Radiosensitivity of the human oxygenated cervical spinal cord based on analysis of 357 cases receiving 4 MeV x rays in hyperbaric oxygen

Relationship between Curative Radiation Therapy of Paravertebral Tumors and the Incidence of Radiation Myelitis

The literature about radiation damage to the spinal cord is reviewed and 4 of own cases are presented. It is pointed out that in curative radiation therapy the risk of myelitis that accompanies higher spinal cord doses must be weighed against the therapeutic gains. However, in view of the lack of information about radiation damage to the spinal cord, this may be quite difficult.

The development of stereotactic body radiotherapy in the past decade: a global perspective

In the past 10 years, there has been an exponential increase in the incorporation of stereotactic body radiotherapy, also known as stereotactic ablative radiotherapy, into the armamentarium against various types of cancer in different settings worldwide. In this article in the 10th year anniversary issue of Future Oncology, representatives from the USA, Canada, Japan, Germany, The Netherlands, Australia and Singapore will provide individual perspectives of the development of stereotactic body radiotherapy in their respective countries.

The radiation dose-response of the human spinal cord

PURPOSE

To characterize the radiation dose-response of the human spinal cord.

METHODS AND MATERIALS

Because no single institution has sufficient data to establish a dose-response function for the human spinal cord, published reports were combined. Requisite data were dose and fractionation, number of patients at risk, number of myelopathy cases, and survival experience of the population. Eight data points for cervical myelopathy were obtained from five reports. Using maximum likelihood estimation correcting for the survival experience of the population, estimates were obtained for the median tolerance dose, slope parameter, and alpha/beta ratio in a logistic dose-response function. An adequate fit to thoracic data was not possible. Hyperbaric oxygen treatments involving the cervical cord were also analyzed.

RESULTS

The estimate of the median tolerance dose (cervical cord) was 69.4 Gy (95% confidence interval, 66.4-72.6). The alpha/beta = 0.87 Gy. At 45 Gy, the (extrapolated) probability of myelopathy is 0.03%; and at 50 Gy, 0.2%. The dose for a 5% myelopathy rate is 59.3 Gy. Graphical analysis indicates that the sensitivity of the thoracic cord is less than that of the cervical cord. There appears to be a sensitizing effect from hyperbaric oxygen treatment.

CONCLUSIONS

The estimate of alpha/beta is smaller than usually quoted, but values this small were found in some studies. Using alpha/beta = 0.87 Gy, one would expect a considerable advantage by decreasing the dose/fraction to less than 2 Gy. These results were obtained from only single fractions/day and should not be applied uncritically to hyperfractionation.

Normal-tissue toxicities of thoracic radiation therapy: esophagus, lung, and spinal cord as organs at risk

The evolution of therapeutic approaches for lung cancer illustrates the trend for treatment intensification, with hopes that dose-intense chemotherapy regimens, higher radiation therapy (RT) doses, or novel fractionation schemes will result in prolongation of survival. Current chemotherapy- and RT-intense regimens may not be intensified further without addressing dose-limiting toxicities such as esophagitis. It is important to understand factors pre-disposing to esophagitis so that strategies to minimize its severity can be investigated. Pulmonary complications such as pneumonitis and fibrosis from RT (with or without chemotherapy) are dose and volume dependent. Methods to better identify the target tissues and improved RT-delivery systems may facilitate increasing target doses or reducing doses to adjacent normal tissues. Biologic predictors may allow clinicians in the future to individualize RT treatment based on a patient's toxicity risk profile. Radiation myelopathy is still the most feared radiation complication of lung cancer treatment. The authors address the known parameters that influence the incidence of thoracic radiation myelopathy and the putative factors that could be considered when a clinician may be required to push the spinal cord dose in favor of tumor control.

Decreasing the dosimetric effects of misalignment when using a mono-isocentric technique for irradiation of head and neck cancer

PURPOSE

The purpose of this study was to quantify and develop methods to decrease inhomogeneities created with field edge mismatch when using a mono-isocentric beam-split technique.

METHODS AND MATERIALS

We validated techniques to determine dose across a half-blocked field edge and quantified potential sources of systematic matchline error. Then, two methods were used to evaluate matchline doses. The first used film dosimetry data from a half-beam field and a spreadsheet. Duplication and reversal provided two columns, each representing a beam-split field edge. Summation simulated perfect abutment and shifting created various gaps and overlaps. The second method involved obtaining dose profiles at midfield along the ray perpendicular to abutted, overlapped, and gapped beam-split fields on six linear accelerators. To enlarge the penumbra, we designed several field edge modifiers, then re-evaluated matchline doses. The field edge modifiers applicability to a 3-field head and neck treatment technique was also examined.

