Time course of loss of residual radiation damage in murine skin assessed by retreatment

ICRP publication 118: ICRP statement on tissue reactions and early and late effects of radiation in normal tissues and organs--threshold doses for tissue reactions in a radiation protection context

This report provides a review of early and late effects of radiation in normal tissues and organs with respect to radiation protection. It was instigated following a recommendation in Publication 103 (ICRP, 2007), and it provides updated estimates of 'practical' threshold doses for tissue injury defined at the level of 1% incidence. Estimates are given for morbidity and mortality endpoints in all organ systems following acute, fractionated, or chronic exposure. The organ systems comprise the haematopoietic, immune, reproductive, circulatory, respiratory, musculoskeletal, endocrine, and nervous systems; the digestive and urinary tracts; the skin; and the eye. Particular attention is paid to circulatory disease and cataracts because of recent evidence of higher incidences of injury than expected after lower doses; hence, threshold doses appear to be lower than previously considered. This is largely because of the increasing incidences with increasing times after exposure. In the context of protection, it is the threshold doses for very long follow-up times that are the most relevant for workers and the public; for example, the atomic bomb survivors with 40-50years of follow-up. Radiotherapy data generally apply for shorter follow-up times because of competing causes of death in cancer patients, and hence the risks of radiation-induced circulatory disease at those earlier times are lower. A variety of biological response modifiers have been used to help reduce late reactions in many tissues. These include antioxidants, radical scavengers, inhibitors of apoptosis, anti-inflammatory drugs, angiotensin-converting enzyme inhibitors, growth factors, and cytokines. In many cases, these give dose modification factors of 1.1-1.2, and in a few cases 1.5-2, indicating the potential for increasing threshold doses in known exposure cases. In contrast, there are agents that enhance radiation responses, notably other cytotoxic agents such as antimetabolites, alkylating agents, anti-angiogenic drugs, and antibiotics, as well as genetic and comorbidity factors. Most tissues show a sparing effect of dose fractionation, so that total doses for a given endpoint are higher if the dose is fractionated rather than when given as a single dose. However, for reactions manifesting very late after low total doses, particularly for cataracts and circulatory disease, it appears that the rate of dose delivery does not modify the low incidence. This implies that the injury in these cases and at these low dose levels is caused by single-hit irreparable-type events. For these two tissues, a threshold dose of 0.5Gy is proposed herein for practical purposes, irrespective of the rate of dose delivery, and future studies may elucidate this judgement further.

[Reirradiation of normal tissues: preclinical radiobiological data]

Reirradiation represent an unfrequent particular clinical situation. The risk/benefit ratio assessment must be taken into account, considering both clinical and dosimetric aspects. There is a relatively limited amount of preclinical data available to date and clinicians should cautiously perform reirradiations in selected indications. This review summarizes the experimental data available on reirradiation of normal tissues, the consequences on early and late toxicities as well as the intrinsic limitations of these models.

Dynamics of micronuclei in rat skin fibroblasts after X irradiation

In a previous study, we demonstrated DNA damage, expressed as micronuclei, in binucleate dermal fibroblasts obtained from human skin 2-9 weeks after fractionated radiotherapy. Here we assessed micronuclei in X-irradiated skin fibroblasts from 9-14-week-old female Lewis rats as a function of time after a single dose of radiation to determine the lifetime of such damage in the skin. After irradiation with 5, 10, 15 and 18 Gy, formation of micronuclei at 1 day or 2 months postirradiation increased up to about 10 Gy, with evidence for a plateau at higher doses. The time course of micronuclei present in the skin fibroblasts demonstrated a plateau region (approximately 20 days after 18 Gy and about 2 months after 10 Gy) before the number of micronuclei started to decline. Residual micronuclei were observed for more than 1 year after irradiation. Monomicronucleated cells predominated in fibroblasts from nonirradiated skin, whereas in fibroblasts from irradiated skin, multimicronucleated cells predominated and persisted (together with monomicronucleated cells) in the residual levels of damage at late times. The results suggest that DNA damage in dermal fibroblasts can be assayed by the micronucleus assay in samples from irradiated skin up to 1 month after irradiation for doses up to at least 10 Gy. Further studies are needed to define the dose-response relationship in detail.

