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Pig skin was used as a model to study the effectiveness of two topically applied creams, Lipochromin and Levosinum, in modifying the development of both early and late radiation damage to pig skin. Irradiated skin sites that received daily topical application of Levosinum or Lipochromin after exposure were compared with sites on the contralateral flank of the same animal that received irradiation only. Irradiation was with graded doses of 90Sr/90Y beta-rays. Incidence of moist desquamation (acute) and ischaemic dermal necrosis (late) were used as end-points. The latency period for the development of moist desquamation and its healing time was also assessed. The latency period for the development of moist desquamation in this model ranged from 4.00-6.75 weeks. There was no significant difference between the cream treatment and control sites. Application of Levosinum shortened the healing time of moist desquamation at each dose level by 5-10 days. In three out of four dose levels used, this shortening of the healing time was statistically significant (p0.03). Treatment with these topical applications also reduced the incidence of late dermal necrosis and increased the ED50 values for the incidence of dermal necrosis. This increase in ED50 values was equivalent to a dose modification factor of 1.11-1.13.

Data describing the response of several normal tissues to fractionated irradiation, in terms of a biphasic repair of sub-lethal damage, have now been published. Typical results of such analyses have been taken and applied to a conventional radiotherapy protocol of 60 Gy in 30 daily fractions. The effect of using a four field treatment plan is shown to reduce the biological effect of the radiation schedule by increments dependent upon the time interval between each field in a treatment fraction, with a 10% reduction in the extrapolated dose response (ERD) resulting from a 5 min interfield interval. When applied to tissues having the same repair characteristics as pig skin this reduction in ERD is predicted to result in an approximately 25% reduction in the probability of acute morbidity from a protocol of 60 Gy in 30 fractions. These results imply that the basic LQ model, which is unable to correct for interfield intervals, overestimates the effect on normal tissues of radical clinical protocols, most of which use more than a single field. Increasing the interfield interval could be used to reduce the normal tissue side effects from radical radiotherapy when multiple fields are used.

The early and late effects of X rays on lymphatic microcirculatory function have been investigated in pig skin. Paired estimates of lymph clearance were performed in irradiated and unirradiated sites on the flank skin of anaesthetized Large White pigs at 3, 6, 9, 12, 26, 39, 52, 64 and 78 weeks after a single dose of 18 Gy of X rays. The dermal clearance of 99Tcm-colloid was used to assess lymph flow. The results demonstrated two waves of impaired lymphatic clearance which correlated well with the gross morphological changes observed. The first wave occurred between 6 and 12 weeks after irradiation and corresponded to a period of ischaemia and oedema. The second wave, at 52 weeks, corresponded with dermal thinning and subcutaneous atrophy. X irradiation resulted in an impairment of lymph drainage both within and outside the field of irradiation. Impaired lymphatic drainage probably contributes to the gross and histological changes observed in the skin after X irradiation.

Localized irradiation of the skin and subcutaneous tissues with large single doses of gamma-rays can induce delayed effects characterized by fibrosis which invades the irradiated tissues. In this study the depth of penetration of muscle fibrosis was measured in the pig 30 weeks after irradiation of the skin surface with single doses of 192Ir gamma-rays of 16-256 Gy. Irradiation was directed either to the outer side of the thigh or to the back, close to the mid-dorsal line. Fibrosis only developed in irradiated muscle after doses that induced moist desquamation of the skin in the acute phase of the reaction, i.e. after skin surface doses of 48-64 Gy. In skeletal muscles, the limit of fibrotic expansion was reached at a depth dose of 14 +/- 4 Gy (+/- SD) for skin surface doses exceeding 48 Gy.

There is currently considerable interest in the possible modulation of normal tissue responses to radiation, to improve the therapeutic ratio in radiotherapy. Recent studies in pig skin have suggested that essential fatty acids, administered after radiation, can modulate the expression of both acute and late responses in this tissue [1,2]. There is also evidence that non-steroidal anti-inflammatory drugs will also modify the response of normal skin to irradiation [3].