RESULTS

Film-determined dose profiles provide similar information across a beam-split field edge as an ionization chamber. With the mono-isocentric beam-split technique, a 4-mm overlap or gap produces inhomogeneities nearly 60% above or below the intended dose. A 2-mm overlap or gap produces inhomogeneities nearly 30% above or below the intended dose. A customized penumbra generator decreased the magnitude of these inhomogeneities to 20% and 10%, respectively.

CONCLUSION

The two methods of evaluating matchline dose described above gave similar results. When using the mono-isocentric half-field technique, small misalignments produce worrisome regions of inhomogeneity. Our penumbra generator substantially decreases the magnitude of the dose inhomogeneities, although the volume receiving an inhomogeneous dose increases.

The effect of single doses of radiation on mouse spinal cord

We have used a mouse model to study spinal cord injury following single doses (12 to 75 Gy) of radiation. The spinal cord (T9,10-L4,5) of C3Hf/Sed//Kam mice was irradiated and response graded using four levels of neurological change. Findings were: (a) the doses required to paralyze 50% of animals (ED50) were 19.79, 20.77, and 21.85 Gy for mild, partial, and complete paralysis, respectively, as measured 200-360 days after radiation. (b) Most damage was progressive but it was not necessarily so; after doses up to 28 Gy recovery was occasionally seen. (c) Latency depended on the dose and the level of damage. Following doses around the ED50, paralysis occurred between 180 to 300 days. (d) There were significant fluctuations in the dose-latency relationship at doses less than 35 Gy. Latency may be not a reliable endpoint to compare biological effects of radiation in this dose range. (e) The radiosensitivity of mouse spinal cord was similar to that reported for rats. (f) Histologically, demyelination was the dominant lesion in the paralyzed animals. We conclude that the mouse is a good animal model to study radiation damage to the spinal cord, at least when a 2.2 cm length is irradiated.

Neurological damage in patients irradiated twice on the spinal cord: a morphologic and electrophysiological study

We reviewed the files of 950 patients treated for Hodgkin's disease since 1966 and were able to find five patients treated with radiochemotherapy and irradiated twice on volumes including a cord segment, at various time intervals, and surviving until now. Seven patients with comparable clinical and therapeutic features, but not reirradiated on the cord, were chosen as a control group and were examined with the same diagnostic procedures. The cumulative cord dose in the reirradiated patients was recalculated and ranged from 50 to 70 Gy. All these patients and the control cases were followed up for more than 10 years and presented no or only minor neurological symptoms. We compare the results of both magnetic resonance imaging (MRI) and electrophysiological studies (spinal and scalp recorded somatosensory evoked potentials--SEPs) in an attempt to define the characteristics of the subclinical damage present in these patients. While no cord abnormality was demonstrated with MRI, electrophysiological studies evidenced a clear difference between cases and controls, as far as the D10-P1 conduction time and SEPs average amplitude are concerned. Advantages and drawbacks of a wider use of electrophysiological methods in research work on cord radiation damage are presented, along with the possible implications of the results obtained for the understanding of the pathogenesis and of the dose dependence of radiation myelitis (RM).

Radiation myelopathy: a review

Radiation myelopathy is a serious potential complication associated with radiation therapy to the spinal cord. Its exact cause is unknown. It represents a spectrum of syndromes, the most common of which are acute transient radiation myelopathy and chronic progressive radiation myelitis. A review of the literature indicates that cord damage is related to a number of controllable factors including dose fraction size, treatment time, total dose, length of cord irradiated, technical and clerical errors. Other less well understood factors that appear to contribute to the risk such as combined chemoradiotherapy effects and hyperthermia should also be recognized. A better awareness and understanding of these factors should ultimately lead to safer and more effective treatments.

The latent period in clinical radiation myelopathy

Seventy-seven papers containing data on more than 300 cases of radiation myelopathy have been analyzed. The data suggest that the latent periods are similar in the cervical and thoracic levels of the spinal cord and are bimodally distributed. Myelopathy of lumbar cord apparently has a shorter latent period. As in controlled animal experiments, the latent period decreases with increasing dose. Furthermore, the variation in latent periods also decreases with dose. It is also seen that retreated patients and pediatric or adolescent patients have greatly reduced latent periods. The implications of these findings as they compare with the animal data are discussed.

