Fractionation sensitivity of the rat cervical spinal cord during radiation retreatment

[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.

Optimal radiotherapy dose schedules under parametric uncertainty

We consider the effects of parameter uncertainty on the optimal radiation schedule in the context of the linear-quadratic model. Our interest arises from the observation that if inter-patient variability in normal and tumor tissue radiosensitivity or sparing factor of the organs-at-risk (OAR) are not accounted for during radiation scheduling, the performance of the therapy may be strongly degraded or the OAR may receive a substantially larger dose than the allowable threshold. This paper proposes a stochastic radiation scheduling concept to incorporate inter-patient variability into the scheduling optimization problem. Our method is based on a probabilistic approach, where the model parameters are given by a set of random variables. Our probabilistic formulation ensures that our constraints are satisfied with a given probability, and that our objective function achieves a desired level with a stated probability. We used a variable transformation to reduce the resulting optimization problem to two dimensions. We showed that the optimal solution lies on the boundary of the feasible region and we implemented a branch and bound algorithm to find the global optimal solution. We demonstrated how the configuration of optimal schedules in the presence of uncertainty compares to optimal schedules in the absence of uncertainty (conventional schedule). We observed that in order to protect against the possibility of the model parameters falling into a region where the conventional schedule is no longer feasible, it is required to avoid extremal solutions, i.e. a single large dose or very large total dose delivered over a long period. Finally, we performed numerical experiments in the setting of head and neck tumors including several normal tissues to reveal the effect of parameter uncertainty on optimal schedules and to evaluate the sensitivity of the solutions to the choice of key model parameters.

Radiobiological effects of total body irradiation on the spinal cord

Total body Irradiation (TBI) is often used for conditioning, prior to bone marrow transplantation. Doses of 8-14 Gy in 1-8 fractions over 1-4 days are administered using low dose rate external beam radiotherapy (EBRT). When necessary, consolidation EBRT using conventional doses, fractionation and dose rate is given. The irradiated volume usually contains critical organs such as spinal cord. The purpose of this study was to assess the biologic effect of TBI on the spinal cord in terms of EQD(2) (equivalent dose given in fractions of 2 Gy). EQD(2) values were calculated using the linear-quadratic generalized incomplete repair (IR) model that incorporates IR between fractions and low dose rate irradiation corrections and accounts for mono and bi-exponential repair. Three fractionation schemes were studied as function of dose rate: 8 Gy in 1 and 2 fractions and 12 Gy in 8 fractions. For the 12 Gy in 8 fractions scheme, the influence of dose rate on EQD(2) was limited because the effect of IR between fractions dominates. For the 8 Gy in 1 fraction scheme, significant sparing of the spinal cord may be achieved for low dose rate (5-20 cGy/min). The extent of effects depends on the parameters used. The IR model provides a useful mathematical framework for examination of the effects of fractionated treatments of varying dose rate. Reliable experimental data are needed for accurate assessment of radiation damage to the spinal cord following fractionated low dose rate TBI.

The impact on oncology of the interaction of radiation therapy and radiobiology

The quantitative analysis of the cell dose-survival curves, the randomness of cell killing by radiation, the probabilistic basis of the response to irradiation of tumors and normal tissues, the understanding of the biological mechanisms underlying to this response, the rationale for dose-time and fractionation in radiotherapy, the introduction of the concepts of tumor control probability (TCP) and biologically effective dose (BED), the relationships TCP-dose, BED-alpha/beta BED-fraction size and BED-treatment time, the problems associated with the accelerated regeneration of surviving tumor clonogens during the course of fractionated radiotherapy, the new demands of knowledge on oncology and radiation biology derived from heterogeneous dose distributions in conformal radiation therapy programs and the definition of the biological basis of normal tissues tolerance to reirradiation are, probably, the most important contributions of radiobiology to clinical radiotherapy in the last twenty five years. Radiotherapy is today a scientific discipline based on the interplay of mathematics, physics, biology and oncology. The knowledge of the basic concepts of radiobiology is essential for daily radiotherapy practices and for all oncologists. The most efficient use of multimodality treatments in cancer therapy cannot be done without a clear understanding of these principles.

