of a single seed vary within the measurement error from the seed specifications provided by the manufacturer . The steep dose-rate gradient around the seed implanted in or adjacent to the tumor provides a high dose rate and cumulative tumor dose close to the seed, and thus to the tumor but a relatively low dose rate and cumulative dose farther away from the seed and thus in normal brain tissue. For example, for the 2 mCi 125I brachytherapy seed the cumulative radiation dose is 138 Gy after 13 weeks at 5 mm central lateral. At 5 mm below the seed, where we defined the 'minimal tumor dose', the initial dose rate is 1.7 cGy/hr and the cumulative dose is 23.0 Gy after 13 weeks.
The BED of permanent seed implants is a point of ongoing discussion . The main concern is the uncertainty about the biological significance of the residual source activity after 1.5 source decay half-times of 125I (half-time = 59.4 days) . For both ethical and evaluation purposes, we terminated our study at 13 weeks (91 days) after implant of a 2 mCi 125I source. After this relatively short period, we consider the BED approximations as proposed by Dale and Jones  to be safe for low-dose rate permanent implants removed after 90 days (see appendix for calculation details). These calculations yield a BEDtumor 2.5 mm distal of the seed of 91.2 Gy, equivalent to a conventional external beam radiotherapy dose of 76 Gy in 200 cGy daily fractions, which is more than clinically feasible.
Although dose-rate effects are of major importance for dose rates of 40 cGy/hr and higher, which is the range of clinical dose rates, this is probably of less importance for the permanent 125I implant (1–5 cGy/hr) . During the 91 days of implantation of the 2 mCi source, the dose rate 5 mm below the source decays from 1.7 cGy/hr to 0.6 cGy/hr.
It is not clear whether these biological models can be extrapolated to even lower dose rates after 2–3 decay half-times for seeds with a lower initial activity, such as the 0.5 mCi sources that we use clinically for prostate cancer implants , or in tumors with a very high doubling time. This is not necessarily a disadvantage of our mouse brachytherapy model; on the contrary, the model allows in vivo study of these uncertainties of clinical brachytherapy.
Guide-screw implanted 125I brachytherapy seeds can be used for experimental clinically significant irradiation of an orthotopic glioblastoma implanted in the nude mouse brain. The method is feasible, gives a high tumor control probability and an improved survival rate of the U251-NG2 glioblastoma-bearing mouse, without fatal irradiation toxicity.
The major concern of the model is the steep dose-rate gradient around the 125I seed with a correspondingly high dose inhomogeneity in the tumor. Because of the very high tumor control rate obtained in this model with little margin for any gain from additive experimental drugs, we recommend to use weaker 125I sources (1 mCi) for future studies.
Calculation of the Biological Equivalence Dose (BED) of permanent iodine-125 implants (according to Dale & Jones, 1998):
where n = number of daily fractions, d = fraction dose (in Gray), and the α/β ratio is the parameter for cellular survival. It is reasonable to assume an α/β-ratio of 3 Gy2 for normal tissue and 10 Gy2 for rapidly proliferative tissue, such as tumor and squamous cell epithelium.
Average repair-half-lives for mammalian tissues are usually 0.5–3 h, with increasing evidence that tumor recovery half-lives are probably shorter than those for late reacting normal tissues. Assuming a tumor recovery half-life T1/2 = 1.5 h, than μ = 0.462 per hour.
The decay half-life T1/2 of iodine-125 is 59.4 days = 1425.6 hours. The decay constant is 0.000486 per hour.
C = 1 - e-λT
We thank Dr. Martha Enger-Chekenya and Prof. Dr. Rolf Bjerkvig, University of Bergen, Norway for kindly providing us the U251-NG2 human glioma cell line.
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