Reirradiation tolerance of the immature rat spinal cord

Spinal Reirradiation-Mediated Myelopathy: A Systematic Review and Meta-Analysis

Reirradiation of the spine is carried out in 42% of patients who do not respond to treatment or have recurrent pain. However, there are few studies and data on the effect of reirradiation of the spine and the occurrence of acute and chronic side-effects caused by reirradiation, such as myelopathy, in these patients. This meta-analysis aimed to determine the safe dose in terms of biological effective dose (BED), cumulative dose and dose interval between BED1 and BED2 to decrease or prevent myelopathy and pain control in patients undergoing radiation therapy in the spinal cord. A search was carried out using EMBASE, MEDLINE, PUBMED, Google Scholar, Cochrane Collaboration library electronic databases, Magiran, and SID from 2000 to 2022 to recognise qualified studies. In total, 17 primary studies were applied to estimate the pooled effect size. The random effects model showed that the pooled BED in the first stage, the BED in the second stage and the cumulative BED1 and BED2 were estimated at 77.63, 58.35 and 115.34 Gy, respectively. Studies reported on dose interval. The results of a random effects model showed that the pooled interval was estimated at 13.86 months. The meta-analysis revealed that using appropriate BED1 and/or BED2 in a safe interval between the first and second phases of treatment can have an influential role in preventing or reducing the effects of myelopathy and regional control pain in spinal reirradiation.

[Repair and time-dose factor: The example of spinal cord irradiation]

The question whether a reirradiation is possible, with either curative of palliative intent, is a frequent issue and a true therapeutic challenge, in particular for a critical organ sensitive to cumulative dose, such as the spinal cord. Preclinical experimental data, based on debatable models that are hardly transferable to patients, suggest that there is a possibility of reirradiation, beyond the classical threshold for dose constraints, taking into account the "time-dose factor". Although the underlying biological mechanisms are however uncertain, scarce clinical data seem to confirm that the tolerance of spinal cord to reirradiation does exist, provided that a particular attention to total dose is given. In the context where modern stereotactic irradiation facilities expand therapeutic perspectives, we review the literature on possibilities of reirradiation, through the example of spinal cord reirradiation.

Incidence and dosimetric parameters of pediatric brainstem toxicity following proton therapy

BACKGROUND

Proton therapy offers superior low and intermediate radiation dose distribution compared with photon-based radiation for brain and skull base tumors; yet tissue within and adjacent to the target volume may receive a comparable radiation dose. We investigated the tolerance of the pediatric brainstem to proton therapy and identified prognostic variables.

MATERIAL AND METHODS

All patients < 18 years old with tumors of the brain or skull base treated from 2007 to 2013 were reviewed; 313 who received > 50.4 CGE to the brainstem were included in this study. Brainstem toxicity was graded according to the NCI Common Terminology Criteria for Adverse Events v4.0.

RESULTS

The three most common histologies were ependymoma, craniopharyngioma, and low-grade glioma. Median patient age was 5.9 years (range 0.5-17.9 years) and median prescribed dose was 54 CGE (range 48.6-75.6 CGE). The two-year cumulative incidence of toxicity was 3.8% ± 1.1%. The two-year cumulative incidence of grade 3 + toxicity was 2.1% ± 0.9%. Univariate analysis identified age < 5 years, posterior fossa tumor location and specific dosimetric parameters as factors associated with an increased risk of toxicity.

CONCLUSION

Utilization of current national brainstem dose guidelines is associated with a low risk of brainstem toxicity in pediatric patients. For young patients with posterior fossa tumors, particularly those who undergo aggressive surgery, our data suggest more conservative dosimetric guidelines should be considered.

