Evidence for humoral effects on the radiation response of rat foot skin.

The influence of perturbation of the physiologic state of the whole body on the outcome of radiation exposure has been examined in a rat foot model. Irradiation was carried out using 60Co gamma-rays. Moist desquamation was used as an endpoint. Rats were given a priming dose of 2 Gy, 4 Gy or 7 Gy to their whole body except their hind feet (partial body priming dose). After a variable time period both hind feet of these animals were irradiated with graded doses of 60Co gamma-rays. The incidence of moist desquamation in the irradiated feet of these animals was compared with the incidence of moist desquamation in animals that had not received the initial partial body priming dose. It was noticed that the incidence of moist desquamation in the rat foot skin of animals that received 7 Gy partial body priming dose 4 h prior to irradiation of their hind feet was significantly less than moist desquamation in control animals. The ED(50) value of 22.53+/-0.16 Gy for moist desquamation of the foot skin of control animals was significantly lower (p<0.01) than the value of 25.25+/-0.29 Gy obtained for animals that received a partial body priming dose of 7 Gy 4 h prior to irradiation of their hind feet. It was concluded that the response of rat foot skin to radiation was not purely the result of epidermal stem cell kill and that it can be modified by alterations in the overall physiological state of the animal's body brought about by a priming dose to the whole of the animal's body except the hind feet.

Changes in epidermal radiosensitivity with time associated with increased colony numbers.

Epidermal clonogenic cell survival and colony formation following irradiation were investigated and related to radiosensitivity. A rapid in vivo/in vitro assay was developed for the quantification of colonies arising from surviving clonogenic cells in pig epidermis after irradiation. Bromodeoxyuridine (BrdU)-labelled cells in full thickness epidermal sheets were visualized using standard immunohistochemistry. In unirradiated skin, approximately 900 BrdU-positive cells mm(-2) were counted. In a time sequence experiment, BrdU-positive cell numbers increased from an average of 900 cells mm(-2) to approximately 1400 cells mm(-2) after BrdU-labelling for 2-24 h. In irradiated skin, colonies containing >/=16 BrdU-positive cells were seen for the first time at days 14/15 after irradiation. The number of these colonies per cm(2) as a function of skin surface dose yielded a cell survival curve with a D(0)-value (+/-SE) of 3.9+/-0.6 Gy. This relatively high D(0)-value is possibly due to a rapid fall off in depth dose distribution for the iridium-192 source and consequently a substantial contribution of hair follicular epithelium to colony formation. At 14/15 days after irradiation, the ED(50) level of 33.6 Gy for the in vivo response of moist desquamation corresponded with 2.7 colonies cm(-2). Surprisingly, the number of colonies increased with time after irradiation with an estimated doubling time of approximately 4 days, while the D(0)-value remained virtually unchanged. This increase in colony numbers could be due to migration of clonogenic cells, to the recruitment of dormant clonogenic cell survivors by elevated levels of cytokines, or to both. Although frequent biopsying caused increased cytokine levels, which had a systemic effect on unirradiated skin, it had no influence on colony formation in irradiated skin. Smaller colonies, containing 4-8 cells or 9-15 cells, were abundant, particularly after higher doses, which resulted in higher D(0)-values. The majority of these small colonies were abortive and did not progress to larger colonies. There was no statistical evidence for significant variations in the interanimal responses.

How valid is the assumption of equal effect per fraction?

