The effect of field size on the reaction of pig skin to single doses of X rays

Skin Response to Single and Fractionated Irradiation: Dynamic Modeling Approach

A biologically motivated mathematical model of the dynamics of swine skin epidermis after single acute irradiation () is extended to describe the effects of fractionated irradiation on this vital body system. The extended model, as the initial one, is a system of nonlinear ordinary differential equations whose variables and parameters have clear biological meaning. The performed studies of the model reveal a correlation between the dynamics of the dimensionless concentration of corneal cells of the upper skin layer and the dynamics of the fraction of the area of swine skin epidermis without moist desquamation under fractionated irradiation similar to that after single acute irradiation (). On the basis of these results, the formula, which allows computation of the dynamics of the moist desquamation in irradiated skin proceeding from the respective dynamics of skin epidermal cells, is proposed. It is demonstrated that the modeling predictions of the moist reaction in swine skin under fractionated irradiation in the wide range of total doses agree, on qualitative and quantitative levels, with the respective experimental data. All this bears witness to the validity of employment of the developed model, after appropriate identification, in the investigation and prediction of the effects of fractionated irradiation on skin epidermis in humans.

Epidermal homeostasis and radiation responses in a multiscale tissue modeling framework

The surface of the skin is lined with several thin layers of epithelial cells that are maintained throughout a lifetime by a small population of stem cells. High dose radiation exposures could injure and deplete the underlying proliferative cells and induce cutaneous radiation syndrome. In this work we propose a multiscale computational model for skin epidermal dynamics that links phenomena occurring at the subcellular, cellular, and tissue levels of organization, to simulate the experimental data of the radiation response of swine epidermis, which is very similar to human epidermis. Incorporating experimentally measured histological and cell kinetic parameters, we obtain results of population kinetics and proliferation indices comparable to observations in unirradiated and acutely irradiated swine experiments. At the sub-cellular level, several recently published Wnt signaling controlled cell-cycle models are applied and the roles of key components and parameters are analyzed. This integrated model allows us to test the validity of several basic biological rules at the cellular level and sub-cellular mechanisms by qualitatively comparing simulation results with published research, and enhances our understanding of the pathophysiological effects of ionizing radiation on the skin.

Dynamics of acutely irradiated skin epidermal epithelium in swine: modeling studies

A mathematical model, which describes the dynamics of acutely irradiated skin epidermal epithelium in swine, is developed. This model embodies the key mechanisms of regulation of skin epidermal epithelium and the principal stages of development of its cells (basal, prickle, and corneal). The model is implemented as a system of nonlinear ordinary differential equations, whose variables and parameters have clear biological meaning. The modeling results for the dose- and time-dependent changes in basal and prickle cell populations are in a good agreement with relevant experimental data. The correlation between the experimental data on the dynamics of moist reaction in acutely irradiated swine skin epidermal epithelium and the corresponding modeling results on the dynamics of corneal cells is revealed. Proceeding from this, the threshold level of corneal cells, which indicates the appearance of the moist reaction, is found. All this bears witness to the validity of employment of the developed model, after appropriate identification, in the investigation and prediction of radiation effects on skin epidermal epithelium in humans.

Development of a porcine skin injury model and characterization of the dose-dependent response to high-dose radiation

A porcine skin model was developed to characterize the dose-dependent response to high-dose radiation. The dorsal skin of a mini pig was divided into four paraspinal sections, with 11 small irradiation fields (2 cm × 2 cm) in each section, and a single fraction of 15, 30, 50 or 75 Gy was delivered to each section using a 6 MeV electron beam. A spectrophotometer measured gross skin changes, and a biopsy for each radiation dose was performed in the 1st, 2nd, 4th, 6th and 9th weeks for histology, immunostaining with anti-CD31, and western blotting with IL-6 and TGF-β1 to determine the degree of skin damage. After a 4-week latency period, erythema and dry desquamation, moist desquamation, and ulceration appeared at 4, 6 and 9 weeks, respectively. Gross skin toxicity was more pronounced, occurred early and continued to progress with irradiation >50 Gy, whereas complete healing was observed 12 weeks after 15 Gy. Spectrophotometry showed erythema indices rapidly increased during the first 4 weeks after irradiation. The number of eosinophils began rising sharply at 4 weeks and normalized after reaching peaks at 7-8 weeks. Microvessel density showed a biphasic pattern with a transient peak at 1 week, a nadir at 4-6 weeks, and maximum recovery at 9 weeks. Increase in the levels of IL-6 and TGF-β1 was detected soon after irradiation. Most of these parameters indicated complete healing of the skin 12 weeks after 15 Gy. Our porcine skin model provides an effective platform for studying high-dose radiation-induced skin injury, in particular histologic and molecular changes, during the early latency period.