Proposal of human spinal cord reirradiation dose based on collection of data from 40 patients

PURPOSE

Driven by numerous reports on recovery of occult radiation injury, reirradiation of the spinal cord today is considered a realistic option. In rodents, long-term recovery was observed to start at approximately 8 weeks. However, prospective clinical studies are lacking. Therefore, a combined analysis of all published clinical data might provide a valuable basis for future trials.

METHODS AND MATERIALS

We collected data from 40 individual patients published in eight different reports after a comprehensive MEDLINE search. These represent all patients with data available for dose per fraction and total dose of each of both treatment courses. We recalculated the biologically effective dose (BED) according to the linear-quadratic model using an alpha/beta value of 2 Gy for the cervical and thoracic cord and 4 Gy for the lumbar cord. In this model, a dose of 50 Gy given in single daily fractions of 2 Gy is equivalent to a BED of 100 Gy(2) or 75 Gy(4). For treatment with two daily fractions, a correction term was introduced to take incomplete repair of sublethal damage into account.

RESULTS

The cumulative doses ranged from 108 to 205 Gy(2) (median dose, 135 Gy(2)). The median interval between both series was 20 months. Three patients were treated to the lumbar segments only. The median follow-up was 17 months for patients without myelopathy. Eleven patients developed myelopathy after 4-25 months (median, 11 months). Myelopathy was seen only in patients who had received one course to a dose of > or =102 Gy(2) (n = 9) or were retreated after 2 months (n = 2). In the absence of these two risk factors, no myelopathy developed in 19 patients treated with < or =135.5 Gy(2) or 7 patients treated with 136-150 Gy(2). A risk score based on the cumulative BED, the greatest BED for all treatment series in a particular individual, and interval was developed. Low-risk patients remained free of myelopathy and 33% of intermediate-risk patients and 90% of high-risk patients developed myelopathy.

CONCLUSION

On the basis of these literature data (and with due caution), the risk of myelopathy appears small after < or =135.5 Gy(2) when the interval is not shorter than 6 months and the dose of each course is < or =98 Gy(2). We would recommend limiting the dose to this level, whenever technically feasible. However, it appears prudent to propose the collection of prospective data from a greater number of patients receiving doses in the range of 136-150 Gy(2) to assess the safety of higher retreatment doses for those patients in whom limited doses might compromise tumor control.

Modulation of rodent spinal cord radiation tolerance by administration of platelet-derived growth factor

PURPOSE

To examine the role of platelet-derived growth factor (PDGF) for ameliorating radiation myelopathy of the cervical spinal cord in a rodent model.

METHODS AND MATERIALS

After developing the technique for cannulation of the basal cistern, initial animal experiments were conducted to test the feasibility of intrathecal continuous infusion of PDGF in a model of cervical spinal cord irradiation in adult Fisher F-344 rats and to determine the most effective dose level of PDGF. Subsequently, the dose-modification factor was determined in a larger group of rats. Irradiation was given in 2 fractions (16 Gy followed by 14-24 Gy) and animals were examined for the development of paresis.

RESULTS

The initial dose-finding experiment revealed significant differences in the incidence of radiation myelopathy (100% in saline-treated control rats, 25% with the most effective dose of PDGF, up to 100% with less effective doses). The most effective dose of PDGF was 0.014 mug per day. Subsequent experiments revealed a median effective dose (ED(50)) of 35.6 Gy (95% confidence interval, 34.7-36.5 Gy) for animals receiving this dose of PDGF in contrast to 33.8 Gy (33.4-34.3 Gy) for the control group (p = 0.003). The dose-modification factor obtained with this dose of PDGF was 1.05.

CONCLUSIONS

Intrathecal administration of PDGF concomitant to irradiation of the cervical spinal cord in rats was feasible. Treatment with PDGF significantly increased the tolerance of the spinal cord. The PDGF experiments should be viewed as a proof of principle that brief therapeutic intervention in the earliest phase of damage induction can reduce late effects in the spinal cord. They form the basis for further studies of growth factor administration in this particular model.

Application of the linear-quadratic model to combined modality radiotherapy

PURPOSE

Methods of performing dosimetry for a combined modality radiotherapy (CMRT) consisting of a targeted radionuclide therapy (TRT) and separately delivered external beam therapy (EBT) have been established using the biologically effective dose (BED). However, a concurrent delivery of the two therapies may influence the radiobiologic effect of the treatment resulting from interaction between the therapies, and this situation has been modeled to assess the likely consequences of this regime.