BACKGROUND AND PURPOSE: The validity of the assumption of equal biological effect with dose per fraction in fractionated radiotherapy has been examined for the acute skin reaction in a rat foot model using a variable number of 2-Gy daily fractions followed by graded top-up doses. MATERIAL AND METHODS: Mature female rats were used. Both hind feet of each rat were irradiated with a range of fractionated and top-up doses of 60Co gamma-rays. The dose-related incidence of moist desquamation was used as an end-point. Quantal data for the incidence of moist desquamation were analysed using probit analysis and ED50 (+/-SE) values were obtained. The results were also compared with predicted values obtained from the application LQ-model. RESULTS: After a single 2-Gy fraction followed by top-up doses 24 h later, the dose effect curve for the top-up doses used was shifted to lower doses as expected and the ED50 for moist desquamation of 19.78 +/- 0.13 Gy was 1.16 Gy less than the ED50 of 20.94 +/- 0.15 Gy for large single dose exposure alone. This implied that only approximately 58% of the initial 2-Gy fraction was effective, and the rest was repaired within a 24-h interval between the 2 Gy and top-up doses. However, after two or three 2-Gy daily fractions the dose effect curves for the subsequent top-up doses moved to the higher doses again and the ED50 for top-up dose increased to 20.33 +/- 0.21 and 20.75 +/- 0.11 Gy, respectively. A further increase in the number of 2-Gy daily fractions shifted the dose effect curves for the top-up doses to lower doses and ED50 values for the top-up doses decreased progressively. CONCLUSIONS: The findings were not in keeping with values predicted based on the assumption of equal effect per fraction and could not be explained by the use of a single alpha/beta ratio in the LQ-model.

Boron neutron capture irradiation of the rat spinal cord: histopathological evidence of a vascular-mediated pathogenesis.

A histopathological study was carried out on the spinal cord of rats after boron neutron capture (BNC) irradiation. Rats were irradiated with thermal neutrons alone or in combination with borocaptate sodium (BSH) or p-boronophenylalanine (BPA). Spinal cords were examined 1 year after irradiation, or at earlier times in rats developing myelopathy. Considered overall, the pathology of the spinal cord after BNC irradiation was comparable with that reported previously after X irradiation of the spinal cord in the identical strain of rat. When BSH was used as the neutron capture agent, the biologically effective dose of radiation delivered to the CNS parenchyma was a factor of -2.7 lower than that delivered to the vascular endothelium. In effect, the blood vessels were selectively irradiated using this BNC modality. The resultant pathology was similar to that observed after irradiation with thermal neutrons alone or in the presence of BPA, situations in which the CNS vasculature was not selectively irradiated. This points to the vascular endothelium as being the critical target cell population, damage to which results in the development of the lesions seen in the spinal cord after BNC irradiation and, by inference, after irradiation with more conventional modalities.

Response of the central nervous system to boron neutron capture irradiation: evaluation using rat spinal cord model.

The response of the central nervous system to boron neutron capture irradiation, with either p-boronophenylalanine (BPA) or borocaptate sodium (BSH) as neutron capture agents, has been assessed using a rat spinal cord model. The mean latency times for the development of myelopathy after irradiation with the thermal neutron beam-alone, or in combination with BPA or BSH, were 23.7 +/- 0.3, 21.8 +/- 0.4 and 19.6 +/- 0.4 weeks, respectively. The radiation-induced lesion in the spinal cord was characterised by white matter necrosis. Due to the variations in the microdistribution of different neutron capture agents in body tissues, it was considered inappropriate to define the biological effectiveness of the high LET radiation, resulting from the 10B(n, alpha)7Li neutron capture reaction, relative to photon radiation, using the term 'relative biological effectiveness' (RBE). The term 'compound biological effectiveness' (CBE) factor was used as an alternative. ED50 values for the various irradiation modalities were calculated from probit fitted dose effect curves. Expressed as total physical absorbed doses these values were 13.6 +/- 0.4, 30.3 +/- 2.7 and 13.8 +/- 0.5 Gy after irradiation with the thermal neutron beam alone, or the thermal neutron beam in combination with BSH or BPA, respectively. The RBE of the thermal neutron beam was 1.4 +/- 0.04. The microdistribution of the two neutron capture agents played a crucial role in the determination of the overall biological effect, after thermal neutron activation. BSH, which is excluded from the CNS parenchyma by the blood brain barrier, had a low CBE factor value of 0.46 +/- 0.5. BPA, on the other hand, which crosses the blood brain barrier and distributes in the CNS parenchyma, had a higher CBE factor value of 1.33 +/- 0.16.