An athymic rat model of cutaneous radiation injury designed to study human tissue-based wound therapy

PURPOSE

To describe a pilot study for a novel preclinical model used to test human tissue-based therapies in the setting of cutaneous radiation injury.

METHODS

A protocol was designed to irradiate the skin of athymic rats while sparing the body and internal organs by utilizing a non-occlusive skin clamp along with an x-ray image guided stereotactic irradiator. Each rat was irradiated both on the right and the left flank with a circular field at a 20 cm source-to-surface distance (SSD). Single fractions of 30.4 Gy, 41.5 Gy, 52.6 Gy, 65.5 Gy, and 76.5 Gy were applied in a dose-finding trial. Eight additional wounds were created using the 41.5 Gy dose level. Each wound was photographed and the percentage of the irradiated area ulcerated at given time points was analyzed using ImageJ software.

RESULTS

No systemic or lethal sequelae occurred in any animals, and all irradiated skin areas in the multi-dose trial underwent ulceration. Greater than 60% of skin within each irradiated zone underwent ulceration within ten days, with peak ulceration ranging from 62.1% to 79.8%. Peak ulceration showed a weak correlation with radiation dose (r = 0.664). Mean ulceration rate over the study period is more closely correlated to dose (r = 0.753). With the highest dose excluded due to contraction-related distortions, correlation between dose and average ulceration showed a stronger relationship (r = 0.895). Eight additional wounds created using 41.5 Gy all reached peak ulceration above 50%, with all healing significantly but incompletely by the 65-day endpoint.

CONCLUSIONS

We developed a functional preclinical model which is currently used to evaluate human tissue-based therapies in the setting of cutaneous radiation injury. Similar models may be widely applicable and useful the development of novel therapies which may improve radiotherapy management over a broad clinical spectrum.

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.

Radiation therapy in the elderly

The purpose of this review is to highlight aspects of radiation oncology specifically related to aging and caring for the older patient with cancer. Particular emphasis is placed on the preclinical and clinical studies focusing on the efficacy and toxicity of RT in this population. Special techniques are also reviewed that have particular relevance to the treatment of the elderly.

Dose-volume effects in rat thoracolumbar spinal cord: an evaluation of NTCP models

PURPOSE

To evaluate models for normal-tissue-complication probability (NTCP) on describing the dose-volume effect in rat thoracolumbar spinal cord.

METHODS AND MATERIALS

Single-dose irradiation of four field lengths (4, 1.5, 1.0, and 0.5 cm) was evaluated by the endpoints paresis and white-matter necrosis. The resulting dose-response data were used to rank phenomenological and tissue architecture NTCP models.

RESULTS

The 0.5-cm field length showed a steep increase in radiation tolerance. Statistical analysis of the model fits, which included evaluation of goodness of fit (GOF) and confidence intervals, resulted in the rejection of all the models considered. Excluding the smallest field length, the Schultheiss (D(50) = 21.5 Gy, k = 26.5), the relative seriality (D(50) = 21.4 Gy, s = 1.6, gamma(50) = 6.3), and the critical element (D(50,FSU) = 26.6 Gy, gamma(50,FSU) = 2.3, n = 1.3) model gave the best fit.

CONCLUSION

A thorough statistical analysis resulted in a serial or critical-element behavior for the field lengths of 1.0 cm and greater. Including the 0.5-cm field length, the radiation response markedly diverged from serial properties, but none of the models applied acceptably described this dose-response relationship. This study suggests that the commonly assumed serial behavior of the spinal cord might be valid for daily use in external- beam irradiation.