Cellular kinetics of murine lung: model system to determine basis for radioprotection with keratinocyte growth factor

PURPOSE

Normal tissue toxicity remains a dose limitation for cancer radiotherapy and chemoradiotherapy. Growth factors offer a novel means of mitigating normal tissue radiotoxicity. In particular, keratinocyte growth factor (rHuKGF), whose proliferative activity is restricted to epithelial cells, holds promise on the basis of the findings of preclinical models of epithelial cytoprotection and the clinical developments to date. We report the radioprotection of murine lung by an increase in tissue cellularity after rHuKGF-induced proliferation.

METHODS AND MATERIALS

Flow cytometric and image analysis techniques after bromodeoxyuridine labeling were used to estimate proliferative parameters. Our specialized analytical methods measure not only labeling indexes, but also the durations of S and G(2)+M phases, potential doubling times, and the net cell production rate. Image analysis techniques were used to identify the specific cell types that were proliferating (type II pneumocytes).

RESULTS

Lung labeling index control values (0.5%) rose to a maximum (5.5%) at 3 days after intratracheal rHuKGF, returning to normal by Day 7. The potential doubling time fell from 66 days to 4.4 days. The net cell production rate rose from a control value of 1%/d to >15%/d by Day 3. This resulted in a nearly twofold increase in alveolar epithelial cellularity, which remained significantly elevated on Day 7. Saline-treated control animals exhibited no significant changes in the proliferative parameter values or cellularity. On the basis of these data, mice were irradiated, solely to the thorax, with ranges of single doses of 250 kVp X-rays 7 days after either intratracheal administration of 5 mg/kg rHuKGF or phospate-buffered saline. This interval was chosen because the proliferative response of the type II cells was finished but the cellularity of the lung remained increased. Pretreatment with rHuKGF extended the latent period before onset of pneumonitis after all radiation doses. rHuKGF treatment 7 days before thoracic irradiation significantly protected against pneumonitis (median effective dose 13.7 Gy, 95% confidence limit 13.4-14.0) compared with the control pretreatment with phosphate-buffered saline (median effective dose 12.8 Gy, 95% confidence limit 12.6-13.1).

CONCLUSION

The data showed that an increase in tissue cellularity, caused by rHuKGF treatment before irradiation, protected the lung from damage due to pneumonitis.

Re-irradiation in conjunction with liposomal doxorubicin for the treatment of skin metastases of recurrent breast cancer: a radiobiological approach and 2 year of follow-up

Thirty patients with local relapses after radical mastectomy and radiotherapy and undergoing infusion of liposomal doxorubicin (40 mg/m(2) monthly for 6 months) were randomized to receive re-irradiation. Radiotherapy was with either 17 fractions of 1.8 Gy, 5 days a week (N=15, group A) or 4 Gy plus two fractions of 3 Gy the 1st week and six fractions of 3 Gy given every second day (N=15, group B). Eight patients from group A (53.3%) and nine patients (60%) from group B demonstrating a clinically complete response (P=0.9). Grade I/II acute skin toxicity was monitored in 26.6% of patients in group A versus 73.3% in group B. The radiation schedule of group A seems superior for grade I/II acute (P=0.027) and late (P=0.015) skin toxicity. The linear quadratic model enabled the prediction of tumor response as well as normal skin reactions.