METHODS AND MATERIALS

A general form of the linear-quadratic model with a dose protraction factor was applied to concurrent delivery of EBT and TRT. Contributions to total BED from intra- and intermodality effects were calculated, and parameter values varied to determine conditions under which the intermodality contributions were likely to be most significant. A Poisson model of tumor control probability (TCP) was used to assess the predicted effect of concurrent delivery on treatment outcome.

RESULTS

In general, over a wide range of parameter values, the effect of intermodality interactions in CMRT is small, increasing total BED delivered to tumor by approximately 1%, and producing a negligible increase in TCP. Synergistic effects could be greater in normal tissues if high doses were received from both therapies, with intermodality terms increasing total BED delivered by approximately 6% in the general case, and by approximately 18% for the case of slow repair in the spinal cord. A significant synergistic effect was predicted between EBT and I-125 seed therapy of the prostate when values of alpha/beta = 1.2 Gy, alpha = 0.026 Gy, mu = 0.36 h(-1) and N(0) = 138 clonogens were used, with TCP increasing from approximately 0.5 to 0.6.

CONCLUSIONS

Under most clinical conditions, the relative temporal delivery of these two therapies is unlikely to significantly influence the overall radiobiologic effect to tumor at the cellular level. Synergistic effects may, however, be more significant in normal tissues and for tumors with low values of alpha/beta and alpha.

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.

Programming of radiotherapy in the treatment of non-small-cell lung cancer--a way to advance care

Radical radiotherapy, the mainstay of treatment for early inoperable non-small-cell lung cancer, is most commonly given in daily fractions, Monday to Friday, to a total dose of 60-70 Gy over 6-8 weeks. Since the 1980s, novel fractionation schedules have been explored with the aim of improving local tumour control and survival without increasing late morbidity. There have been two main approaches. In hyperfractionated radiotherapy the dose per fraction is reduced and the total dose increased to give improved tumour control without increased late morbidity. Hyperfractionation schedules, with more than one fraction per day have been successfully evaluated, but so far significant benefit has not been achieved when compared with conventional radiotherapy plus chemotherapy. In accelerated radiotherapy the overall duration of radiotherapy is reduced to overcome repopulation of tumour cells during the course of treatment. In all the different regimens of accelerated radiotherapy a common feature is giving two or more fractions on some or all treatment days and, in some cases, a lower dose per fraction is also incorporated. CHART (continuous hyperfractionated accelerated radiotherapy) is the most novel and accelerated schedule tested, and a randomised controlled trial showed a significant survival advantage from CHART compared with conventional radiotherapy. Changes in the fractionation of radiotherapy must be combined with other approaches such as neoadjuvant and concomitant chemotherapy, hypoxic-cell modifiers, and conformal radiotherapy, so that care of patients with non-small-cell lung cancer can be further advanced.

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.

Total biological effect on late reactive tissues following reirradiation for recurrent nasopharyngeal carcinoma

PURPOSE

To assess the additional damage of normal tissues attributable to reirradiation and the magnitude of partial recovery following the initial course.

METHODS AND MATERIALS

Symptomatic late complication rates (excluding xerostomia) in 3635 patients receiving one course (Group 1) and 487 patients receiving two courses of external radiotherapy (Group 2) for nasopharyngeal carcinoma were retrospectively analyzed and compared.

RESULTS

Group 2 had significantly lower actuarial complication-free survival rates than Group 1: 48% versus 81% at 5 years. The post-retreatment incidence was significantly affected by biologically effective dose (BED) (assuming an alpha/beta ratio of 3 Gy) of the first course: hazard ratio (HR) = 1.04 per Gy(3) (p = 0.01), but only marginally by that of the second course: HR = 1.01 per Gy(3) (p = 0.06). If the summated BED was taken as the dose unit, it was estimated that a total BED of 143 Gy(3) would induce a 20% incidence at 5 years, while the corresponding dose projected from Group 1 was 111 Gy(3). The gap effect was insignificant in the overall analyses, but a trend of decreasing risk with increasing interval was observed in patients with gap > or = 2 years: HR = 0.86 per year (p = 0.07).