[Evaluation of late radiation-induced changes in superficial microcirculation after acute beta-irradiation. II. Prognostic importance of cutaneous Doppler laser]

OBJECTIVE

The changes that occur in the tissular microcirculation after accidental acute irradiation account for some of the early effects of such irradiation, especially at the cutaneous level. The prognostic importance of the cutaneous laser doppler was tested in an experimental model of acute beta-irradiation.

METHODS

Ten pigs were given beta-irradiation with a high single localized dose of 90Sr/90Y (32 or 64 Gy, 7 mg/cm2) delivered to the flank, and were evaluated 2, 7, 14, 21 and 28 days thereafter. Each individual was its own control. The local microcirculation was measured in the resting state and during thermal stimulation at 42 degrees C, using a Periflux cutaneous Doppler laser with p413 probes. Three periods of six minutes each were continuously recorded: period 1 (P1) represented basal resting cutaneous perfusion, with the slope p corresponding to the increase in perfusion when two minutes of thermal stimulation at 42 degrees C began; P2 to plateau perfusion during this stimulation; and P3 to perfusion on the return to equilibrium.

RESULTS

After acute beta-irradiation in the pig, all the cutaneous microcirculation parameters measured (P1, p, P2 and P3) had risen at day 2 in the irradiated area by a factor of 2 to 4, depending on the dose (p < 0.001), compared to the adjacent control area. On the other hand, as from day 7, the resting and the stimulated microcirculation varied little, except for a reduction of the slope p by a factor of 2 (p < 0.05) after the strongest radiation dose.

CONCLUSION

After acute irradiation, the increase in the resting cutaneous microcirculation may correspond to immediate but transitory capillary vasodilatation that accompanies the initial erythema in accidental irradiation. The absence of vascular response to thermal stimulation seems to be a good means of reaching an early diagnosis of delayed cutaneous radiation necrosis.

The volume effect in radiotherapy--its biological significance

Volume-related effects are an important consideration in clinical radiotherapy. The early rationale for the need to consider treatment volume has become clouded by the lack of clarity and a misinterpretation of some early clinical findings. In particular, there is a need to separate our understanding of biologically iso-effective radiation responses from the clinical concept of normal tissue tolerance, as they relate to changes in treatment volume. Animal data, including those for large animals, are reviewed. These animal studies indicate the need for caution in extrapolating retrospective clinical data to new treatment situations, since the conclusions reached may have been dictated by dogma and not by careful consideration of different factors that may have been involved. These include anatomical and physiological factors, and variations in the dose distribution pattern to a specific organ or tissue. These biological factors could limit the general applicability of simple approaches based on mathematical models to the volume effect relationship in radiotherapy.

Factors influencing the degree of erythematous skin reactions in humans

Dose-response relationships have been studied using an ordinal visual scale and reflectance spectrophotometry data from 123 treatment sites on 110 patients treated with 10 dose fractions over 12-14 days. Dose rates varied between 3 and 240 Gy/h and total doses of between 25 and 41 Gy were given using teletherapy apparatus. We found qualitative scoring of erythematous skin reactions to be subject to considerable inter- and intra-observer variation. Reflectance spectrophotometry provided more reproducible information, some of which was undetectable by naked eye. Baseline erythema readings were significantly higher in male patients and at anatomical sites of previous heavy UV exposure. In addition, a pronounced decline in erythema readings during the second week of therapy and 'reciprocal vicinity' (abscopal) effects adjacent to the field, undetected by the eye, were observed in a subset of patients. Meaningful dose-response relationships could be derived only from reflectance data with peak change from the pretreatment baseline measure providing the best discrimination. Peak erythema measures following treatment were found to depend on the age and gender of the patient as well as the treatment site and its baseline erythema measurement. This was independent of the total dose administered or the instantaneous dose rate at which it was delivered. The rate of erythema development was also dose rate dependent but only weakly dependent on the biological dose intensity (Gy equiv./day) of the treatment course. The data raise the question of whether irradiation-induced erythema is exclusively a secondary phenomenon occurring as a result of basal cell killing. The short repair half time value of 0.06 h obtained by direct analysis is perplexing and may reflect a dose rate-dependent physiological vasodilatory response to irradiation and/or a multi-component cellular repair process.

