No previous partial-cord irradiation study has included a pre-clinical model in which the spinal cord was similar in size to humans and the irradiation conditions were similar to those encountered in clinical spinal radiosurgery. It is unknown exactly how the physical size of the spinal cord influences radiation tolerance in the setting of partial-cord irradiation, but if viable oligodendrocytes and oligodendrocyte precursor cells can only migrate 2-3 mm from unirradiated tissue, as suggested16-19
, the size of the human spinal cord could limit the extent of remyelination and the encouraging results from rat studies may be less relevant.
The complex reactions of biological systems to radiation made this study impossible to perform in non-biological models. The Yucatan minipig was the animal of choice for this study. The pig has been used for radiobiological studies involving many anatomical sites of interest including the skin20
and spinal cord23
because these structures have many anatomical and physiological similarities to their counterparts in humans 24
. Mature pigs used to study the effects of irradiated length on the response of the spinal cord showed frank paralysis, vascular changes, white matter demyelination and necrosis comparable to those seen in humans14,25,26
. The spinal cord tolerance to single-fraction, uniform irradiation was investigated in two previous pig studies at Oxford University. The first study compared the response of “mature” (37-42.5 weeks) versus “immature” (15.5-23 weeks) animals23
and the second study14
investigated the “length effect.” Pigs defined as “mature” (40-47 weeks) were used in the current study because one Oxford study23
found that only transient neurologic changes occurred in “immature” pigs after doses that paralyzed mature animals suggesting that young pigs have a greater ability to repair or compensate for radiation damage.
While many studies have been performed to investigate parameters that affect spinal cord tolerance, only the most relevant studies are compared here. Outlines of study designs from Bijl, et. al.13
, the Oxford study14
, and the current study are shown in . Based on results of the Oxford study in pigs14
and on studies in rats by Bijl, et. al.13
, and van Luijk, et. al.12
, it was hypothesized that the ED50
for spinal cord tolerance to lateral partial-cord irradiation in pigs would be at least 30 Gy. It is unclear why the ED50
for the present study was only 20.0 Gy. Migration of functional cells or their precursors from uninjured tissue at the periphery of a radiation injury has been proposed as one mechanism that accounts for an increase in spinal cord tolerance when short lengths are irradiated18,27
. The ability of normal remyelinating cells to migrate up to 2 mm from a narrow rim of healthy tissue surrounding an area of demyelination has been shown by Franklin et. al. in a rat model19
while Bijl, et. al. have suggested a critical migration distance of 2-3 mm18
. It may be that the steep dose gradient created by using the ‘shoot through’ method with a proton beam12
coupled with the relatively small diameter of the rat spinal cord (3.5 mm) allowed for migration of remyelinating cells from the non-irradiated side of the spinal cord to the irradiated side whereas the pig spinal cord is too large (8-11 mm diameter) for effective migration. This explanation conflicts somewhat with the “bath and shower” experiments by Bijl, et. al., who showed even doses as low as 4 Gy to tissue surrounding a high-dose radiation spinal cord injury negatively influence the ability of the high-dose lesion to recover17
. Preliminary results (n=22) from our companion study investigating spinal cord tolerance to uniform irradiation, suggest that the advent of motor deficit appears to be independent of irradiated volume in the lateral direction.
Spinal cord tolerance study parameter comparison.
The current study is similar to the Oxford study14
in that both studies incorporated mature female pigs but most other aspects differ. The most significant differences are the dose distribution and dose rate used. While the Oxford study used parallel opposed fields to create a uniform dose distribution across the spinal cord, the current study delivered a steep dose gradient (≈8% per mm) across the spinal cord in the lateral direction. The ED50
of the present study (20.0 Gy) is much less than reported for the Oxford study14
(27-28 Gy) despite the steep dose gradient used in the present study. This finding is most likely due to the low dose rate (0.21-0.30 Gy/min) used in the Oxford study14
. Spinal cord tolerance has been shown to increase dramatically from 21.3 Gy to 27.2 Gy as dose rate is reduced from 1.79 Gy/min to 0.245 Gy/min in a rat model 28,29
. Spinal cord tolerance appears to be constant in the range of typical clinical dose rates from 1.79 Gy/min up to 10-15 Gy/min18,27,28
. The ED50
(20.0 Gy) for the current study is in good agreement with published spinal cord tolerance data for pigs (if adjusted for the dose-rate effect)14
, guinea pigs30
that receive uniform spinal cord irradiation under similar conditions (field length ≥ 1.6 cm and dose rate > 1.7 Gy/min).
