Tolerance of rodent tails to necrosis after "daily" fractionated X rays or D-T neutrons

Consequential late effects in normal tissues

Unconventional, more aggressive irradiation protocols are usually associated with aggravation of acute reactions. In recent clinical studies, this has resulted in modulation of late effects in the same organ. This phenomenon has been termed consequential late effect (CLE). Correlations between acute and late effects have been reported in a number of tissues. Moreover, some radiobiological parameters may be used to differentiate between consequential and generic late effects: Dose fractionation and overall treatment time have a similar effect on acute and consequential responses, but opposing effects on generic late effects. Modulation of acute effects will affect the consequential component of late sequelae. Similarly, it will be influenced by the irradiated volume if a volume effect exists for the acute response. Moreover, markers for the acute response should be predictive for consequential effects. The present review gives preclinical and clinical evidence for CLE. These are predominantly found in organ systems where the acute response (of the epithelial lining) is associated with an impairment of the barrier against mechanical or chemical stress, which may cause additional trauma to the underlying tissues. Therefore, CLE are mainly found in the urinary and intestinal system, in mucosa and, to some extent, in skin. In these tissues with a consequential component of the late sequelae, amelioration of the acute response to irradiation may be a useful approach to minimize late side effects of effective radiation therapy.

The effect of combined modality treatment with ionising radiation and TPPS-mediated photodynamic therapy on murine tail skin

The effect on normal skin of combined modality treatment with 300 kV X-rays and photodynamic therapy (PDT) using the photosensitising drug meso-tetra (sulphonatophenyl) porphine (TPPS) was studied using the mouse tail necrosis assay. Prior treatment with a tolerance dose of PDT produced a significant increase in the probability of necrosis following graded doses of ionising radiation. A tolerance dose of X-rays administered prior to graded doses of PDT also produced a significant rise in the necrosis rate. TPPS appeared to have a radiosensitising effect but, as the animals were kept in subdued light, the low dose of PDT they therefore received may provide an alternative explanation. The effect of prolonging the interval between the modalities on the necrosis rate did not appear to be related to the time course of either the changes in blood flow produced by each modality, measured by xenon clearance studies or the development of the skin reaction following X-ray irradiation.

The relative biological effectiveness of fractionated doses of fast neutrons (42 MeVd----Be) for normal tissues in the pig. I. Effects on the epidermis and dermal vascular/connective tissues

The effects of fractionated doses of fast neutrons (42 MeVd----Be) on the early epithelial and later dermal response of pig skin have been assessed and compared with those after X irradiation. For the early epithelial reaction, i.e. moist desquamation, the relative biological effectiveness (RBE) of the neutron beam increased with the decreasing size of the X-ray dose/fraction. There was an experimentally observed upper RBE value of approximately 2.75 for X-ray doses/fraction of between 2 and 5 Gy. For the late reaction of ischaemic dermal necrosis the RBE was greater than 3.0 for X-ray doses/fraction of less than 3 Gy and, based on the assumptions made in the linearquadratic model of cell survival, an upper limiting RBE of 4.32 +/- 0.39 was calculated for infinitely small doses/fraction. These findings were compared with other radiobiological data and the conclusions drawn from the results of clinical trials. It was concluded that for the sparing of late effects in skin and subcutaneous tissues, relative to acute reactions, a relatively small number of fractions in a short overall treatment time may be optimal for fast neutron therapy.

Residual skin injury after repeated irradiation: differences observed using healing, macrocolony, and microcolony endpoints

Following three repeated tolerance doses to mouse tail skin, residual injury was characterized by a 35% reduction in the iso-effective dose compared to age-matched controls, using healing or macrocolony endpoints. In contrast, the reduction was only 9%, measured using microcolony formation. The colony data showed that the reduction was a constant dose, not a dose-modifying effect. The residual injury is interpreted as due to a reduced density of microcolony-forming cells in the epidermis, and these are less capable of macrocolony formation and hence of re-epithelialization in the repeatedly-irradiated epidermis.

Protective effect of hypoxia in the ram testis during single and split-dose X-irradiation

Spermatogonial stem-cell survival in the ram was studied after single (6 Gy) and split-dose (2 x 3 Gy, interval 21-24 h) X-irradiation both under normal and hypoxic conditions. Hypoxia was induced by inflation of an occluder implanted around the testicular artery. The occluders were inflated about 10 min before irradiation and deflated immediately after. Stem-cell survival was measured at 5 or 7 weeks after irradiation by determination of the Repopulation Index (RI) in histological testis sections. The RI-values after fractionated irradiation were only half those after single dose irradiation. Hypoxia had a protective effect on the stem-cell survival. After split-dose irradiation under hypoxic conditions two times more stem cells survived than under normal oxic conditions; the RI-values increased from 34% (oxic) to 68% (hypoxic). This effect of hypoxia was also found after single dose irradiation where the RI-values increased from 68% (oxic) to 84% (hypoxic). The development of the epithelium in repopulated tubules was also studied. Under hypoxia, a significantly higher fraction of tubules with complete epithelium was found after single (38 vs. 4%) as well as after split-dose irradiation (12 vs. 0%).