Damage and morbidity from pneumonitis after irradiation of partial volumes of mouse lung

PURPOSE

The aims of this study were to: (a) define the relationship of dose and volume irradiated to damage and morbidity in mouse lung, (b) determine the threshold volume for morbidity after partial lung irradiation; and (c) determine whether the response to radiation of mouse lung is independent of the region irradiated.

METHODS AND MATERIALS

C3Hf/Kam female mice were used in this study. The fractional volume of the lung to be irradiated was determined by two methods, weights and computed tomography (CT) scanning. Two experiments were performed to define the volume effect and to determine whether the response of the mouse lung to radiation was homogeneous. In the first experiment, single doses of x-rays ranging from 12 to 20 Gy were given to partial volumes of 84%, 70%, and 40% including the base, 50%, 33%, and 17% including the apex, to 43% in the middle, and to the sum of 57% as 17% in the apex and 40% in the base. In the second experiment, the same volumes of 50% and 70-75% in the apex and base of the lung were irradiated with single doses ranging from 12-19.25 Gy. Morbidity from radiation pneumonitis was quantitated by two end points, breathing rate and lethality between 12 and 32 weeks after irradiation. Damage was assessed by histopathological evidence of pneumonitis.

RESULTS

Clear well-defined dose-response curves were obtained for both breathing rate and lethality after all volumes irradiated. There was a clear volume-dependent shift of the dose-response curves for breathing rate and lethality at 28 weeks after irradiation, the end of the pneumonitis phase of damage, to higher doses compared with these data after whole-lung irradiation. In addition, the slopes of the dose-response curves for irradiation of partial lung volumes were more shallow compared to those after whole-lung irradiation. Increases in breathing rate correlated with lethality when the volume irradiated was equal to or greater than 50% of the reference volume. However, after irradiation of volumes smaller than 40%, breathing rate increases were not accompanied by death. A heterogeneous response of the mouse lung to radiation was observed in the first experiment and confirmed by the second experiment. For a given volume irradiated, the isoeffect dose was always less for the base than for the apex of the lung. The threshold volume for breathing rate changes was less than 17 and 40% when the irradiated volumes involved the apex and base, respectively. For lethality, the threshold volume was between 40 and 70% for the base and greater than 50% for the apex of the lung. Finally, damage as assessed by histological evidence of pneumonitis was observed in the irradiated area only.

CONCLUSIONS

(a) The volume effect was resolvable in mice, (b) the volume effect in mouse lung exhibits a clear threshold for morbidity, (c) the threshold volume for morbidity is dependent on the end point, (d) the response of mouse lung is heterogeneous, dependent on the site irradiated, and is always greater for the same volumes irradiated in the base than the apex, and, (e) histopathological damage does not always produce observable morbidity.

Effects of field size on the survival of pig epidermal colony-forming in situ after electron irradiation

The survival of colony-forming cells in pig epidermis after irradiation was measured using different electron field sizes. The sensitivity of the colony-forming cells was characterized by D(o) = 2.5-3.0 Gy. There was an effect of field size, described approximately by: Dose (to give five colonies/cm2) (Gy) = (31 +/- 2) x Area-(0.048 +/- 0.015). The effect of field size was less than found previously using tolerance to skin reactions (area exponent = 0.16). This work indicates for the first time that the effects of different large field sizes in skin can be detected at the level of colony-forming cell survival.

Modification of late dermal necrosis in the pig by treatment with multi-wavelength light

Low-level light from a multi-wavelength light source has been used to prevent late X-ray-induced dermal necrosis in the pig. Skin fields, measuring 4 cm x 4 cm on the flank, were irradiated with graded doses of X rays and the incidence of late dermal necrosis at 10-16 weeks after irradiation was scored. The control skin sites were irradiated only with 250 kV X rays but the test skin sites were subsequently exposed to low-level light. Local light exposure was from an array of gallium aluminium arsenide diodes, which produced wavelengths of 660, 820, 880 and 950 nm, pulsating at 5 kHz. Light treatment was given three times a week, from 6-16 weeks after X irradiation. Each treatment session was 1 min, which was equivalent to energy density of 1.08 Jcm-2. Light treatment increased the ED50, the dose which causes dermal necrosis in 50% of the irradiated skin fields, from 20.10 +/- 0.12 Gy to 21.94 +/- 0.30 Gy. This difference, although small, was highly significant (p < 0.001) and was equivalent to a dose modification factor (DMF) of 1.09. The effect of light treatment was minimal at incidence levels of less than the 50% but greater at higher levels of effect. These findings suggest that low-level light, when applied appropriately, may be useful in the prevention of late X-ray-induced damage to the dermis.

