Radiation tolerance and fractionation sensitivity of the developing rat cervical spinal cord

Determination of the proton RBE in the rat spinal cord: Is there an increase towards the end of the spread-out Bragg peak?

BACKGROUND AND PURPOSE

To determine the relative biological effectiveness (RBE) of protons in the rat spinal cord as a function of linear energy transfer (LET) and dose.

MATERIALS AND METHODS

The rat cervical spinal cord was irradiated with single or two equal fractions (split doses) of protons at four positions (LET 1.4-5.5 keV/µm) along a 6 cm spread-out Bragg peak (SOBP). From dose-response analysis, TD₅₀- (dose at 50% effect probability) and RBE-values were derived using the endpoint of radiation-induced myelopathy.

RESULTS

Along the SOBP, the TD₅₀-values decreased from 21.7 ± 0.3 Gy to 19.5 ± 0.5 Gy for single and from 32.3 ± 0.3 Gy to 27.9 ± 0.5 Gy for split doses. The corresponding RBE-values increased from 1.13 ± 0.04 to 1.26 ± 0.05 (single doses) and from 1.06 ± 0.02 to 1.23 ± 0.03 (split doses).

CONCLUSIONS

For the relative high fractional doses, the experimental RBE at the distal edge of the proton SOBP is moderately increased. The conventionally applied RBE of 1.1 appears to be valid for the mid-SOBP region, but the higher values occurring more distally could be of clinical significance, especially if critical structures are located in this area. Further in vivo studies at lower fractional doses are urgently required.

Late normal tissue response in the rat spinal cord after carbon ion irradiation

Background

The present work summarizes the research activities on radiation-induced late effects in the rat spinal cord carried out within the “clinical research group ion beam therapy” funded by the German Research Foundation (DFG, KFO 214).

Methods and materials

Dose–response curves for the endpoint radiation-induced myelopathy were determined at 6 different positions (LET 16–99 keV/μm) within a 6 cm spread-out Bragg peak using either 1, 2 or 6 fractions of carbon ions. Based on the tolerance dose TD₅₀ of carbon ions and photons, the relative biological effectiveness (RBE) was determined and compared with predictions of the local effect model (LEM I and IV). Within a longitudinal magnetic resonance imaging (MRI)-based study the temporal development of radiation-induced changes in the spinal cord was characterized. To test the protective potential of the ACE (angiotensin converting enzyme)-inhibitor ramipril™, an additional dose–response experiment was performed.

Results

The RBE-values increased with LET and the increase was found to be larger for smaller fractional doses. Benchmarking the RBE-values as predicted by LEM I and LEM IV with the measured data revealed that LEM IV is more accurate in the high-LET, while LEM I is more accurate in the low-LET region. Characterization of the temporal development of radiation-induced changes with MRI demonstrated a shorter latency time for carbon ions, reflected on the histological level by an increased vessel perforation after carbon ion as compared to photon irradiations. For the ACE-inhibitor ramipril™, a mitigative rather than protective effect was found.

Conclusions

This comprehensive study established a large and consistent RBE data base for late effects in the rat spinal cord after carbon ion irradiation which will be further extended in ongoing studies. Using MRI, an extensive characterization of the temporal development of radiation-induced alterations was obtained. The reduced latency time for carbon ions is expected to originate from a dynamic interaction of various complex pathological processes. A dominant observation after carbon ion irradiation was an increase in vessel perforation preferentially in the white matter. To enable a targeted pharmacological intervention more details of the molecular pathways, responsible for the development of radiation-induced myelopathy are required.

Extent and kinetics of recovery of occult spinal cord injury

PURPOSE

To obtain clinically useful quantitative data on the extent and kinetics of recovery of occult radiation injury in primate spinal cord, after a commonly administered elective radiation dose of 44 Gy, given in about 2 Gy per fraction.

METHODS AND MATERIALS

A group of 56 rhesus monkeys was assigned to receive two radiation courses to the cervical and upper thoracic spinal cord, given in 2.2 Gy per fraction. The dose of the initial course was 44 Gy in all monkeys. Reirradiation dose was 57.2 Gy, given after 1-year (n = 16) or 2-year (n = 20) intervals, or 66 Gy, given after 2-year (n = 4) or 3-year (n = 14) intervals. Two animals developed intramedullary tumors before reirradiation and, therefore, did not receive a second course. The study endpoint was myeloparesis, manifesting predominantly as lower extremity weakness and decrease in balance, occurring within 2.5 years after reirradiation, complemented by histologic examination of the spinal cord. The data obtained were analyzed along with data from a previous study addressing single-course tolerance, and data from a preliminary study of reirradiation tolerance.

RESULTS

Only 4 of 45 monkeys completing the required observation period (2-2.5 years after reirradiation, 3-5.5 years total) developed myeloparesis. The data revealed a substantial recovery of occult injury induced by 44 Gy within the first year, and suggested additional recovery between 1 and 3 years. Fitting the data with a model, assuming that all (single course and reirradiation) dose-response curves were parallel, yielded recovery estimates of 33.6 Gy (76%), 37.6 Gy (85%), and 44.6 Gy (101%) of the initial dose, after 1, 2, and 3 years, respectively, at the 5% incidence (D(5)) level. The most conservative estimate, using a model in which it was assumed that there was no recovery between 1 and 3 years following initial irradiation and that the combined reirradiation curve was not necessarily parallel to the single-course curve, still showed an overall recovery equivalent to 26.8 Gy (61%). The spinal cords of symptomatic monkeys consistently revealed a mixture of white matter necrosis and vascular injury, but the majority of spinal cords of asymptomatic animals did not exhibit overt lesions detectable by light microscopy.