Tissue tolerance to reirradiation

There are increasing requests for delivering a second course of radiation to patients who develop second primary tumors within or close to previous radiotherapy portal or late in-field recurrences. Rational treatment decisions demand rather precise knowledge on long-term recovery of occult radiation injury in various organs. This article summarizes available experimental and clinical data on the effects of reirradiation to the skin, mucosa, gut, lung, spinal cord, brain, heart, bladder, and kidney. The data reveal that, in general, acutely responding tissues recover radiation injury within a few months and, therefore, can tolerate another full course of radiation. For late toxicity endpoints, however, tissues vary considerably in their capacity to recover from occult radiation damage. The heart, bladder, and kidney do not exhibit long-term recovery at all. In contrast, the skin, mucosa, lung, and spinal cord do recover subclinical injury partially to a magnitude dependent on the organ type, size of the initial dose, and, to a lesser extent, the interval between radiation courses. The available clinical data have inspired many radiation oncologists to undertake systematic studies addressing the efficacy and toxicity of reirradiation in various clinical settings. Hopefully, systematic scoring, collection, and analysis of patient outcome will produce quantitative data useful for clinical practice.

Reirradiation tolerance of the rat heart

PURPOSE

To investigate the influence of reirradiation on the tolerance of the heart after a previous irradiation treatment.

METHODS AND MATERIALS

Female Wistar rats were locally irradiated to the thorax. Development of cardiac function loss was studied with the ex vivo working rat heart preparation (20). To compare the retreatment experiments, initial, and reirradiation doses were expressed as the percentage of the extrapolated tolerance dose (ETD) (1).

RESULTS

Local heart irradiation with a single dose led to a dose-dependent and progressive decrease in cardiac function. The progressive nature of irradiation-induced heart disease is shown to affect the outcome of the retreatment, depending on both the time interval between subsequent doses and the size of the initial dose. The present data demonstrate that hearts are capable of repairing a large part of the initial dose of 10 Gy within the first 24 h. However, once biological damage as a result of the first treatment is fixed, the heart does not show any long-term recovery. At intervals up to 6 months between an initial treatment with 10 Gy and subsequent reirradiation, the reirradiation tolerance dose slightly decreased from 74% of the ETDref (at 24-h interval) to 68% of the ETDref (at 6-month interval). Between 6 and 9 months, reirradiation tolerance dose dropped more even to 43% of the ETDref. Treatment of the heart with an initial dose of 17.5 Gy, instead of 10 Gy, 6 months prior to reirradiation, also led to a further decrease of the reirradiation tolerance dose (< 38 vs. 68% of the ETDref).

CONCLUSIONS

The outcome of the present study shows a decreased tolerance of the heart to reirradiation at long time intervals (interval > 6 months). This has clinical implications for the estimation of reirradiation tolerance in patients whose mediastinum has to be reirradiated a long time after a first irradiation course.

Re-irradiation tolerance in the rat spinal cord: influence of level of initial damage

The influence of the level of initial radiation damage on the long term recovery and re-irradiation tolerance in the rat spinal cord was investigated. Rats were irradiated with 0, 10, 20, 30 and 36 daily fractions of 2.15 Gy initially representing 0, 25, 50, 75 and 90% of cord tolerance. After an interval of 20 weeks, retreatments were given using graded single doses of X-ray. The end-point was paralysis of the forelimbs due to white matter necrosis. Latent times to paralysis were inversely proportional to the level of initial injury and retreatment doses. The retreatment ED50S were 19.0, 17.0, 15.7, 14.0 and 11.8 Gy for the control animals and animals irradiated initially with 10, 20, 30 and 36 fractions of 2.15 Gy respectively. Using the extrapolated response dose (ERD) concept, alpha/beta of 3.0 Gy, the retreatment ED50S in % ERD were 81, 70, 58 and 42% after initial doses of 25, 50, 75 and 90% ERD respectively. The level of initial injury appeared to influence the proportion of residual injury. For an initial injury of 25 and 90% of ERD, the respective residual injury was 74 and 65% of the initial damage; for an initial injury of 50 and 75% ERD, the residual injury decreased to 59 and 57% respectively. It is concluded that there was significant long-term recovery in the rat spinal cord, and that the level of initial radiation damage influenced both the retreatment tolerance and the time to expression of injury.