Prevention of X-ray-induced late dermal necrosis in the pig by treatment with multi-wavelength light

Low-level light from a multi-wavelength array of light sources has been used to prevent late X-ray-induced dermal necrosis in the pig. Skin fields, measuring 4 x 4 cm on the flank, were irradiated with a single dose of 23.4 Gy of X-rays. This X-ray dose was associated with the development of a 100% incidence of dermal necrosis, 10-16 weeks after irradiation. These irradiated skin sites were subsequently exposed to light of 660, 820, 880, and 950 nm wavelengths from a gallium aluminium arsenide multiple wavelength multidiode cluster probe (Biotherapy Medical Laser, 3ML), three times a week, from 4 to 16 weeks or 6 to 16 weeks after X-irradiation. The skin fields were exposed to the light pulsating at either 2.5 Hz or 5 kHz. With light pulsating at 5 kHz, energy densities of 0.22, 0.54, 1.08 2.16, 4.32, and 10.8 J/cm2 were used. Treatment with light pulsating at 2.5 Hz, 6-16 weeks after X-irradiation, or treatment with light pulsating at 5 kHz, 4-16 weeks after X-irradiation, did not have a significant effect on the incidence or the latency for the development of ischemic dermal necrosis irrespective of the exposure time to light at each treatment. With light pulsating at 5 kHz, no effect of light dose was observed. However, the overall incidence of dermal necrosis was significantly reduced (P = 0.001) to 52% in the X-irradiated fields receiving treatment with 5 kHz light, 6-16 weeks after X-irradiation.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies of dose-fractionation on early and late responses in pig skin: a reappraisal of the importance of the overall treatment time and its effects on radiosensitization and incomplete repair

Studies in pig skin have examined the effects of dose fractionation on the acute radiation response. The variation in ED50 values for moist desquamation for doses given as 1-48 fractions over less than or equal to 16 days were best fitted by a log-log plot of iso-effect dose against the number of fractions; the slope of this plot indicated a fraction number exponent (N) of 0.42 +/- 0.007. Based on the assumptions made in applying the linear-quadratic (LQ) model, the alpha/beta ratio was found to decrease with decreasing per fraction: for doses given as 6-27 Gy per fractions the alpha/beta ratio was 8.74 +/- 0.48 Gy, whereas for doses of 2.55-6 Gy per fraction it was only 0.85 +/- 0.29 Gy. A simple approach to a time factor could not be used to calculate iso-effect doses for acute reactions in pig skin when treatment time was increased from less than or equal to 16 days to 28-39 days. This was due to the opposing effects of radiosensitization and repopulation when the cell cycle time of epidermal basal cells was shortened. For late dermal necrosis in pig skin, repair of sublethal damage was not completed in 24 hr. This finding has a significant effect on the interpretation of the results of fractionation studies using this late endpoint. Expressed in terms of a simple power-law function, there was a significant change in the fraction number exponent "N" from 0.43 +/- 0.007 to 0.37 +/- 0.006 for the complete and incomplete repair data, respectively. Many of the fractionation effects reported for acute and late damage to pig skin would appear to be in excellent agreement with those for human skin.