Re-irradiation of mouse skin: similarity of dose reductions for healing and macrocolony endpoints

The amount of residual injury in mouse tail skin, assessed by the decrease in re-irradiation dose for equal effect, was similar whether assessed using healing or colony endpoints (17-21% after single priming doses). There were tendencies towards an increased sensitivity of the colony-forming cells by a factor of about 2, and less residual injury after multifractionated priming doses. These observations are compatible with a lower alpha/beta ratio characterising the response to dose fractionation for residual injury than for the acute healing response.

Lack of differential sparing of late ischaemic atrophy and early epidermal healing, after dose fractionation of mouse tails down to 2.6 Gy per fraction

Long-term atrophy of irradiated mouse tails began after about 5 months, and the incidence rose steadily to the end of the lifespan. The major associated histological change was atherosclerosis in the single tail artery. The incidence of the ischaemic atrophy was dependent on the size of the irradiated volume. The probability of ischaemic atrophy assessed at 3 years after irradiation was little dependent on the dose. The fractionation effect was described by alpha/beta congruent to 30 Gy, which was not lower than the range of values applicable for healing of the early epidermal reactions on the tail. Hence the general finding of a sparing of late effects in tissues using low doses per fraction was not observed in these experiments using dose fractions down to 2.6 Gy and the present endpoints.

The radiosensitivity of microcolony- and macrocolony-forming cells in mouse tail epidermis

A new microcolony technique is described for measuring the survival of colony-forming cells in mouse tail epidermis. The survival curve is characterised by D0 = 2.70 +/- 0.12 Gy. The number of microcolonies per cm2 is similar to the number of macrocolonies after high doses, which shows for the first time that all microcolonies (greater than or equal to 32 cells) in epidermis develop into macrocolonies. At low doses the number of macrocolonies underestimates the number of colony-forming cells because of coalescence of microcolonies to form macrocolonies. This results in a lower apparent sensitivity of macrocolony-forming cells by a factor of about 1.5. About 3% of basal cells in tail epidermis appear to be capable of colony formation.

Acute and late damage in mouse tail following hyperthermia and X-irradiation

Tissue injury following X-irradiation of adult mouse tail was assessed by: (a) the severity of the acute skin reaction, which appeared on day 13 and peaked at approximately 20 days after treatment; (b) the occurrence of necrosis by three months; and (c) the incidence of "late' necrosis which developed between 6.5 and 11 months after treatment, in tails whose gross appearance was normal at 3 months. Hyperthermia (44.0 or 44.5 degrees C for 30 min, by hot-water immersion) given 9 days after radiation, i.e. close to the time of appearance of the acute skin reaction, resulted in an increased thermal response and, in addition, an increase in tissue response for all three waves of radiation injury studied. The times of expression of injury remained unaltered and the degrees of thermal enhancement of both early and "late' necrosis were similar to that measured for the acute skin reaction following radiation. All values were significantly lower than the thermal enhancement ratio (TER), measured as a result of a primary interaction between heat and radiation when the two modalities are given in close association. The observed increase in "late' injury could be accounted for by thermal potentiation of the acute radiation response; there was no evidence for a significant exacerbation by a "late' or progressing thermal injury.

Late occurring lesions in the skin of rats after repeated doses of X-rays

Late radiation damage, characterized by atrophy and necrosis in the skin and subcutaneous tissues, has been demonstrated in both the tail and feet of rats. The incidence of necrosis increased with total dose. These total doses, in the range 72-144 Gy, were given as 4-8 treatments of 18 Gy, each dose separated from the next by an interval of 28 days. This treatment protocol minimized acute epithelial skin reactions. The same regime applied to the skin on the back of rats resulted in a very severe acute reaction occurring after the second to fifth dose of 18 Gy. This was surprising since back skin, like tail skin, is less sensitive to large single doses of radiation than that of the foot. The late radiation reaction in the foot and tail of rats are compared and contrasted with other attempts to assess late effects in rodent skin and with late changes seen in pig skin.