The role of stereotactic body radiotherapy and stereotactic radiosurgery in the re-irradiation of metastatic spinal tumors

Stereotactic body radiotherapy (SBRT) and stereotactic radiosurgery (SRS) are advanced radiotherapy delivery techniques that allow for the delivery of high-dose per fraction radiation. Advances in imaging technology and intensity modulation have allowed SRS and SBRT to be used for the treatment of tumors in close proximity to the spinal cord and cauda equina, in particular spinal metastases. While the initial treatment of spinal metastases is often conventional palliative radiotherapy, treatment failure is not uncommon, and conventional re-irradiation may not be feasible due to spinal cord tolerance. SBRT and SRS have emerged as important techniques for the treatment of spinal metastases in the proximity of previously irradiated spinal cord. Here we review the current data on the use of SBRT and SRS spinal re-irradiation, and future directions for these important treatment modalities.

Re-irradiation: outcome, cumulative dose and toxicity in patients retreated with stereotactic radiotherapy in the abdominal or pelvic region

The purpose of the present study was to explore the outcome, cumulative dose in tumor and organs at risk and toxicity after extra-cranial stereotactic re-irradiation. Twenty-seven patients were evaluated who had been re-irradiated with stereotactic body radiotherapy (SBRT) after conventional radiotherapy (CRT). The dose summation of the SBRT and CRT plans was done by dose point calculations accounting for fraction size by the linear-quadratic model. Efficacy and toxicity was scored by looking at the reduction in tumor size, pain and bleeding. Symptomatic response was observed in 96% of the patients. The median maximum SBRT dose to the tumor was 90 Gy₃ (range: 42-420 Gy₃). The median cumulative dose for the rectum, bowel and bladder resulted in 104 Gy₃, 98 Gy₃ and 113 Gy₃, respectively. No grades 5, 4 and 3 acute and late toxicity was observed. In conclusion: re-irradiation to the same region using extra-cranial stereotactic radiotherapy is feasible and resulted in a 96% symptomatic response with low toxicity.

Nasopharyngeal carcinoma: salvage of local recurrence

Local control of nasopharyngeal carcinoma has substantially improved with advancing radiotherapy technology and appropriate combination with chemotherapy. However, when local recurrence occurs, this is one of the most difficult challenges. Aggressive treatment is indicated because long term salvage is achievable particularly for early recurrence, but high risk of complications is a serious concern. Treatment options include different methods of surgery and/or re-irradiation with/without chemotherapy. Available information in the literature is grossly inadequate; most reports compose of small series of highly selected patients with heterogeneous characteristics and treatment. No randomized trials have been performed to evaluate the therapeutic ratio of different treatment methods. This article reviews available treatment options, their therapeutic benefits and risks of complications; the objective is to provide information for developing treatment recommendations and suggestions for future improvement.

Spinal cord tolerance in the age of spinal radiosurgery: lessons from preclinical studies

Clinical implementation of spinal radiosurgery has increased rapidly in recent years, but little is known regarding human spinal cord tolerance to single-fraction irradiation. In contrast, preclinical studies in single-fraction spinal cord tolerance have been ongoing since the 1970s. The influences of field length, dose rate, inhomogeneous dose distributions, and reirradiation have all been investigated. This review summarizes literature regarding single-fraction spinal cord tolerance in preclinical models with an emphasis on practical clinical significance. The outcomes of studies that incorporate uniform irradiation are surprisingly consistent among multiple small- and large-animal models. Extensive investigation of inhomogeneous dose distributions in the rat has demonstrated a significant dose-volume effect while preliminary results from one pig study are contradictory. Preclinical spinal cord dose-volume studies indicate that dose distribution is more critical than the volume irradiated suggesting that neither dose-volume histogram analysis nor absolute volume constraints are effective in predicting complications. Reirradiation data are sparse, but results from guinea pig, rat, and pig studies are consistent with the hypothesis that the spinal cord possesses a large capacity for repair. The mechanisms behind the phenomena observed in spinal cord studies are not readily explained and the ability of dose response models to predict outcomes is variable underscoring the need for further investigation. Animal studies provide insight into the phenomena and mechanisms of radiosensitivity but the true significance of animal studies can only be discovered through clinical trials.