Dose-volume correlation in radiation-related late small-bowel complications: a clinical study

The effects of the volume of irradiated small bowel on late small-bowel tolerance was studied, taking into account the equivalent total dose and type of pre-irradiation surgical procedure. A method was developed to estimate small-bowel volumes in the high-dose region of the radiation treatment using CT-scans in the treatment position. Using this method small-bowel volumes were measured for three-field and AP-PA pelvic treatments (165 cm3 and 400 cm3, respectively), extended AP-PA pelvic treatment (790 cm3), AP-PA treatment of para-aortic nodes (550 cm3) and AP-PA treatment of para-aortic and iliac nodes (1000 cm3). In a retrospective study of 111 patients irradiated after surgery for rectal or recto-sigmoid cancer to a dose of 45-50 Gy in 5 weeks, extended AP-PA pelvic treatment (n = 27) resulted in a high incidence of severe small-bowel complications (37%), whereas for limited (three-field) pelvic treatment (n = 84) the complication rate was 6%. These complication data together with data from the literature on postoperative radiation-related small-bowel complications were analysed using the maximum likelihood method to fit the data to the logistic form of the dose-response relation, taking the volume effect into account by a power law. The analysis indicated that the incidence of radiation-related small-bowel complications was higher after rectal surgery than after other types of surgery, which might be explained by the development of more adhesions. For both types of surgery a volume exponent of the power-law of 0.26 +/- 0.05 was established. This means that if the small-bowel volume is increased by a factor of 2, the total dose has to be reduced by 17% for the same incidence of small-bowel complications.

The skin: its structure and response to ionizing radiation

The response of the skin to ionizing radiation has important implications both for the treatment of malignant disease by radiation and for radiological protection. The structural organization of human skin is described and compared with that of the pig, with which it shows many similarities, in order that the response of the skin to ionizing radiation may be more fully understood. Acute radiation damage to the skin is primarily a consequence of changes in the epidermis; the timing of the peak of the reaction is related to the kinetic organization of this layer. The rate of development of damage is independent of the radiation dose, since this is related to the natural rate of loss of cells from the basal layer of the epidermis. Recovery of the epidermis occurs as a result of the proliferation of surviving clonogenic basal cells from within the irradiated area. The presence of clonogenic cells in the canal of the hair follicle is important, particularly after non-uniform irradiation from intermediate energy beta-emitters. The migration of viable cells from the edges of the irradiated site is also significant when small areas of skin are irradiated. Late damage to the skin is primarily a function of radiation effects on the vasculature; this produces a wave of dermal atrophy after 16-26 weeks. Dermal necrosis develops at this time after high doses. A second phase of dermal thinning is seen to develop after greater than 52 weeks, and this later phase of damage is associated with the appearance of telangiectasia. Highly localized irradiation of the skin, either to a specific layer (as may result from exposure to very low energy beta-emitters) or after exposure to small highly radioactive particles, 'hot particles', produces gross effects that become visibly manifest within 2 weeks of exposure. These changes result from the direct killing of the cells of the skin in interphase after doses greater than 100 Gy. Dose-effect curves have been established for the majority of these deterministic endpoints in the skin from the results of both experimental and clinical studies. These are of value in the establishment of safe radiation dose limits for the skin.

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.

The effects of single doses of X rays on the mechanical properties of pig skin in vivo

Changes in the mechanical properties of pig skin have been studied in vivo, using a dermal extensometer, after irradiation with a single dose of 18 Gy of X rays. There was no significant change in the stiffness of irradiated skin, when compared with unirradiated skin, until 9 weeks after irradiation when the irradiated skin was significantly stiffer. This effect was also found at 12 and 15 weeks after irradiation. When the increase in skin thickness, as a consequence of oedema, was taken into account a significant increase in the unrelaxed elastic modulus of irradiated skin was only seen at 12 and 15 weeks after irradiation. There were no significant changes in force relaxation, after extension of the skin, over this time period. After the resolution of oedema, which was associated with a significant 20% reduction in the thickness of irradiated skin relative to unirradiated skin, the mechanical properties of irradiated skin were not markedly different from those of unirradiated skin. However, between 30 and 39 weeks after irradiation there was a further wave of dermal thinning, resulting in a total reduction in the thickness of irradiated skin relative to unirradiated skin of 26%. This was associated with a rapid rise in the skin stiffness and unrelaxed elastic modulus by approximately 65 and approximately 140%, respectively. It was only at these late times after irradiation that the force relaxation of the skin was modified significantly. At 9 and 12 weeks after irradiation the reduction in skin stiffness and the unrelaxed elastic modulus were dose related. Based on the percentage of fields showing a significant reduction in these biomechanical parameters, ED50 values of between 12 and 14.5 Gy were established. This would appear to be a sensitive method for assessing radiation-induced dermal changes since few gross changes are observed in this dose range.

