K. Kian Ang

Inhalation anesthesia in experimental radiotherapy: a reliable and time-saving system for multifractionation studies in a clinical department.

An inhalation anesthesia system has been employed to overcome several of the limitations associated with the use of sodium pentobarbital and other i.p. administered anesthetics in experimental radiotherapy. The described method is reliable and time-saving. The depth and duration of anesthesia are easily controllable. Only 4 deaths have occurred with more than 6000 animal exposures. The use of polystyrene jigs is shown to provide adequate thermal isolation. Oxygen as a carrier of the anesthetic agent is expected to prevent a reduced tissue oxygenation and its radiobiological consequences. The whole system is constructed as a mobile unit in which up to 16 mice or rats can be anesthetized simultaneously and irradiated in a single field with clinical treatment equipment during short time intervals between patient irradiations. The described advantages of this method make it specially suited for experiments with protracted fractionation schedules.

Lack of evidence for increased tolerance of rat spinal cord with decreasing fraction doses below 2 Gy

The radiation tolerance of the spinal cord, both in man and in rats, has been shown to depend strongly on the size of the dose per fraction. With fraction doses down to about 2 Gy, the spinal cord tolerance can be predicted by a modified Ellis formula: D approximately N0.43. More recently alternative isoeffect formulas were based on the linear-quadratic (LQ) model of cell survival where the effect of dose fractionation is characterized by the ratio alpha/beta which varies from tissue to tissue. For the spinal cord, as well as for other late responding tissues, the ratio alpha/beta is small, in contrast to most acutely responding tissues. Both the Ellis-type formula, and to a lesser extent the LQ-model, predict a continuously increasing tolerance dose with decreasing fraction size. From the LQ model, the concept of flexure dose has been derived, which proposes the limit of effective fractionation to be about 0.1 alpha/beta. At this dose per fraction no significant further gain in tolerance would be detected. From previous experiments on the rat cervical spinal cord with doses per fraction down to about 2 Gy, the ratio alpha/beta was determined to be 1.7 Gy, and the LQ-model would predict a rise in tolerance with a reduction in fraction size to far below 2 Gy. Based on these predictions clinical studies have been initiated assuming a significantly increased tolerance by reduction of fraction size to about 1 Gy. However, in the present experiments no evidence was found for such an increase in tolerance with fraction sizes below 2 Gy.

Direct Estimation of Latent Time for Radiation Injury in Late-responding Normal Tissues: Gut, Lung, and Spinal Cord

Mixture models are proposed for simultaneous analysis of the latency and fractionation characteristics of radiation injury in late-responding normal tissues. The method is an extension of the direct analysis for quantal response data. Conceptually, the application of the mixture model is based on the biological observation that over a wide range of doses a proportion of the irradiated subjects will never express damage. Mixture models allow the time of occurrence to be utilized in the analysis. Furthermore, this type of model takes time-censored observations into account in a natural way and provides an adequate framework for modelling and analysis of effect-dependent latency. Mixture models with complete and incomplete repair are applied to dose-incidence data for four late endpoints in rodents: death from radiation-induced pneumonitis, leg paralysis after spinal-cord irradiation, and radiation-induced rectal stenosis and anal discharge. Radiation-induced pneumonitis had an effect-dependent latency. The modelling of this phenomenon correlates well with the results of histologic studies. Interestingly, the ratio of hazard rates was not constant for this endpoint. The dominating feature in the latency of radiation injury to the spinal cord was a strong dependency on dose per fraction. After correction for this effect a tendency towards a longer latent time for lower effect levels was observed. For the rectal complications, there was no difference between latency with radiation only vs. radiation combined with cis-platin.

Is the rate of repair of radiation-induced sublethal damage in rat spinal cord dependent on the size of dose per fraction?

In the present study the possible dependency of the kinetics of repair of sublethal damage in rat spinal cord on the fraction size has been further investigated. A wide range of sizes of dose per fraction (1.7-17.5 Gy) has been given with interfraction intervals varying from 0.5 to 24 hr. A direct method for analysis of quantal response and an incomplete-repair (IR) model for survival after fractionated exposures with short intervals were used to interpret the data. The half time of repair (T1/2) was found to be 1.6, 1.6 and 1.9 hrs for fraction sizes of approximately 4, 9, and 14 Gy respectively. There appears to be no significant effect of fraction size on the rate of repair. A clinically relevant feature observed from these experimental data is that in this tissue it takes more than 4 hrs for repair of sublethal damage, induced by a dose of approximately 4 Gy, to approach completion (i.e., sparing beyond the limit of the experimental resolution). This has to be taken into account when several fractions are to be given each day. Another feature noted from the analysis of these results is that the alpha/beta determined from the complete repair data is considerably smaller than that estimated from the incomplete repair data (interval less than or equal to 4 hrs). The nature of the inconsistency is discussed.

The kinetics of repair of sublethal damage in the rat cervical spinal cord during fractionated irradiations

The kinetics of repair of sublethal damage were investigated in the cervical spinal cord of rats. Two and 4 fractions have been given with intervals ranging from 20 min to 24 h. The occurrence of paralysis within 7 months after irradiation (due to white matter necrosis) was used as the endpoint. From dose-response curves, ED50 values (dose at which 50% of the animals develop paralysis) were determined, from which the proportion of the dose repaired (FR) during the different intervals can be calculated. It was found that the rate of cellular repair of sublethal damage was exponential, with a half-time of approximately 110 min after a fraction size of 11-15 Gy and 85 min after 7-11 Gy. The corresponding time to complete cellular repair was calculated to be 8 and 6 h, respectively. This suggests that the rate of cellular repair is faster after smaller fractions.