CONCLUSION

Combined analysis with the data of the previous studies yielded firm evidence that the spinal cord has a large capacity to recover from occult radiation injury induced by a commonly prescribed elective dose. This finding strengthens the rationale for selective use of radiotherapy to treat second primary tumors arising in previously irradiated tissues or late recurrences. However, some caution should be exercised in applying quantitative experimental data, because the length of follow-up in these experiments was limited to 2-2.5 years after reirradiation, whereas human myelopathy cases occasionally occur after longer latency. Because there is a large variation in long-term recovery among tissues, the tolerance of other tissues at risk should also be taken into account in prescribing therapy.

Radiation therapy and the management of intramedullary spinal cord tumors

The use of radiation therapy in the management of intramedullary spinal cord tumors remains controversial. Several studies indicate that the use of postoperative radiation therapy modestly improves both local control and survival in spinal cord ependymomas and astrocytomas. Modern treatment planning and imaging allow more accurate target definition and respect for related normal tissue tolerances.

Radiation response of the rat cervical spinal cord after irradiation at different ages: tolerance, latency and pathology

PURPOSE

The investigation of the age dependent single-dose radiation tolerance, latency to radiation myelopathy, and the histopathological changes after irradiation of the rat cervical spinal cord.

METHODS AND MATERIALS

Rats, ages 1-18 weeks, were irradiated with graded single doses of 4 MV photons to the cervical spinal cord. When the rats showed definite signs of paresis of the forelegs, they were killed and processed for histological examination.

RESULTS

The radiation dose in paresis due to white matter damage in 50% of the animals (ED50) after single dose irradiation was about 21.5 Gy at all ages > or = 2 weeks (mean 21.4 (mean 21.4 Gy; 95% CI 21.0, 21.7 Gy). Only the ED50 at 1 week was significantly lower (19.5 Gy; 18.7, 20.3 Gy). The latency to the development of paresis clearly changed with the age at irradiation, from about 2 weeks after irradiation at 1 week to 6-8 months after irradiation at age > or = 8 weeks. The white matter damage was similar in all symptomatic animals studied. The most prominent were areas with diffuse demyelination and swollen axons, often with focal necrosis, accompanied by glial reaction. This was observed in all symptomatic animals, irrespective of the age at irradiation. Expression of vascular damage appeared to depend on the age at irradiation. No vascular damage was observed in the rats irradiated at 1 week, clearly altered blood vessels were seen in animals symptomatic 10 weeks after irradiation at > or = 3 weeks, and vascular necrosis occurred after > or = 6 months in some rats irradiated at > or = 8 weeks.

CONCLUSION

Although the latency to myelopathy is clearly age dependent, single dose tolerance is not age dependent at age > or = 2 weeks in the rat cervical spinal cord. The white matter damage is similar in all symptomatic animals studied, but the vasculopathies appear to be influenced by the age at irradiation. It is concluded that white matter damage and vascular damage are separate phenomena contributing to the development of radiation myelopathy, expression of which may depend on the radiation dose applied and the age at irradiation.

Repair capacity of adult rat glial progenitor cells determined by an in vitro clonogenic assay after in vitro or in vivo fractionated irradiation

Demyelination is one of the pathological conditions identified as a late response of the central nervous system (CNS) to irradiation. We have proposed that radiation-induced depletion of glial stem cells, which are the source of myelinating cells in the CNS, would lead to a lack of replacement of senescent or otherwise damaged oligodendrocytes. This impaired process of cell renewal would result in a decline of oligodendrocytes, i.e. demyelination. In the present study the repair capacity of glial stem cells was investigated and compared with the repair capacity of the CNS in vivo using functional endpoints. For this purpose, glial stem cells, derived from the adult rat optic nerve, were subjected to fractionated irradiation in vivo and in vitro and their survival was quantified with an in vitro clonogenic assay. The data were analysed by three different methods, all based on the LQ-model (single dose survival curve; 'beta RR', 'Fe-plot'). The resulting value of the beta-parameter of adult glial stem cells is consistent with values obtained for functional endpoints after irradiation of the CNS in vivo. The alpha/beta-ratio (4.9-7.3 Gy) of adult glial stem cells, however, is higher than the alpha/beta-ratio (approximately 2 Gy) obtained for CNS in vivo and is closer to that of an acute responding tissue.

Repair kinetics of radiation damage in the developing rat cervical spinal cord

The kinetics of repair of sublethal radiation damage was examined in the cervical spinal cord of developing, 1-week-old rats. Split-dose irradiation treatments were given with time intervals of 0.5-96 h. The data, supplemented with fractionation data from previous experiments, were analysed using direct analysis based on the incomplete repair (IR) model. The best fit to the monoexponential repair model resulted in an estimated half-time of repair (T1/2) of 1.5 (1.3-1.8) h. No indications of a slow or second component of repair could be detected in the 1-week-old cervical spinal cord. This is in contrast with literature reports of experiments with the adult rat cervical spinal cord, suggesting bi-exponential repair, with 65% of the damage repaired with a slow repair T1/2 of about 4 h. Two-step methods, although statistically inferior to direct analysis, are still in use for analysis of repair experiments. A number of two-step methods used for data analysis in previous reports concerning sublethal damage repair, are dose (un)repaired, proportion of dose unrepaired, and proportion of damage unrepaired. It is argued that of the methods discussed, only analysis of the data expressing the results as proportion unrepaired damage (delta Eu) and using split-dose experiments, does not result in introduction of an artificial second repair T1/2 in tissues with a high fractionation sensitivity. Two-step analysis of the present data using delta Eu suggested monoexponential repair with a T1/2 value of 1.5 (SE 0.2) h.