An investigation into the combined effects of X-irradiation and 3MHz ultrasound-induced hyperthermia on pig skin

The thermal enhancement of radiation-induced damage in pig skin has been investigated. Heating at 43 degrees C for 60 min was produced by a scanned 3MHz ultrasound transducer, immediately after single doses of X rays. The ED50 values for the dermal reactions of dusky/mauve erythema and necrosis after irradiation alone were 18.6 +/- 0.5 Gy and 20.5 +/- 0.4 Gy, respectively. The reduction in the ED50 values to 15.3 +/- 0.4 Gy and 17.7 +/- 0.5 Gy after irradiation plus heating was significant and suggested a thermal enhancement ratio (TER) of between 1.15 and 1.22. These TER values were within the range obtained in both pig and rat skin using other 'dry' heating methods. This would suggest that the non-thermal effects of ultrasound do not influence the thermal enhancement of x-irradiation damage.

Treatment volume and tissue tolerance

Hypotheses regarding tissue organization and radiation response were described and, on the basis of simple modeling, tentative conclusions were drawn regarding the influence on "tolerance" doses of tissue organization and the volume of an organ irradiated. (1) A functional subunit (FSU) may be defined structurally (e.g. as in a nephron), or only functionally, as the largest unit of cells capable of being regenerated from a surviving clonogenic cell without loss of the specified function. (2) Functional subunits may be arranged in parallel or in series: in parallel they give rise to graded dose responses, whereas in series, they give rise to threshold binary, or quantum responses. (3) Tolerance doses are a function of the number and radiosensitivity of target cells in an FSU, tissue organization and the functional reserve (i.e. the proportion of functional subunits necessary for adequate organ function). (4) An influence of treatment volume on "tolerance" doses is more likely to depend upon tissue organization than upon differences in cellular radiosensitivity. (5) The volume of tissue irradiated would be expected to be irrelevant to the "tolerance" of tissues showing a graded dose response (e.g. skin desquamation), except when the injury becomes severe, when tolerance for a large severe wound is likely to be less than for a small volume of injury, for non-radiobiological reasons. (6) Tissues with FSU's arranged in series, for example, spinal cord or peritoneal sheath along small bowel, should show a threshold-binary response. Sigmoid dose response curves should have a lower threshold and be steeper the larger the treatment volume. The effect of increase in volume is greatest with changes in small volumes: once a large number of FSU's are being irradiated, a further increase in volume has little effect on the position or slope of the probability curve for such complications. (7) Because their sigmoid dose-response curves are step, threshold-binary tissues are sensitive to small increases in "biologically effective" dose. For example, when the spinal cord is treated with large dose fractions the biological effectiveness per unit of dose increases sharply. Thus, using a large volume together with large dose fractions, as could happen in palliative prescriptions, could augment the danger of myelitis.(ABSTRACT TRUNCATED AT 400 WORDS)

Experimental irradiation of the rat ureter: the effects of field size and the presence of contrast medium on incidence and latency of hydronephrosis

Following X-irradiation of a 1.5 cm length of rat ureter, hydronephrosis developed after doses down to 10 Gy. The estimated ED50 was 11.8 Gy. In the dose range 37.4 Gy to 17.5 Gy there was a significant increase in latency with decreasing dose, but at lower doses the latency did not increase further. Reducing the length of ureter irradiated to 0.5 cm or 0.8 cm caused a decrease in incidence of hydronephrosis and longer latency periods. The ED50 for rats irradiated to 0.5 cm of ureter was 29.6 Gy. The possibility that secondary radiation produced as a result of interaction between X-radiation and iodinated contrast medium might affect the radiation induction of hydronephrosis was investigated. No difference was found between groups of rats irradiated with or without injection of contrast media.

