The effect of age on the latency of radiation myelopathy

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.

Split dose carbon ion irradiation of the rat spinal cord: Dependence of the relative biological effectiveness on dose and linear energy transfer

PURPOSE

To measure the relative biological effectiveness (RBE) of carbon ions relative to 15 MeV photons in the rat spinal cord for different linear energy transfers (LET) to validate model calculations.

METHODS AND MATERIALS

The cervical spinal cord of rats was irradiated with 2 fractions of carbon ions at six positions of a 6 cm spread-out Bragg-peak (SOBP, 16-99 keV/μm). TD50-values (dose at 50% complication probability) were determined from dose-response curves for the endpoint radiation induced myelopathy (paresis grade II) within 300 days after irradiation. Using previously published TD50-values for photons (Karger et al., 2006; Debus et al., 2003), RBE-values were determined and compared with predictions of two versions of the local effect model (LEM I and IV).

RESULTS

TD50-values for paresis grade II were 26.7 ± 0.4 Gy (16 keV/μm), 24.0 ± 0.3 Gy (21 keV/μm), 22.5 ± 0.3 Gy (36 keV/μm), 20.1 ± 1.2 Gy (45 keV/μm), 17.7 ± 0.3 Gy (66 keV/μm), and 14.9 ± 0.3 Gy (99 keV/μm). RBE-values increased from 1.28 ± 0.03 (16 keV/μm) up to 2.30 ± 0.06 at 99 keV/μm. At the applied high fractional doses, LEM I fits best at 16 keV/μm and deviates progressively toward higher LETs while LEM IV agrees best at 99 keV/μm and shows increasing deviations, especially below 66 keV/μm.

CONCLUSIONS

The measured data improve the knowledge on the accuracy of RBE-calculations for carbon ions.

Dose-volume effects in the rat cervical spinal cord after proton irradiation

PURPOSE

To estimate dose-volume effects in the rat cervical spinal cord with protons.

METHODS AND MATERIALS

Wistar rats were irradiated on the cervical spinal cord with a single fraction of unmodulated protons (150-190 MeV) using the shoot through method, which employs the plateau of the depth-dose profile rather than the Bragg peak. Four different lengths of the spinal cord (2, 4, 8, and 20 mm) were irradiated with variable doses. The endpoint for estimating dose-volume effects was paralysis of fore or hind limbs.

RESULTS

The results obtained with a high-precision proton beam showed a marginal increase of ED50 when decreasing the irradiated cord length from 20 mm (ED50 = 20.4 Gy) to 8 mm (ED50 = 24.9 Gy), but a steep increase in ED50 when further decreasing the length to 4 mm (ED50 = 53.7 Gy) and 2 mm (ED50 = 87.8 Gy). These results generally confirm data obtained previously in a limited series with 4-6-MV photons, and for the first time it was possible to construct complete dose-response curves down to lengths of 2 mm. At higher ED50 values and shorter lengths irradiated, the latent period to paralysis decreased from 125 to 60 days.

CONCLUSIONS

Irradiation of variable lengths of rat cervical spinal cord with protons showed steeply increasing ED50 values for lengths of less than 8 mm. These results suggest the presence of a critical migration distance of 2-3 mm for cells involved in regeneration processes.

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.

[Late post-irradiation cervical spinal cord disease. A case]

A case of delayed progressive radiation myelitis (DPRM) which begin 11 months after naso pharyngeal carcinoma radiation, in a young man, is reported. The initial manifestation is often a Brown-Sequard's syndrome progressing to complete and permanent myelopathy, with notable absence of localized or radicular pain. The parenchymal change of the spinal cord in radiation myelopathy can be easily visualized with magnetic resonance imaging (MRI) however there may be cases in which MRI appearance alone does not distinguish specially between tumor and radiation necrosis with absolute confidence: therefore, DPRM is by necessity a diagnosis of exclusion, based on clinical, paraclinical results and course of disease. Corticosteroid therapy is accompanied by a significant remission of symptoms. The evolution is characterized by a worse prognosis, prevention is absolutely necessary based on perfect radiation technic, knowledge on tolerance of spinal cord to irradiation (time-dose-volume factors) and other risks factors (chemotherapy, age and vascular disease).