Fractionation sensitivity of the rat cervical spinal cord during radiation retreatment

Data concerning the fractionation sensitivity of normal tissues during radiation retreatment are limited. Experiments were performed to investigate whether the fractionation sensitivity of the rat cervical spinal cord is changed during retreatment 6 months after a first dose of 15 Gy, representing about half the biologically effective dose for induction of paresis. After a 6 months interval, the long-term recovery from the first treatment was about 45%. The fractionation sensitivity of the rat cervical spinal cord during reirradiation was not significantly different from the fractionation sensitivity of not previously irradiated control rats, with an alpha/beta ratio of 2.3 Gy in control rats and 1.9 Gy during reirradiation of the spinal cord. An additional observation from these experiments was the presence of incomplete repair after fractionated treatment with 2 fractions of 3 Gy per day with 10-h intervals.

Reirradiation tolerance of the immature rat spinal cord

The dose dependence and time course of long-term recovery in the cervical spinal cord of 3-week-old rats was investigated, and compared with the recovery in adults rats. At intervals of 1 to 168 days after initial irradiation of the cervical spinal cord at the age of 3 weeks, reirradiation experiments were performed to test the pattern of long-term recovery in immature spinal cord. The single dose ED50 for white matter mediated paresis was about 21 Gy for 3-week-old as well as adult rats, although the latency to paresis development increased from about 90 days in 3-week-old rats to about 250 days in adult rats. The main long-term recovery was seen during the first month after the initial radiation treatment at 3 weeks. This is in contrast to long-term recovery in adult rats, in which the main recovery took place between 2 and 6 months after the first irradiation. Calculations according to the LQ model showed that the extra dose that can be given to the cervical spinal cord after a 1-6 months interval in the 3-week-old rats reaches a maximum of about 20% of the total biological effect resulting in paresis. In adult rats the extra dose that can be given to the cervical spinal cord after a 6 months interval represents about 40% of the total biological effect. These studies show that time course as well as extent of long-term recovery from radiation treatment not only depends on tissue and species, but also on age.

A simple method for fitting curves to dose-effect data for functional damage

Dose-effect curves are used extensively to assess how tissues respond to radiation. One method of obtaining these is to fit a curve to the values of some measured effect plotted against dose using non-linear least-squares regression. This paper reports the use of a generalized (four-parameter) sigmoid equation fitted to all the individual data points, rather than to the mean values for each dose group, which eliminates the need to incorporate weighting of the data. The equation allows an analytical solution for values of isoeffect doses, which can be used, for example, to determine dose enhancement ratios, or equivalent remembered doses in top-up experiments. The regression approach can also determine both standard errors and 95% confidence limits on the mean predicted effect values from the fit to the data at all doses, and these define a uniform envelope of errors about the best-fit line, from which an error in an isoeffect dose can be assessed. This approach has been used to fit dose-effect data from a variety of normal tissues and tumours with highly satisfactory results.

Re-irradiation of mouse kidneys: a comparison of re-treatment tolerance after single and fractionated partial tolerance doses

Mouse kidneys were bilaterally irradiated with fractionated doses of 15 X 1.2 Gy, 15 X 1.6 Gy or 15 X 2.0 Gy. After an interval of 26 weeks the mice were re-irradiated with a range of single doses and functional kidney damage was measured (from clearance of [51Cr]EDTA) at monthly intervals until 36 weeks after re-irradiation. Re-irradiation tolerance was assessed from a comparison of dose-response curves for renal damage in re-treated mice compared with age-matched controls, which received only the re-treatment single doses. Fractionated irradiation markedly reduced the tolerance to subsequent re-irradiation after 6 months. The data indicated a continuous progression of the initial damage, even when the doses used were below the threshold level required to give measurable functional damage. These results were qualitatively similar to those previously published for re-irradiation of mouse kidneys after single doses. A quantitative comparison of the re-treatment tolerance after fractionated and single-dose irradiation indicated, however, that fractionated irradiation to partial tolerance was less damaging than predicted on the basis of the linear-quadratic (LQ) model. These results suggest either the failure of the LQ model for renal irradiation damage at low doses per fraction or, possibly, the presence of two subpopulations in the kidney: one population responsible for overt functional damage and the second population leading to subthreshold damage not normally expressed as functional impairment.