Monte carlo simulation of an X-ray pixel beam microirradiation system

Monte Carlo simulations are used in the development of a nanotechnology-based multi-pixel beam array small animal microirradiation system. The microirradiation system uses carbon nanotube field emission technology to generate arrays of individually controllable X-ray pixel beams that electronically form irregular irradiation fields having intensity and temporal modulation without any mechanical motion. The microirradiation system, once developed, will be incorporated with the micro-CT system already developed that is based on the same nanotechnology to form an integrated image-guided and intensity-modulated microirradiation system for high-temporal-resolution small animal research. Prospective microirradiation designs were evaluated based on dosimetry calculated using EGSnrc-based Monte Carlo simulations. Design aspects studied included X-ray anode design, collimator design, and dosimetric considerations such as beam energy, dose rate, inhomogeneity correction, and the microirradiation treatment planning strategies. The dosimetric properties of beam energies between 80-400 kVp with varying filtration were studied, producing a pixel beam dose rate per current of 0.35-13 Gy per min per mA at the microirradiation isocenter. Using opposing multi-pixel-beam array pairs reduces the dose inhomogeneity between adjacent pixel beams to negligible levels near the isocenter and 20% near the mouse surface.

A phase II study of primary reirradiation in squamous cell carcinoma of head and neck

BACKGROUND AND PURPOSE

In this prospective study, the effect of a second course of primary radiotherapy on locoregional control, survival and toxicity was investigated, in patients who underwent a second course of high dose irradiation for second primary or locoregional recurrent squamous cell head and neck carcinoma (HNSCC) in a previously irradiated area.

PATIENTS AND METHODS

A total of 34 patients with second primary (n=26) or locoregional recurrent (n=8) tumours were treated with a second course of high dose radiotherapy. Patients were selected for re-irradiation in case of inoperable and/or unresectable tumours. In most cases, the target volume for re-irradiation was confined to the gross tumour volume (GTV). No elective radiotherapy was applied in the former high-dose area. A total dose of 46 Gy was applied to elective areas with a boost up to 60 Gy with conventional fractionation. The median follow-up period was 32 months.

RESULTS

The locoregional control rate after 2 years was 27%. The 3-year overall survival was 22%. The most frequently reported acute side-effect was acute mucositis resulting in swallowing complaints. Pharyngeal and oesophageal late morbidity was also the most important late side-effect. In general, acute and late radiation-induced morbidity remained within acceptable limits.

CONCLUSIONS

In conclusion, primary re-irradiation appears to be feasible in terms of acute and late radiation-induced toxicity. To improve outcome in terms locoregional control and survival, future studies should be focussed on optimising radiation schedules and the addition of concomitant chemotherapy.

Prophylactic hyperbaric oxygen treatment and rat spinal cord re-irradiation

Normal tissue injury may lead to severe, life threatening, late side effects after therapeutic use of irradiation. Neurological complications caused by radiation of the spinal cord are ascribed to progressive, irreversible damage to the vasculature. Hyperbaric oxygen (HBO) is known to induce angiogenesis in irradiated tissue and has been proven to reduce late radiation injury in several normal tissues when applied during the latent period before complications become manifest. In the present study: (1). the prophylactic potential of HBO; (2). optimal timing of HBO therapy after spinal cord irradiation, i.e. during the latent period; and (3). effect of HBO on the re-irradiation tolerance of the spinal cord were investigated. The rat cervical spinal cord was locally X-ray irradiated with ten fractions of 6.5 Gy in 11 days. Five treatment groups (n=10) included: irradiation alone and irradiation followed by 30 HBO treatments (100% oxygen at 240 kPa for 90 min) during latency, with HBO starting either immediately, 5, 10 or 15 weeks after the primary irradiation course. One year after the primary treatment, the same spinal cord volume was re-irradiated with 20 Gy single dose. During life span, the animals were observed on the incidence of myelitis and the duration of the latent period. The actuarial analysis revealed no significant difference in neurological complications free survival between the irradiation alone and the irradiation+HBO treatment groups. A tendency towards radiosensitization was found in the group in which the primary irradiation course was immediately followed by the HBO treatment course. The data show that HBO applied during the latent period of progressively developing irradiation damage to the spinal cord does not increase the re-irradiation tolerance of this tissue.