Dose-incidence curves for tumour control and normal tissue injury, in relation to the response of clonogenic cells

A probit analysis has been made of data from the literature on local control of tumours and injury to normal tissue as a function of dose of radiation. Fifteen series were analysed for local tumour control in man and ten series for complications. The analysis yielded values for the D50 dose (50% incidence of effect) and the probit width (K), a measure of the steepness of the dose-incidence curve. The same analyses were made of data for rodents. Broadly, K was proportional to D50 in the ratio 1:7, with no major differences between tumours and reported complications. D50 was plotted as a function of dose per fraction for four normal tissues and two tumours in rodents. D50 decreased more rapidly with increasing dose per fraction for the normal tissues than for tumours. The probit width, K, varied inversely with increasing dose per fraction for normal tissues and this contrasted with the tumour response. Thus with increasing dose per fraction, the threshold for effect decreased and the steepness of the ensuing dose-incidence curve increased, relatively more rapidly for normal tissue than for tumour. These curves of gross response have been analysed also by the double negative log method of Gilbert [23], in an attempt to estimate the number and survival characteristics of "tissue-rescuing cells". These were calculated to be less than 1 in 10(4) of the numbers of clonogenic cells measured by excision assays. The D0 values of the derived survival curves for these tissue-rescuing cells were higher than those measured by excision assays.

Tolerance of the mature human central nervous system to photon irradiation

Tolerance of mature human brain to photon irradiation is described. Isoeffect curves have been derived for tolerance of large and small volumes of brain, by examination of doses determined empirically and in clinical use. These have been compared with isoeffect curves of thoracic myelitis, optic nerve and chiasm damage, and brain necrosis. The results show that the best-fitting Ellis-type equations, when five fractions per week are used, have low exponents for overall treatment time and high exponents for the number of increments (N), and are similar to published data on rat myelitis. Of the equations used to test the relationship between total dose and number of increments, the power curve was the best fit. Mean values of exponents for N derived for brain and spinal cord tolerance were 0.4 or more. These were similar to values obtained for optic nerve and chiasm damage, though the data are more limited for this complication. Brain necrosis is observed at slightly higher doses probably because of a difference in the end-point observed rather than because of any fundamental difference in tissue response. Evidence is presented to suggest that some repopulation may occur in widely spaced schedules. The use of the Ellis equation derived from connective-tissue data is inappropriate for central nervous system tissue, and its use may lead to a substantial risk of overdosage. A plea is made for adequate documentation of treatment details when human data are reported. The importance of dose per fraction is emphasized.

Radiation myelopathy in patients treated for carcinoma of bronchus using a six fraction regime of radiotherapy

The adoption of a six-fraction regime of radiotherapy for patients with locally advanced carcinoma of the bronchus was followed by the appearance of radiation myelitis in eight cases. These were among a group of 130 patients given radiotherapy with anterior and posterior treatment fields, without shielding of the spinal cord. Radiation myelitis was found only in those where the calculated spinal-cord dose exceeded 3350 cGy (rad). The possible precipitating factors in the eight patients who suffered myelopathy were compared with those in the remaining 62 patients who also received spinal-cord doses calculated to be greater than 3350 cGy (rad). Only one difference was found--the haemoglobin concentration was significantly higher in those who suffered neuropathy compared with those who did not (P = 0.05).

Late radiation damage to the central nervous system: a radiobiological interpretation

The evidence in support of the two opposing views as to the pathogenesis of late radiation damage to the central nervous system after therapeutic doses of radiation are briefly reviewed. Analysis of the radiation dose levels used by various investigators holding opposing views as to pathogenesis of CNS lesions they reported, suggests that both groups of protagonists were in part correct. Both vascular and glial changes are important in the development of late radiation damage to the nervous system; the preponderance of one or the other type of cell damage depends upon the radiation dose. Vascular effects occur at lower dose levels but after a longer latent period than effects mediated through damage to neuroglia. Current radiobiological concepts and their possible importance in establishing the radiosensitivity of the central nervous system are discussed. The origins of both vascular and neuroglial cell mediated changes are discussed in terms of the reproductive loss of radiation damaged cells. This view is supported by some recent experimental findings. The radiobiological basis of some recently investigated treatment modalities aimed at improving the control of intracranial gliomata by radiotherapy are also considered.