The follow-up period of the current study (53-75 weeks) is considered long enough to observe the initial phase of radiation myelopathy (4-6 months) as commonly reported for guinea pigs30
. The latent period observed for the onset of motor deficits in the current study (10-35 weeks) was in good agreement with the Oxford study14
(7.5-16.5 weeks). The latency period for pigs is shorter than the mean response time (10.3 months) reported for three cases of human myelopathy following single-fraction spine radiosurgery but response times do overlap11
. It is possible that more cases of radiation myelopathy would have been observed with a longer follow-up period; the Oxford study reported two late responding pigs at 65 and 75 weeks following irradiation to doses very close to ED50
. The one non-responding animal in the 20 Gy dose group (pig #12, ) of the current study was followed out to 75 weeks but no neurologic changes were noted.
The histological changes observed in the lateral white matter are very similar to lesions observed in other studies in pigs23
. Lesions consist of diffuse demyelination, with focal areas of coalescing necrosis in the higher dose groups. In the current study, the animal showing transient neurological signs and subsequently followed for at least a year after irradiation, showed some degree of glial scar formation, the extent and location of which are in agreement with the transient and relatively minor character of the motor impairment.
The spine of the mature Yucatan minipig is similar in size to humans allowing for inclusion of the same image-guided positioning and irradiation conditions encountered in clinical spinal radiosurgery. The accuracy of the Novalis Body
image-guidance system has been reported by Verellen, et. al., as a mean three-dimensional displacement vector of 0.6 mm with an overall standard deviation of 0.9 mm when 2 mm thick CT slices are used33
. Solberg, et. al.34
used the Novalis Body
system to target an anthropomorphic head phantom and observed a mean (Std. Dev.) three-dimensional vector displacement of 1.1 mm (0.42 mm) in end-to-end positioning verification tests. Vinci, et. al., compared measured versus calculated positions of the 80% isodose line following Novalis Body
image-guided irradiation of a head phantom and concluded that the addition of 1.25, 1.0, and 1.0 mm margins (A-P,R-L,S-I) is appropriate for a gross tumor volume/planning tumor volume expansion to cover delivery error35
. The shape of the dose distribution used in the current study makes the dose-volume histogram relatively insensitive to 1-2 mm shifts in the anterior/posterior and rostral/caudal directions. A lateral 1 mm shift of the treatment isocenter towards the spinal cord would result in an increase in the maximum spinal cord dose of approximately 1-2% while a 1 mm shift in the opposite direction would result in a decrease of the maximum spinal cord dose of 3-4%. Respiration-induced spinal motion was not considered in this study because it has been reported to be negligible in swine under similar setup conditions36
In conclusion, results indicate that for a dose distribution with a steep lateral gradient (≈8% per mm), pigs have an ED50
that closely resembles that for pigs (if adjusted for the dose-rate effect)14
, guinea pigs30
that receive uniform spinal cord irradiation. While this study does not ultimately determine the tolerance of the human spinal cord to radiosurgery, this study could not be performed in a human model. Studies have shown that pigs have many anatomic and physiologic similarities to humans24
and results of radiobiological studies have shown good agreement with the limited data from irradiated humans23
. The porcine model provides a homogeneous population to perform a systematic investigation of radiation dose-volume effects on the spinal cord under conditions encountered in clinical spinal stereotactic radiosurgery and continuing studies are likely to elucidate the mechanism of injury and opportunities for intervention.