Retreatment of the spinal cord with palliative radiotherapy

PURPOSE

We conducted this retrospective review of patients whose spinal cord was irradiated twice to evaluate the outcome in terms of palliation and long-term side effects.

METHODS AND MATERIALS

Eight patients (4 females, 4 males; median age: 67 years) were identified whose spinal cord had been irradiated twice between July 1990 and July 1997, usually for the management of bone metastases. All patients were followed up until their death from progressive disease. The Karnofsky performance score at the time of retreatment ranged from 20% to 90%. Total dose for the first treatment ranged from 29 to 50 Gy (median: 38 Gy) with single doses 1.25-3 Gy; the total dose for the retreatment ranged from 29 to 38 Gy (median: 30 Gy) with a single dose 1.8-4 Gy. The cumulative dose ranged from 59 to 88 Gy (median: 67.5 Gy). The overlap in the site of retreatment consisted of 1-3 segments, whereas in one patient, 2 single segments were treated twice. The outcome in terms of progressive disease, the palliative effects, and the development of myelopathy was assessed retrospectively.

RESULTS

The median interval to reirradiation was 30 months (range: 6-63 months), and the median follow-up after the last treatment was 16 months (range: 5-44 months). After reirradiation, 4/7 patients experienced complete pain relief, 2/7 patients experienced minor pain relief, and only 1 patient showed no change. Two patients with paraparesis experienced complete recovery. All patients tolerated retreatment very well. No serious acute side effects requiring any therapy were seen. During follow-up, no patient showed treatment-induced neurologic abnormalities affecting motor and sensory function, and all patients were able to walk and were continent for stool and urine.

CONCLUSION

On the basis of the findings in this limited number of patients, it is not possible to give clear and general recommendations concerning the optimal total dose and fraction size that will have maximal palliative effects and minimal side effects. However, for the purpose of palliation, retreatment within the dose range used at our institution should be considered, taking the patient's expected life span and clinical symptoms into account.

Late effects of ionizing radiation on the microvascular networks in normal tissue

Damage to the microvascular networks constitutes one of the most important components of ionizing radiation damage to normal tissue. Previously, we have reported the early (3, 7 and 30 days postirradiation) effects of ionizing radiation on the structure and function of normal tissue microvascular networks. Here we report on the late effects of ionizing radiation on the structural and functional changes in microvascular networks in locally irradiated (single 10-Gy dose) hamster cremaster muscles observed 60, 120 and 180 days postirradiation; age-matched animals were used as controls. As in the previous study, intravital microscopy was used to measure structural and functional parameters in complete microvascular networks in vivo. A factorial design was used to examine the effects of radiation status, time postirradiation, and network vessel type on the structure and function of microvascular networks. Our results indicate that the progression of radiation-induced microvascular damage continues during the late times but that there is partial recovery from radiation damage within 6 months postirradiation. Red blood cell flux, red blood cell velocity, and capillary blood flow in irradiated networks at 180 days postirradiation were significantly greater than control levels. As at the early times, all vessel types were not damaged equally by radiation at every time.

Tissue tolerance to reirradiation

There are increasing requests for delivering a second course of radiation to patients who develop second primary tumors within or close to previous radiotherapy portal or late in-field recurrences. Rational treatment decisions demand rather precise knowledge on long-term recovery of occult radiation injury in various organs. This article summarizes available experimental and clinical data on the effects of reirradiation to the skin, mucosa, gut, lung, spinal cord, brain, heart, bladder, and kidney. The data reveal that, in general, acutely responding tissues recover radiation injury within a few months and, therefore, can tolerate another full course of radiation. For late toxicity endpoints, however, tissues vary considerably in their capacity to recover from occult radiation damage. The heart, bladder, and kidney do not exhibit long-term recovery at all. In contrast, the skin, mucosa, lung, and spinal cord do recover subclinical injury partially to a magnitude dependent on the organ type, size of the initial dose, and, to a lesser extent, the interval between radiation courses. The available clinical data have inspired many radiation oncologists to undertake systematic studies addressing the efficacy and toxicity of reirradiation in various clinical settings. Hopefully, systematic scoring, collection, and analysis of patient outcome will produce quantitative data useful for clinical practice.