Radiation myelopathy of the lumbo-sacral spinal cord

The clinical findings in four cases of radiation injury to the lumbosacral spinal cord and cauda equina, occuring after radiotherapy of malignant testicular tumours with radiation doses of 1597-1670 ret, are reported. The main feature in each case was a flaccid paraparesis, without sensory loss in three cases, while one patient presented a doubtful sensory deficit. Symptoms of sphincter dysfunction were absent. In all cases the symptoms caused a marked and persistent incapacity. It is suggested that the principal site of radiation injury is the proximal parts of the lower motor neuron.

Hyperbaric oxygen in radiation therapy

The importance of oxygen with low LET radiations has been established beyond any doubt in many different systems, both plant and animal. While some studies, especially head and neck tumors, are impressive, it has not been demonstrated unequivocally that radiation under hyperbaric conditions is superior to well fractionated, well conceived, conventional radiotherapy. Any resulting gain in survival from the addition of hyperbaric oxygen will be limited, especially with more advanced stages of disease. Well controlled studies, especially with earlier stage disease, are still necessary. It would be worthwhile to undertake such trials, especially with tumors of the head and neck constituting the most promising site of study, as others have noted, since even a 5% to 10% improvement in survival would mean many lives saved. Continued trials with hyperbaric oxygen, oxygen in other forms, neutrons and other particles, and radiation sensitizing drugs are all justified in an attempt to overcome the oxygen effect on human tumors.

Delayed myeloradiculopathy produced by spinal X-irradiation in the rat

Rats were subjected to 3,500 r of X-irradiation in a single dose while breathing oxygen at 1 ATM pressure. Comparison was made between the delayed effects of irradiating thoracic, lumbar, and the cauda equina fields. The lumbar field involved the alpha-motoneurons and spinal roots supplying the sciatic nerve, while the cauda equina field involved these spinal roots but spared the alpha-motoneurons in the spinal cord. Thoracic irradiation produced paraplegia after an interval of 127-150 days. In the irradiated zone, the spinal cord was severely damaged, but the thoracic spinal roots were spared. Lumbar irradiation produced paraplegia after an interval of 83-211 days. In the irradiated zone, the alpha-motoneurons were largely spared, the spinal cord showed mild to moderate white matter damage, but the most severe damage was of the lumbosacral spinal roots. The posterior roots were more affected than the anterior. In longer interval cases the degeneration of the roots appeared to be due to focal devitalization. Evidence is advanced that root degeneration had been progressing for at least 4 weeks before the onset of paraplegia. In the cauda equina series the lumbosacral spinal root changes were similar to those in the lumbar series. This study indicates that different levels of the neuraxis have different degrees of susceptibility to X-irradiation. The thoracic cord appears more susceptible than the lumbosacral; the lumbosacral roots appear more susceptible than the thoracic; the posterior roots are more susceptible than the anterior. These findings may have relevance to the study of radiation damage in man, even though the dose schedule used in this experimental study differs greatly from that used for radiotherapy.

Neurologic complications after irradiation of the cervical spinal cord for malignant tumour of the head and neck

A total of 165 patients with tumours of the head and neck were irradiated via fields including the entire cervical portion of the spinal cord. Eight patients (4.8 per cent) developed mild reversible signs of radiation myelitis. Only one of these cases was found among the 44 patients who received a dose to the spinal cord of over 5 000 rad via fields of less than 16 cm in length; 7 cases were patients with Hodgkin's disease who were given up to 3 700 rad via mantle fields. A survey of previous reports on transverse spinal lesions provoked by irradiation revealed a possibility of overdosage in several cases, and dose tolerance limits mentioned previously should accordingly be applied with caution.

Pathology of radiation myelopathy

After nothing the rarity of papers describing the pathology of delayed radiation necrosis of the spinal cord, the clinical and pathological findings from four cases are presented. The main pathological features are asymmetric demyelination of the lateral columns and to a lesser degree the posterior and anterior columns of white matter, with coagulative necrosis at the level of irradiation which affected the grey matter to a lesser degree. There is ascending and descending secondary tract degeneration, and poor glial response in the lesions themselves. Vascular changes, mainly hyalilne thickening of arteriolar walls, are present, but not in degree sufficient to explain the primary lesion. The discussion of the pathogenesis of the myelopathy weighs the merits of a primary vascular lesion against those of a primary effect of the radiation on neural tissue. The latter is favoured.