CONCLUSION

The major determinant of post-retreatment complication is the severity of damage during the initial course. The sum of total doses tolerated is higher than that expected with a single-course treatment, suggesting occurrence of partial recovery (particularly in those reirradiated after an interval of 2 years or more).

'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.

Boron neutron capture therapy: re-irradiation response of the rat spinal cord

PURPOSE

To evaluate the retreatment response of the CNS to BNC irradiation using a rat spinal cord model.

MATERIALS AND METHODS

Fischer 344 rats were irradiated with single doses of 6 MeV X-rays which were 22, 40 or 80% of a total effect (TE). An additional group of rats was irradiated with a single exposure of thermal neutrons in the presence of the neutron capture agent boronophenylalanine (BPA) to a dose that represented 82% of the TE. After an interval of 26 weeks, animals were re-irradiated using various single doses of thermal neutrons in combination with BPA.

RESULTS

The re-irradiation ED50 doses represented 77, 80 or 50% of the TE after an initial X-ray dose of 22, 40 or 80% of the TE, respectively. The re-irradiation ED50 dose was 55% of the TE after an initial BNC irradiation dose representing 82% of the TE.

CONCLUSION

The level of the initial radiation damage had a direct bearing on the re-irradiation response. Recovery following initial treatment with BNC irradiation was similar to that after initial irradiation with X-rays.

Absence of thoracic radiation myelitis after hyperfractionated radiation therapy with and without concurrent chemotherapy for Stage III nonsmall-cell lung cancer

PURPOSE

To investigate whether thoracic spinal cord dose of 50.4 Gy given via 1.2 Gy b.i.d. fractionation carries a risk of developing radiation myelitis during studies using hyperfractionated radiation therapy (HFX RT) with and without concurrent chemotherapy (CHT).

METHODS AND MATERIALS

Of 300 patients with Stage III nonsmall-cell lung cancer (NSCLC) who were treated on two consecutive Phase III studies, 158 patients received 50.4 Gy to a portion of their spinal cord and survived > 1 year after the beginning of the therapy.

RESULTS

None of these 158 patients developed thoracic radiation myelitis. Therefore, influence of potentially contributing factors on the occurrence of radiation myelitis, such as interfraction interval, or those unproven yet, such as cord length or administration of concurrent CHT, was not possible to investigate.

CONCLUSION

Given the continuing interest in HFX RT and encouraging results obtained in studies in lung cancer, further investigation is needed to get more informations about risks of developing thoracic radiation myelitis with this cord dose.

Kinetics of repair in the spinal cord of the rat

PURPOSE

Split dose experiments were carried out with two 2 Gy fractions per day at intervals ranging from 0.5 to 24 h, in order to investigate both the time to complete repair and the detailed kinetics of repair of sublethal damage in the cervical spine of rats.

MATERIALS AND METHODS

Male rats of the WAG/Rij strain were irradiated at 2 Gy/min with 18 MV photons to a length of 18 mm of cervical spinal cord. Four hundred twenty-three rats were irradiated without top-up doses to investigate whether repair was complete by 24 h or whether any slow repair or proliferation occurred up to 50 days after irradiation. Three hundred seventy-nine rats were also irradiated in split dose (2 Gy + delta t + 2 Gy each day) experiments, with intervals of 0.5, 1, 2, 4, 8 and 24 h. The split dose irradiations were followed by a single top-up dose of 15 Gy (producing about half the total damage).

RESULTS

Repair was complete by 24 h as the ED50 values were the same at 1, 11 and 50 day intervals for two large fractions, and for 10 fractions in 10 or 50 days. A mono-exponential component of repair of T1/2 = 0.25 (95% CI 0.16-0.48) h was determined by direct analysis using all the data and T1/2 = 0.37 (0.28-0.53) h for the split 2 Gy doses with top-up only. A bi-exponential analysis did not fit better. The presence of a second component was demonstrated graphically, with T1/2 of about 6.5 h but with a wide confidence interval from near 0 to 13 h. However, the 24 h ED50 was significantly different from all ED50s except the 8 h value. Considering all data together, an upper limit of about 7 h could be placed on any long component, or else repair could not be complete by 24 h.