'Full dose' reirradiation of human cervical spinal cord

With the progress of modern multimodality cancer treatment, retreatment of late recurrences or second tumors became more commonly encountered in management of patients with cancer. Spinal cord retreatment with radiation is a common problem in this regard. Because radiation myelopathy may result in functional deficits, many oncologists are concerned about radiation-induced myelopathy when retreating tumors located within or immediately adjacent to the previous radiation portal. The treatment decision is complicated because it requires a pertinent assessment of prognostic factors with and without reirradiation, radiobiologic estimation of recovery of occult spinal cord damage from the previous treatment, as well as interactions because of multimodality treatment. Recent studies regarding reirradiation of spinal cord in animals using limb paralysis as an endpoint have shown substantial and almost complete recovery of spinal cord injury after a sufficient time after the initial radiotherapy. We report a case of "full" dose reirradiation of the entire cervical spinal cord in a patient who has not developed clinically detectable radiation-induced myelopathy on long-term follow-up of 17 years after the first radiotherapy and 5 years after the second radiotherapy.

Long-term recovery kinetics of radiation damage in rat spinal cord

PURPOSE

This study aimed to assess the influence of the level of initial injury on the long-term recovery kinetics of radiation damage in the central nervous system using a rat spinal cord model.

METHODS AND MATERIALS

The adult rat spinal cord (C2-T2) was initially given two or three daily fractions of 9 Gy, or three daily fractions of 10.25 Gy. At day 4 or weeks 6, 8, 12, 20, 28, 40, or 52, animals were reirradiated with graded single doses of X rays. The end point was forelimb paralysis caused by white-matter necrosis.

RESULTS

Latent times to paralysis as measured from the date of the initial treatment increased with increasing time interval between initial treatment and reirradiation but decreased with increasing size of initial injury. Retreatment ED50s were 14.1, 14.8, 15.4, 16.3, and 16.2 Gy for animals reirradiated at day 4 and weeks 8, 12, 20, and 28, respectively, after an initial dose of 9 Gy x 2. After 9 Gy x 3, the retreatment ED50s at day 4 and weeks 6, 8, 12, 20, 28, 40, and 52 were 10.0, 9.9, 9.8, 12.0, 13.9, 14.6, 14.7, and 15.5 Gy, respectively. For an initial dose of 10.25 Gy x 3, the retreatment ED50s at day 4 and weeks 8, 12, 20, 28, and 40 were 5.8, 6.1, 8.4, 10.6, 12.2, and 13.3 Gy, respectively. Using the linear-quadratic (LQ) model, alpha/beta of 3.0 Gy, to quantitate the biological effect of the different retreatment schedules, the initial doses of 9 Gy x 2 or 3, or 10.25 Gy x 3 were found to represent 47, 71, and 89% of the extrapolated response dose (ERD), respectively, and no significant increase in tolerance was observed for retreatment given within 8 weeks of initial treatment. Significant long-term recovery was observed thereafter and increased with increasing time interval to retreatment. The retreatment tolerance and radiation damage recovered at different intervals were influenced by the initial dose. Using direct analysis, the recovery kinetics could be best described by introducing a time function consisting of a linear and quadratic time component dependent on initial dose to the LQ model.

CONCLUSION

These results are consistent with the presence of significant long-term recovery of radiation damage in rat spinal cord, and suggest that the size of the initial damage influences the recovery kinetics, and hence the retreatment tolerance.

Reirradiation tolerance of the rat heart

PURPOSE

To investigate the influence of reirradiation on the tolerance of the heart after a previous irradiation treatment.

METHODS AND MATERIALS

Female Wistar rats were locally irradiated to the thorax. Development of cardiac function loss was studied with the ex vivo working rat heart preparation (20). To compare the retreatment experiments, initial, and reirradiation doses were expressed as the percentage of the extrapolated tolerance dose (ETD) (1).