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.

Early and late changes in the functional vascularity of pig skin have been evaluated after X-irradiation. These changes were assessed using a quantitative isotopic clearance technique. Two waves of radiation-induced changes in isotopic clearance were demonstrated. The first wave, observed after an interval of three weeks, was characterized by the faster clearance of the tracer from areas of irradiated skin. The second wave, which reached a peak after 12 weeks, was denoted by a slowing of the clearance rate relative to normal skin. The modification in vascular function observed at 12 weeks was transient, function returning to normal as late radiation damage developed. The changes observed in the pig have many similarities with those recorded in man but are markedly different from those found in rodents. These differences are discussed with respect to the vascular structure of the skin of the animal species investigated.

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.

The importance of the size of the treatment area for the response of the skin to radiation has been studied in the pig. The responses of skin areas of 16 cm3 (4 x 4 cm) and 64 cm2 (16 x 4 cm) were compared after single doses of X rays. In the initial 3-9-week period after irradiation the severity of the erythema reaction, which is associated with epidermal call death, was not influenced by the area of skin irradiated. For the later dermal response (10-16 weeks) a similar result was obtained. The dose required to produce dermal necrosis in 50% of the fields treated (ED50) was approximately 2070 cGy for both field sizes. Additional studies have shown that the ED50 for dermal necrosis was not influenced by the age of animals at the time of irradiation. This was despite considerable differences in the vascular density and blood flow in pig skin with increasing age. The apparent contradiction between the results of this experimental study in the pig, which shows no effect of field size, and currently accepted clinical practice is discussed.

The development of subcutaneous fibrosis after X irradiation in courses of 1, 6 or 30 fractions has been studied quantitatively in the domestic pig. The NSD formula fails to predict the correct variation in dose for changes in the number of fractions so as to produce comparable levels of connective tissue damage even over the clinically used range from 6f./18d. to 3Of./39d.; the error in the prediction may amount to an excess dose of circa 30 per cent. The slope the iso-effect curve between 6 and 30 fractions is 0·46, compared with the combined fractionation and time factors of 0·33 in the NSD formula. The degree of late fibrosis did not correlate with the severity of the early skin reaction. The results suggest that the NSD formula is not suitable for use when planning new treatment schedules in which the number of dose fractions is reduced.

The skin of pigs has been used to study the effect of fractionated irradiation with fast neutrons of mean energy 6 MeV. Six, nine and 15 fractions have been used over a period of three weeks, and two fractions divided by short intervals of time up to 24 hours. The effects of fractionated treatments with X rays are also summarised. The RBE of the neutron beam rises steadily from 2·3 for a single dose to 3·6 for 15 fractions. Observations up to five years after treatment have failed to show any sign of increased late reactions after neutrons. There is less recovery during a fractionated course of treatment with fast neutrons than with X rays, the relative amount of recovery being 0·6. Very little recovery was observed during the first seven hours after treatment with neutrons. Reactions occurring after fractionated treatments with X rays show that the number of fractions is more important than the time over which the treatment is extended, up to 28 days, but that prolongation is nevertheless a sign...

Fifteen fields on one pig were irradiated with 1, 2, 2, 3 and 5 fractions in 1, 5, 2, 3 and 5 days respectively. Skin reactions were recorded up to 80 or 100 days, so that the doses which produced equal reactions could be assessed. Three degrees of damage were analysed for early, medium and late reactions. As an example, the doses to produce one level of reaction over 30–85 days were respectively 2,050, 2,560–2,750, 2,750, 3,360 and 4,060 rads for the fractions listed above. The corresponding values of “DQ” = (Dn — D1)/(n —; 1) were high, 500–700 rads. The values of (Dn − D1)/(n − 1) became smaller with larger numbers of fractions. Two pigs were irradiated to investigate the “reciprocal vicinity” effect. It was found to be small for the field arrangements used, but the sensitivity of pig skin increased in the dorso-ventral direction and corrections were sometimes necessary. Two pigs were irradiated with pairs of equal doses separated by 0 to 24 hours. A recovery pattern was observed similar to that found ...