An investigation of changes in relative biological effectiveness (RBE) with depth for X ray beams generated between 100 and 250 kVp using the mouse foot as biological test system

For the early mouse foot skin reactions an increase in RBE relative to 60Co has been found for the irradiations with X ray beams generated between 100 and 250 kVp at both the surface and depth, respectively. In most cases, the RBE of the late skin reactions for irradiations at the surface and at depth are significantly lower than those for the early reactions. The clinical implications of these results, the effect of a possible hypoxic fraction of cells in the skin, and the choice of a "standard" radiation modality for radiobiological experiments are discussed.

The time-dose relationship for radiation-induced lung damage in pigs

The dose-time relationship of radiogenic pneumopathy was studied in 68 young pigs. The right lungs of the animals were irradiated by means of a telecobalt unit. Dose distribution calculations and the preparation and execution of irradiation were chosen to largely parallel the treatment of patients. The radiation response of the lungs was quantified by chest-X-rays, functional studies and histological and biochemical investigations of the autopsy specimens. The dose which produced unambiguous signs of radiation pneumopathy in at least 50% of the individuals (ED50), was used to draw an iso-effect plot. A steep slope of the iso-effect line was found when the number of fractions was increased from 5 to 15. In contrast, the iso-effect line showed only a small slope when the influence of overall time was investigated. Significant differences between doses for early and late reactions of the lungs could not be detected. The radiogenic lung reaction in pigs is adequately described by the relation D approximately N0.32 X T0.05 and an alpha/beta value of 3.7 Gy. These results are in a good agreement with those derived for mice by other investigators. The good comparability of the biological findings obtained with pigs to man points to the application of this dose-time relation in radiotherapeutical practice.

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.

Split-dose recovery in epithelial and vascular-connective tissue of pig skin

In the first 16 weeks after irradiation, two distinct waves of reaction can be observed in pig skin; the first wave (3-9 weeks) represents the expression of damage to the epithelium while the second is indicative of primary damage to the dermis, mediated through vascular injury. Following beta-irradiation with a strontium-90 applicator, a severe epithelial reaction was seen with little subsequent dermal effects. X-rays (250 kV) on the other hand, produced a minimal epithelial response at doses which led to the development of dermal necrosis after 10-16 weeks. Comparison of single doses with two equal doses separated by 24 h produced a D2-D1 value of 7.0 Gy at the doses which produced moist desquamation in 50% of fields (ED50) after strontium-90 irradiation. After X-irradiation comparison of ED50 doses for the later dermal reaction suggested a D2-D1 value of 4.5 Gy. Over this same dose range of X-rays the D2-D1 value for the first wave epithelial reaction was 3.5 Gy. These values of D2-D1 for epithelial and dermal reactions in pig skin were compared with published data and were examined in relation to the theoretical predictions of a linear quadratic model for tissue target cell survival. The results were broadly in keeping with the predictions of such a model.

The relative biological effectiveness of fast neutrons (42MeVd leads to Be) for early and late normal tissue injury in the pig

Early and late radiation damage has been investigated in a number of normal tissues in the pig after irradiation with single doses of neutrons produced by 42MeV deuterons on beryllium. The results have been compared with data obtained after irradiation with single doses of 250kV X rays. In the skin a low RBE value of approximately 1.2 was obtained for the early (3-9 week) epithelial reaction. For the subsequent dermal vascular response, higher RBE values in the range of 1.35-1.6 were obtained: the RBE decreasing with an increase in the neutron dose. For late skin damage, assessed by the relative reduction in the linear dimensions of an irradiated field, a RBE value of approximately 1.5 was obtained. In the kidney the RBE value, for a neutron dose level (550 cGy) at which renal function was just preserved, was 2.0. A lower value of 1.7 was found for doses resulting in a loss of renal function. The results of 133Xenon clearance studies showed two waves of impaired ventilation function in the irradiated lung. In the acute reaction (3-9 months), at a dose level consistent with just preserving normal ventilation function, the RBE value was less than 1.2. For late lung damage (15-24 months) the RBE value was higher, 1.4. For the rectum, methods are presently only available for assessing acute damage. A RBE of 2.0 was found for neutron doses in the range 350-575 cGy. The RBE values for early endpoints in the skin, lung and gut of the pig are comparable with those published previously for other species, including man. The values for late effects in pig skin and lung were higher than for early damage in those tissues.