RESULTS

Local heart irradiation with a single dose led to a dose-dependent and progressive decrease in cardiac function. The progressive nature of irradiation-induced heart disease is shown to affect the outcome of the retreatment, depending on both the time interval between subsequent doses and the size of the initial dose. The present data demonstrate that hearts are capable of repairing a large part of the initial dose of 10 Gy within the first 24 h. However, once biological damage as a result of the first treatment is fixed, the heart does not show any long-term recovery. At intervals up to 6 months between an initial treatment with 10 Gy and subsequent reirradiation, the reirradiation tolerance dose slightly decreased from 74% of the ETDref (at 24-h interval) to 68% of the ETDref (at 6-month interval). Between 6 and 9 months, reirradiation tolerance dose dropped more even to 43% of the ETDref. Treatment of the heart with an initial dose of 17.5 Gy, instead of 10 Gy, 6 months prior to reirradiation, also led to a further decrease of the reirradiation tolerance dose (< 38 vs. 68% of the ETDref).

CONCLUSIONS

The outcome of the present study shows a decreased tolerance of the heart to reirradiation at long time intervals (interval > 6 months). This has clinical implications for the estimation of reirradiation tolerance in patients whose mediastinum has to be reirradiated a long time after a first irradiation course.

Radiotherapeutic management of spinal metastases

Radiotherapy remains the primary treatment of malignant epidural spinal cord compression. Therapeutic success depends on diagnosis before the development of neurological compromise and the prompt initiation of radiotherapy. Radiotherapy alone is effective in over 85% of cases of spinal cord compression that occur in highly radioresponsive tumors (multiple myeloma, germ cell or lymphoproliferative tumors). In the more common tumors, like breast, prostate, and lung cancer, response to radiotherapy is based on presenting neurologic deficits, extent of disease, duration of symptoms, and overall clinical status, including other sites of metastatic involvement. Surgery is recommended in addition to radiotherapy in selected cases, and further study is needed to better define the prognostic and neurological parameters for the surgical management of spinal cord compression. Improvements in outcome in the treatment of spinal cord compression will require approaches like combined modality therapy because of the limitations primarily imposed by the radiation tolerance of the spinal cord.

High-dose reirradiation of head and neck cancer with curative intent

PURPOSE

This study evaluates the response of new or recurrent head and neck cancers and the response of associated normal tissues to high dose reirradiation with curative intent.

METHODS AND MATERIALS

From 1964 to 1991, 15 patients with in-field new second head and neck cancers and 85 patients with recurrent head and neck cancers have had high-dose reirradiation that overlapped with previously irradiated volumes. Reirradiation was given only to patients with no more than apparent minimal clinical radiation effects from the first radiation course. The reirradiation consisted of external beam only in 82 patients, external beam plus intracavitary or interstitial implant irradiation in 14 patients, and interstitial implant irradiation only in four patients. The combined overlapping dose from both the initial and subsequent irradiation (including brachytherapy) was 69-89 Gy in 14 patients, 90-99 Gy in 15 patients, 100-119 Gy in 27 patients, and 120 Gy or greater in 44 patients. Four patients had areas of overlap that received greater than 180 Gy.

RESULTS

The actuarial 5-year survival was 37% for patients with new second primary cancers and 17% for patients with recurrent cancers. Loco-regional tumor control was achieved in 60% of the patients with new tumors and in 27% of the patients with recurrent tumors. Nine of the 100 patients developed severe adverse normal tissue effects from the reirradiation.

CONCLUSION

High-dose reirradiation of head and neck cancers can be successful curative treatment in a significant proportion of patients. It is associated with substantial but acceptable risks in properly selected patients.