We acknowledge with interest the letter from Drs Guttenberger and Thames commenting on our re-analysis of data on the kinetics of repair of sublethal radiation damage (Hopewell & van den Aardweg, 1988). We welcome this opportunity to respond since the topic has important implications for clinical radiotherapy. Reference to this very recently submitted letter was made in two previous publications by one of the authors (Thames, 1989; Thames et al, 1989). Before discussing the important points raised by their letter, it is first essential to correct an unjustified assertion made in the above-mentioned publications (Thames, 1989; Thames et al, 1989), namely that any analyses based on plots of the percentage unrepaired dose are hampered by the selective exclusion of data points, specifically those with the longer interfraction intervals, and that this biases the results. In the studies carried out on pig skin, no data points were excluded or repeated to obtain a favourable result.

In a recent paper (Berry et al., 19741) pig skin experiments led to the conclusion that the NSD formula failed to correctly represent iso-effect doses in the range of 6–30 fractions. Unfortunately, the authors did not discuss the statistical significance of their results. In fact, my own analysis of their data shows that precisely the opposite conclusion could be drawn, namely, the NSD formula does represent iso-effect doses calculated from their results within experimental error.

Dr. Roger Berry was kind enough to send to me a copy of the article on Excess Late Subcutaneous Fibrosis after Irradiation of Pig Skin Consequent Upon the Application of the NSD Formula. I feel that the following comments are justified. (1) The value of 0·33 as the regression coefficient for the isoeffect line when log dose is plotted against log time has considerable basis in many previous publications. This is used as a basis which, suitably partitioned according to the NSD concept for the first time, made possible for the first time separation of the effects of fractionation and of time (Ellis, 1966; 1968; 1971).

Dr. Frank Ellis has let us see his comments on our paper “Excess late subcutaneous fibrosis after irradiation of pig skin, consequent upon application of the NSD formula”, and we feel that the attention of uncommitted readers should be drawn to certain points: Comment (2) of Dr. Ellis' letter quotes two references which have no bearing on the point he makes—namely that we have not applied the correct dose for our work.

Criticism levelled at the paper by Hopewell and Young (1982a), in which they assert that the skin radiation reaction is not dependent on field size, does not detract from its merit in stimulating a debate on important problems in radiobiology. They implicitly absolve the clinicians from not putting into radiotherapeutic practice their conclusions, but these impinge on fundamental processes in living tissues exposed to ionising radiations and problems of radiation protection.

In a recent article on the potential of Californium-252 in radiotherapy (Hall and Rossi, 1975) the authors reviewed the experimental, physical and biological work that has been done up to 1974. They observed that the RBE values of Cf neutrons relative to γ rays tend to scatter around a common line which indicates a decrease in RBE as a function of increasing dose rate. RBE values for several biological systems such as cells cultured in vitro, pig skin, mouse jejunum, lymphocytic leukaemia cells and mouse lethality, appear to show the same relationship as a function of dose rate. The authors also mentioned that the fit is not perfect because of the well-known RBE variations among different tissues and cell systems. The few points some distance from the line were discussed in more detail and an explanation was provided.

The important study by Hopewell and Young (1982) on the absence of field size effect in reactions of irradiated pig skin claims to give evidence contradicting that of clinical experience. The authors point out the lack of firm quantitative clinical data and the apparent absence of any detailed prospective clinical study. They imply that clinical field size formulae may not be based on firm facts and may, therefore, be invalid. However, if one considers clinical relevance, their study on pig skin reactions was performed under some experimental conditions which are not generally present in clinical practice: (1) The study is based on single dose fractions, and very little human data are available for single doses.