Long-term recovery and reirradiation tolerance of mouse bladder

The aim of this study was to investigate the influence of overall treatment time on the development and repair of radiation injury in the bladder. The bladders of mice were irradiated with 2 equal doses of X rays separated by 1 day, 3 months or 9 months. Additional groups of mice were given 8 or 16 Gy (approximately 20% and 60%, respectively, of a full tolerance dose) and subsequently reirradiated with a range of test doses after 3 or 9 months. Functional bladder damage was assessed from measurements of urination frequency and bladder compliance (measured cystometrically). There was a very early onset of functional damage (within 2 weeks) when mice were reirradiated 9 months after a dose of 16 Gy. Lower initial doses of 8 Gy did not alter the time of expression of damage after reirradiation. The early damage was probably due to stimulated cellular proliferation, which occurred after 16 Gy but not after 8 Gy, causing rapid expression of retreatment injury. Reirradiation tolerance for late bladder damage was inversely related to the dose given in the first treatment but was independent of the interval between treatments. There was no evidence for increased tolerance as the interval between treatments increased from 1 day to 9 months, which suggests that protracting the overall treatment time will not lead to sparing of late radiation damage in the bladder.

The lack of long-term recovery and reirradiation tolerance in the mouse kidney

Mouse kidneys were bilaterally irradiated with X-ray doses of either 6 or 10 Gy (equivalent to approximately 30 and 70 per cent of full tolerance respectively). After an interval of 2 or 26 weeks the mice were retreated with a range of test doses given as single or fractionated irradiation schedules. Functional kidney damage was measured (using clearance of [51Cr]EDTA) before retreatment and at monthly intervals up to 1 year after retreatment. Reirradiation tolerance was assessed from dose-response curves for renal damage in retreated mice compared with that in age matched controls which received only the second treatment. Damage from the initial radiation doses progressed, leading to a decreased reirradiation tolerance with time. There was no evidence for any recovery from functional damage in the interval between 2 and 26 weeks. These studies would strongly suggest that reirradiation of a previously irradiated kidney (even after low initial doses below tolerance) is likely to lead to severe renal damage. Despite the poor reirradiation tolerance at 26 weeks, there was no reduction in the capacity for repair of sublethal injury when fractionated irradiation was given to previously irradiated mice compared with controls.

Residual radiation-induced injury in dermal tissue: implications for retreatment

The evidence for residual radiation-induced injury has been investigated in the dermal vascular/connective tissue of pig skin at intervals of 17-52 weeks after irradiation. The primary irradiation was a single dose of 18 Gy, which represents the upper limit of "tolerance" to X irradiation of pig dermal tissue. Re-irradiation was with graded single doses of X rays in order to establish dose-effect relationships for the incidence of late ischaemic dermal necrosis of the skin; dose-effect curves obtained for previously irradiated skin were compared with those obtained using previously unirradiated skin in the same group of animals. At intervals of 17, 35 and 52 weeks after the primary treatment the resulting ED50 values for dermal necrosis were not significantly different from those obtained for previously unirradiated skin. This suggests little or no effective residual injury at these time intervals after a primary full "tolerance" dose. This conclusion was supported by the findings for the latency time for the development of dermal necrosis, which were similar in re-irradiated and previously unirradiated skin. Epithelial desquamation was not induced by the doses used in these studies, either after the primary treatment or after re-irradiation; however, the early erythema reactions seen in re-irradiated skin were markedly reduced, particularly when this was carried out after 35 and 52 weeks, when compared with skin that had not previously been irradiated. This suggests that the early erythema reaction may be a particularly poor predictor of late effects after the re-irradiation of the skin. Although the present results suggest that dermal and subcutaneous tissues may safely be retreated with a full tolerance dose at relatively early times after an initial radical treatment, caution is recommended in extrapolating these results to other late-responding normal tissues. In other tissues some persistent injury may be present even at very long time intervals after the primary treatment.