Disruption of the blood-brain barrier as the primary effect of CNS irradiation

The blood-brain barrier (BBB) is believed to be unique in organ microcirculation due to the 'tight junctions' which exist between endothelial cells and, some argue, the additional functional components represented by the perivascular boundary of neuroglial cells; these selectively exclude proteins and drugs from the brain parenchyma. This study was designed to examine the effects of irradiation on the BBB and determine the impact of the altered pathophysiology on the production of central nervous system (CNS) late effects such as demyelination, gliosis and necrosis. Rats, irradiated at 60 Gy, were serially sacrificed at 2, 6, 12 and 24 weeks. Magnetic resonance image analysis (MRI) was obtained prior to sacrifice with selected animals from each group. The remaining animals underwent horse-radish peroxidase (HRP) perfusion at the time of sacrifice. The serial studies showed a detectable disruption of the BBB at 2 weeks post-irradiation and this was manifested as discrete leakage; late injury seen at 24 weeks indicated diffuse vasculature leakage, severe loss of the capillary network, cortical atrophy and white matter necrosis. Reversal or repair of radiation injury was seen between 6 and 12 weeks, indicating a bimodal peak in events. Blood-brain barrier disruption is an early, readily recognizable pathophysiological event occurring after radiation injury, is detectable in vivo/in vitro by MRI and HRP studies, and appears to precede white matter necrosis. Dose response studies over a wide range of doses, utilizing both external and interstitial irradiation, are in progress along with correlative histopathologic and ultrastructural studies.

Irradiation and responsiveness to pain stimuli in rats

This study evaluates whether irradiation inhibits responses to pain in an animal model. We found that irradiation with doses of 10 Gy, 15 Gy and 17.5 Gy of the lumbar enlargement of the spinal cord inhibits the behavioural responses to the stimulus of the hot-plate. These doses were otherwise without effects. This data is discussed in view of the effects of irradiation of living cells, and we propose that a modification of pain signal processing is accomplished. Similar considerations apply to the human condition.

Re-irradiation tolerance of rat spinal cord to fractionated X-ray doses

Experiments were performed to assess the re-irradiation fractionation sensitivity in the rat cervical spinal cord. Animals were given initially three daily fractions of 9 Gy representing 75% of tolerance at the ED50 level. After an interval of 20 weeks, they were re-irradiated with graded doses of X-ray in single, 2, 5, 10 and 20 daily fractions, or a single retreatment top-up dose of 12.8 Gy (equivalent to 80% of retreatment tolerance) followed by doses in 1, 2, 4, 10 and 20 daily fractions. The end-point was paralysis of the forelimbs secondary to white matter necrosis. Latent periods to paralysis ranged from 188 to 245 days from initial irradiation, or from 48 to 105 days from re-irradiation. For a given fractionated retreatment schedule, shorter latent times were observed in animals re-irradiated to higher total doses. The re-irradiation ED50 values for single, 2, 5, 10 and 20 fractions were 14.0 (95% CI 13.3, 14.2), 20.5 (19.9, 21.2), 29.1 (28.0, 30.1), 36.3 (35.1, 37.4) and 47.8 Gy (46.2, 48.3), respectively. For re-irradiations with a 12.8 Gy top-up dose followed by doses in single, 2, 4 and 20 fractions, the retreatment ED50 values excluding the 12.8 Gy top-up dose were 4.5 (95% CI 3.0, 5.4), 6.5 (5.6, 7.3), 7.0 (5.0, 8.1) and 10.9 Gy (8.9, 12.5), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Late effects of radiation on the lumbar spinal cord of guinea pigs: re-treatment tolerance

PURPOSE

Using a guinea pig model of lumbar myelopathy, various factors affecting the tolerance of spinal cord to irradiation were assessed: (a) extent of initial injury (b) time interval between priming and test doses (c) animal age at the time of initial radiation treatment.

METHODS AND MATERIALS

A 3 cm section of lumbar spinal cord of guinea pigs was irradiated with fractionated doses of 4.5 Gy gamma rays given as 9 fractions per week. Guinea pigs were primed with 9 x 4.5 Gy in 7 days which is 60% of the ED50 for a continuous course of treatment. After 28 or 40 weeks, animals were retreated with 6-14 fractions of 4.5 Gy. Animals were observed for 2 years following the priming dose and both the incidence and latency of myelopathy recorded.