From 2 Gy to 1 Gy per fraction: sparing effect in rat spinal cord?

Recently published results, from this group, on rat cervical spinal cord, a late responding tissue, indicated no further sparing with lowering the fraction size from 2 to 1.8, 1.5, and 1.3 Gy. In the present experiments a small but probably significant rise in tolerance is suggested, when the dose per fraction was decreased from 2 Gy down to 1 Gy. This rise would however still be much less than what is predicted by the linear quadratic model, based on the experimental data obtained with fraction sizes larger than 2 Gy.

THE EFFECT OF SMALL RADIATION DOSES ON THE RAT SPINAL CORD: THE CONCEPT OF PARTIAL TOLERANCE

To evaluate the tolerance of the rat spinal cord to small radiation doses per fraction, an increasing number of fractions is required for induction of paralysis. The assessment of doses of 1-2 Gy, as used in the clinic, would require that over 100 fractions be given. The validity of replacing part of a fractionated irradiation of the spinal cord by a single large dose has been tested. Fractionated irradiation doses with 18 MeV X rays were followed by a top-up dose of 15 Gy as a single treatment. This is the fraction size of a treatment with two irradiation doses leading to paralysis in 50% of the animals (ED 50). Fractionated treatments were carried out with 2, 5, 10 and 20 fractions followed by the top-up dose of 15 Gy. The isoeffect curve, as a function of the number of fractions, has the same slope as experiments performed without top-up dose. The results show that the quality and quantity of cellular repair is not modified when part of a multifractionated exposure is replaced by a larger top-up dose. An important consequence of this finding is, that in treatments with unequal fraction sizes, the partial tolerances can simply be added. Since a top-up dose can replace a sizable number of irradiation treatments, its application will allow investigations of the extent of sublethal damage repair for fraction sizes as low as 1 Gy.

Influence of overall treatment time in a fractionated total lymphoid irradiation as an immunosuppressive therapy in allogeneic bone marrow transplantation in mice

Three groups of C57/BL/Ka mice received total lymphoid irradiation (TLI) in a total dose of 34 Gy in three different fractionation schedules. In the first group daily fractions of 2 Gy were given during 3 1/2 weeks. In the second group 4 to 5 fractions with 3 1/2 hr interval were given each day, thus delivering 17 fractions in 4 days. In the third group three fractions were given daily for two consecutive days and was repeated two times after 8 or 9 days interval, resulting in a total treatment time of 3 1/2 weeks. The tolerance of all different schedules was excellent. No difference in the peripheral white blood cell and lymphocyte counts nor the degree of immunosuppression as measured by phytohaemaglutinin or concanavalin A induced blastogenesis and mixed lymphocyte reaction were observed at the end of the treatment and up to 200 days. When bone marrow transplantation was performed one day after the end of each schedule, chimerism without signs of graft versus host disease was induced in all the groups. However, from the results in a limited number of animals it seems that concentrated schedules were less effective for chimerism induction. It has been demonstrated that it is possible to reduce drastically the overall treatment time for TLI before bone marrow transplantation. Further investigations are necessary in order to determine the optimal time-dose-fractionation factors and the different parameters involved in the transplantation.

Effect of combined AZQ and radiation on the tolerance of the rat spinal cord

The effect of combined AZQ (aziridinylbenzoquinone) and ionizing radiation on the rat spinal cord has been investigated. The highest dose of AZQ (3 mg/kg) tolerated by rats weighing 250 g was injected intravenously 15 min before a single dose of radiation or the first of 2 fractional doses. The development of paralysis due to white-matter necrosis in rats of each dose group was scored and dose-response curves were constructed. It has been found that a combination of AZQ with radiation did not lead to a significant shift of the dose-response curve as compared to that of radiation alone. Thus, AZQ administered according to the schedule used in the present study did not seem to reduce the radiation tolerance of the central nervous system.

Determination of ionisation chamber collection efficiency in a swept electron beam by means of thermoluminescent detectors and the "two-voltage" method.

Two methods for determining the collection efficiency of a 0.6 cm3 thimble ionisation chamber exposed to the swept electron beam of a linear accelerator Therac 20 Saturne (CGR MeV) have been compared. In one method the chamber signal has been compared to that of simultaneously exposed thermoluminescent LiF dosemeters (TLD), in the other the two-voltage method of Boag, adapted for swept beams, has been used. By variation of the electron energy between 20 and 13 MeV, of the focus-skin-distance (FSD) between 200 and 100 cm and of the monitor rate between 400 monitor units (m.u.) and 100 m.u. per minute, different values could be produced for the peak charge density M. The collection efficiency of the chamber, operating at a standard voltage of 250 V, decreases from 0.99 to 0.84 for a charge density increasing from 0.3 X 10(-4) C/m3 to 7.5 X 10(-4) C/m3, respectively. The maximum deviation observed between the TLD and the two-voltage method adopted for similar M is never more than 2% and mostly smaller than 1%. It can be concluded that, under the present experimental conditions, the calculated ionisation chamber collection efficiency is confirmed by the experimental method using TL dosimetry.