Correlation of the dose-response relationships for epidermal colony-forming units, skin reactions, and healing, in the X-irradiated mouse tail

The sensitivity of epidermal colony-forming units (CFU) in the mouse tail has been compared with the sensitivity of target units responsible for healing the epidermis (EHU). The value of D0 for epidermal CFU was 3.45 +/- 0.36 Gy, much higher than most earlier reports for CFU in dorsal epidermis. The reason for the high D0 is unknown, but it is considered unlikely to be due to marked hypoxia. The value of D0 for EHU was 2.78 +/- 0.51 Gy, which was deduced from the steepness of the dose-incidence curve for the healing of tails. A comparison of the two values of D0 indicates that the inactivation rate of CFU can account for the steepness of the dose-response curve for survival of the tissue. The dose which allowed 50% of tails to heal well corresponded to 3-4 colonies per cm2 of epidermis, to a median peak skin reaction of about 2 (moist desquamation) on an arbitrary scoring scale, and to a slightly lower median skin reaction of 1.7 when the reaction scores were averaged over the period 3 to 6 weeks after irradiation.

Can late effects of neutron irradiation in man be estimated from early responses?

Slowly-developing tissue changes after neutron irradiation should be more easily predicted from acutely-developing injury than is the case with X rays. The difference between tissue responses to neutrons and X rays is that cell survival in both rapidly and slowly responding tissues is a direct logarithmic function of neutron dose, at least up to about 3 Gy, whereas the X-ray dose-survival relationship differs between the two types of tissue: the target cells for late injury are more susceptible to killing from accumulation of sub-lethal X-ray injury and hence the survival curve diverges from its initial essentially linear region more rapidly than does that for the target cells for acute injury.

Biological bases for high RBE values for late effects of neutron irradiation

The late effects of fractionated irradiation with neutrons have been relatively more severe than after x-irradiation. Reasons for the RBEn/x being higher for late than for acute effects may include: (1) Late effects are reduced more by fractionation of X ray doses than are acute effects, whereas, with neutrons, the fractionation response is the same in rapidly-and slowly-responding tissues; (2) Late-responding tissues are less "sensitized" (and are, therefore, relatively protected) by redistribution throughout the division cycle during a fractionated regimen than are acutely-responding tissues: since neutron responses are less affected by cell-cycle distribution than are X ray responses, the relative protection of slowly-responding tissues is less; (3) The target cells for late, but not acute injury, may repair potentially lethal damage after X ray, but not after neutron exposure. Thus, the dissociation of RBE values for acute and late injury reflects mainly the dissociation between acute and late responses to conventional fractionated X ray regimens and, from the point of view of complications of radiotherapy, we should not condemn neutrons but praise X rays. Since fractionation of neutron doses into increments equivalent to those used in X ray therapy does not provide a preferential sparing of slowly-responding tissues, it is reasonable to shorten the overall duration of neutron treatment to deliver the total dose tolerated by the relevant "late-effects" tissue(s) in the shortest time consistent with acceptable acute responses.

Persistent and late occurring lesions in irradiated feet of rats: their clinical relevance

Radiation-induced deformity, as characterized by tissue loss, has been investigated in rat feet. The acute epithelial response and the loss of deeper tissues occur concomitantly after irradiation. The greatest loss of tissue (severe deformity) was produced in feet where the healing of the epithelial reaction was greatly delayed. While deformity will clearly continue to "persist" after the acute reaction has healed it is misleading to refer to this lesion as "late" damage. A late-occurring lesion, not previously described in the literature, can be produced in the rat foot by high doses of radiation delivered in such a way that moist desquamation is avoided, i.e. by extending the total treatment time. The "persistent" and "late" radiation lesions are discussed in relation to other published data on the radiation response of rodent skin in the foot, ear and tail. Parallels are also drawn between reactions in rodents and those in the skin of pig and man.

Dose response experiments using the mouse tail model

Radiation induced tail necrosis in BALB/c mice was used to investigate the effects of tissue temperature and local oxygenation on the radiation response. Single doses of 250 kV roentgen-rays were given over the temperature range 32.5 to 37 degrees C and dose response curves obtained. The temperature dependence of the response was shown to be consistent with changing hypoxic status by the derivation of a simple model; this has practical implications for the treatment of patients using simultaneously combined radiation therapy and hyperthermia.

Dose-rate considerations in the introduction of low-dose-rate afterloading intracavitary techniques for radiotherapy

An investigation has been made into the changes in total dose required as a consequence of proposed increases in dose-rate in low-dose-rate treatments of cancer of the uterine cervix. The relationship between total irradiation time and dose-rate has been measured using an assay based on mouse-tail radionecrosis, with irradiation schedules similar to existing and proposed human cervix treatments. This relationship, which is similar to that observed in other biological systems, predicts that the total dose for epithelial tolerance should be reduced by about one third when the dose-rate is increased from 1.0 to 3.5 Gy per hour. The clinical implications of this finding are discussed.