RESULTS

Young adult guinea pigs (8 wk old) showed both a decreased radiation tolerance and latency compared to old individuals (40 wk old). At 28 or 40 wk after 9 x 4.5 Gy, only about 8% of the initial injury was remembered in young adult guinea pigs.

CONCLUSION

The amount of residual injury was dependent on the initial damage as a proportion of the tolerance dose. The spinal cord shows a greater capacity for long-term recovery than generally appreciated and re-treatment doses clinically prescribed may be lower than necessary.

The effect of intraspinal cytosine arabinoside on the re-irradiation tolerance of the cervical spinal cord of young and adult rats

Intrathecal treatment with cytosine arabinoside (ara-C) in combination with radiation has been used as prophylactic treatment in children with acute lymphatic leukaemia. Animal experiments have shown that ara-C enhances the effect of radiation on the spinal cord when administered shortly before irradiation, and that the long-term recovery after a combined treatment may be impaired. In the present experiments immature, 3-week-old rats, were treated with ara-C and radiation on the cervical spinal cord, and the long-term recovery was examined by reirradiation after different intervals. The endpoint of the study was paresis due to radiation myelopathy. The results showed a clear enhancement of the radiation effect with a dose-modifying factor of 1.2, when ara-C was administered before irradiation. However, no indications for impaired long-term recovery were observed. Additional experiments in adult rats with ara-C treatments during a 6-month interval between two radiation doses also did not suggest any interference between ara-C treatment and long-term recovery of radiation induced injury. It is concluded that for both the adult and immature nervous tissue, only when ara-C is administered intraspinally shortly before irradiation, interaction between ara-C and radiation results in a significant reduction of the isoeffective radiation dose by a factor of 1.2 (1.13-1.37, 95% confidence interval).

Fractionation sensitivity of the rat cervical spinal cord during radiation retreatment

Data concerning the fractionation sensitivity of normal tissues during radiation retreatment are limited. Experiments were performed to investigate whether the fractionation sensitivity of the rat cervical spinal cord is changed during retreatment 6 months after a first dose of 15 Gy, representing about half the biologically effective dose for induction of paresis. After a 6 months interval, the long-term recovery from the first treatment was about 45%. The fractionation sensitivity of the rat cervical spinal cord during reirradiation was not significantly different from the fractionation sensitivity of not previously irradiated control rats, with an alpha/beta ratio of 2.3 Gy in control rats and 1.9 Gy during reirradiation of the spinal cord. An additional observation from these experiments was the presence of incomplete repair after fractionated treatment with 2 fractions of 3 Gy per day with 10-h intervals.

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

To investigate the influence of age at irradiation on single dose radiation tolerance and fractionation sensitivity, the cervical spinal cord of rats was irradiated at the age of 1 week and at 15-18 weeks (adult). While the main histological lesions seem to be comparable after irradiation at the two ages, differences were found in single dose tolerance, latency to paresis due to white matter lesions, and fractionation sensitivity. The 50% effect dose (ED50) for single dose irradiation at one week was 19.5 Gy, which is only 10%, but significantly (p < 0.05), lower than the ED50 of about 21.5 Gy at 3 weeks and above. The latency to paresis was clearly influenced by the age at irradiation. The latency in the rats irradiated at 1 week was about 2 weeks, while for adult rats a latency of about 8 months was observed. The fractionation sensitivity for irradiation at 1 week was lower than the fractionation sensitivity of the adult rats; the alpha/beta value at 1 week was estimated to be 4.5 Gy, while for the adult rats an alpha/beta value of 1.8 Gy was found. As a consequence, the observed small difference in tolerance to single doses between 1 week-old and adult rats is further enhanced after fractionated irradiation. During prolonged irradiation treatments this decreased tolerance may be compensated by a higher proliferation rate in the immature central nervous system. The results of the present experiments indicate that, for a single tissue and endpoint, paresis due to white matter lesions in the rat cervical spinal cord, the latency to expression of damage and the fractionation sensitivity clearly change with age at irradiation.