The relationship between o.e.r., r.b.e. and number of fractions for X-ray- or neutron-induced skin damage

The skin reactions in aerated and hypoxic mouse tails after single or fractionated doses of 250 kV X-rays or fast neutrons (6 MeV deuterons on beryllium) have been measured. The o.e.r. for one to sixteen fractions of X-rays remains constant, while that for one to ten fractions of neutrons decreases with increasing neutron fractionation and decreasing neutron dose/fraction. The o.e.r. for X-rays was 1.7, for single-neutron doses 1.4, and for ten fractions of neutrons 1.25. It was anticipated that the o.e.r. for neutron-induced damage would decrease further as neutron fractionation is increased because the contribution to damage from the highest LET components of dose, the alpha and heavy recoil particles, would increase relative to the lowe LET components. The r.b.e. values obtained for skin damage were higher at all neutron doses/fraction examined in this study on tails than all those previously obtained in studies on skin at other sites on four species. This may be due to the influence of hypoxia on the r.b.e. measurements in the mouse tail.

A cell kinetic model to explain the time of appearance of skin reaction after X-rays or ultraviolet light irradiation

Skin reactions to various doses of X-rays (300 and 10 kV) and ultraviolet light (u.v.) have been compared using hairless mice. Two regions of epidermis with widely differing cell kinetics and gross structure have been compared. Little evidence could be found to support the idea that the early phases of the reaction are dependent on cell cycle time. The data can be explained by a model based on the assumption that epidermis contains only a small fraction of clonogenic (stem) cells and this fraction may vary in different epidermal regions. X-rays appear to exert their greatest destructive action on these clonogenic cells while u.v. is more indiscriminate in its action, killing both clonogenic and non-clonogenic cells.

The tolerance of mouse tails to necrosis after repeated irradiation with X rays

The reduction in the "tolerance" dose for the production of tail necrosis in adult mice seven weeks after irradiation has been measured in tails which had previously received various single or multiple tolerable doses. The major findings are: A. The ability of heavily-irradiated and healed tails to tolerate about a further 90% of the first dose was found to apply between six weeks and ten months after the first irradiation. B. The tolerance dose was reduced to about 65% of the original at the third irradiation, and this dose (approximately) remained tolerable at the fourth, fifth and sixth irradiation, all delivered at six week intervals. C. Subsequent to seven weeks after the sixth dose, late effects were observed as an atrophy of the tail, mostly distal to the healed irradiated region. The occurrence was very marked (occurring in about 40% of tails by eight months) compared with that observed after only one dose (not more than 4% by eight months). Hence, if the assay time was extended to eight months after irradiation to include these late effects, the tolerance dose for the tail after the sixth dose would be reduced to about 55% of the original value.

Radionecrosis of normal tissue: studies on mouse tails

Studies on the radionecrosis of mouse tails demonstrate the following modifications to the dose necessary for necrosis in 50 per cent of tails (the ND50): (a) There is very little reduction in ND50 values for irradiated lengths of tail from 2 cm to almost the whole tail, but there is a sharp increase in dose for lengths less than 1.5 cm. (b) The ND50 is high for unanesthetized mice irradiated in air, due to tissue hypoxia. (c) The hypoxia can be reduced by varying amounts by applying heat to the tail, or by flowing oxygen over the tail surface, or by anesthetizing the animal. (d) The ingress of oxygen through the surface can be reduced by placing a clamp round the proximal tail. These features are discussed with reference to the state and possible position of the target cells, and to the use of this assay technique in comparative studies.

Re-irradiation of rat tails to necrosis at six months after treatment with a "tolerance" dose of x rays or neutrons

Rat tails were re-irradiated to necrosis levels at about six months after various fractionated treatments with 290 kV X rays or 14 MeV neutrons. The X ray dose required to produce necrosis in half of a group of tails (ND50), which had been heavily X-irradiated six months before, was 91+/-4% of the ND50 for aged controls. After prior neutron-irradiation, however, this value was 87+/-4% (neutrons in second treatment) or 75+/-5% (X rays in second course). The "effective" oxygenation of mouse tails at this time after X-irradiation was similar to that of controls; thus these percentage dose values indicate the remarkable tolerance of this organized tissue to a second course of X-irradiation, and the presence of more residual injury in neutron-irradiated tissues.

Sublethal damage in cells of the mouse gut after mixed treatment with X rays and fast neutrons

The effect of combining X ray and neutron irradiations on cell survival in the mouse intestine has been investigated. The irradiations from the two beams were given within minutes or at most a few hours of each other when recovery from sublethal damage was not always complete. The results show that cells surviving the first dose of radiation have accumulated the same amount of recoverable sublethal damage regardless of whether that first dose was with X rays or neutrons. The rate at which the sublethal damage is shed is the same after X rays or neutrons. It is reasoned that there is less sparing of damage by fractionation of neutron dose compared with fractionation of X ray dose not because there is less recovery from sublethal damage after neutrons but because there is relatively more lethal damage; the recovery from any sublethal damage is the same as if it were from X rays. If X rays and neutron doses are separated by times long enough to allow the full repair of sublethal injury then the combined effect is simply additive.