DISCUSSION AND CONCLUSIONS

Two components of repair (0.7 and 3.8 h) have been reported by Ang et al. (Ang, K.K., Jiang, G.L., Guttenberger, R., Thames, H.D., Stephens, L.C., Smith, C.D. and Feng, Y. Impact of spinal cord repair kinetics on the practice of altered fractionation schedules. Radiother. Oncol. 25: 287-294, 1992) in the spinal cord of Sprague-Dawley rats. Two components have also been reported by others more recently. The present results could, with its graphical interpretation, agree in principle, but with a shorter fast component and a longer slow component. A slow component of 5.5 h was reported by Ruifrok et al. (Ruifrok, A.C.C., Kleiboer, B.J. and van der Kogel, A.J. Fractionation sensitivity of rat cervical spinal cord during radiation retreatment. Radiother. Oncol. 25: 295-300, 1992) in a related strain of WAG/Rij rats. The possible presence of a slower component than Ang et al.'s 3.8 h might help to explain the four myelopathies observed in the pilot studies for the CHART clinical trial. The presence of the definite fast component (< 0.5 h) could have important consequences when pulsed brachytherapy is used to replace continuous low dose rate irradiation.

Is pulsed dose rate more damaging to spinal cord of rats than continuous low dose rate?

BACKGROUND AND PURPOSE

Theoretical calculations suggest that pulsed dose-rate irradiation (PDR) should have approximately the same effectiveness as continuous low dose-rate (CLDR) when the same total dose is given in the same overall time, unless large doses per pulse (> 2 Gy) are used and/or non-exponential or very short half-times of repair (< 0.5 h) are present in the irradiated tissues. However, few animal experiments have been reported to test this theory, and some of them gave contradictory results. We have carried out experiments to determine whether PDR irradiation of 18 mm of cervical spinal cord in the rat was more or less effective than CLDR at 0.5-1 Gy/h, when the overall average dose rate during each day of PDR was close to the overall CLDR average dose rate.

MATERIALS AND METHODS

PDR was simulated at a within-pulse dose rate of 4 Gy/h by filtered 18 MV X-rays from a linear accelerator. Two PDR schedules were used, 0.69 Gy at 1 h repetition (9 pulses per day) and 2 Gy at 3 h repetition (4 pulses per day), with overnight intervals of 16 and 15 h, respectively. The CLDR was delivered from iridium-192 wires in two concentric rings around a collar designed to fit the necks of rats so that they could eat and drink during the 72 h that was always the duration of the CLDR. Dose rate was then proportional to total CLDR dose. A range of doses was used to obtain dose response-curves, with a 15 Gy top-up dose (at 2 Gy/min, HDR) given on the day after the end of the PDR or CLDR irradiations. Animals were observed for at least 9 months to see whether fore-limb myelopathy developed. A total of 6-8 rats was irradiated per dose point, in two sets of experiments at an interval of 12 months.

RESULTS

A set of 2 Gy fractions (at HDR) given daily, followed by the same top-up dose of 15 Gy at HDR, was available from a previous experiment for planning. Its ED50 was 61.2 Gy. The ED50 values found for the PDR schedules with 2 Gy at 3 h and 0.69 Gy at 1 h were 59.9 and 60.2 Gy, respectively. These were just 2% more effective than the daily HDR fractions, similar to expectations from theory if two components of repair are present. However, the CLDR irradiations resulted in no myelopathy even after doses up to 68 Gy at 0.94 Gy/h.. Thus PDR over 7 days (not at nights) appears to be more effective than CLDR over 3 days, with an effective dose-modifying factor of at least 1.1 to 1.17.

DISCUSSION AND CONCLUSIONS

Reasons for this absence of effect with CLDR in these experiments are discussed, the most likely explanation being that a substantial component of repair with very short T1/2 (< 0.5 h) was present in spinal cord of these rats. There is evidence from other experiments elsewhere and in our laboratory for such a fast component of repair.

Lack of influence of sequence of top-up doses on repair kinetics in rat spinal cord

BACKGROUND AND PURPOSE

The rat spinal cord model was used to determine whether repair kinetics changed during a course of fractionated radiotherapy if twice daily doses were given either at the initial or final period of a concomitant boost irradiation schedule.

MATERIALS AND METHODS

The rat cervical spinal cord was irradiated from C2-T2 in 870 animals with top-up doses of three daily fractions of 9 Gy representing 75% of the biologic dose at the ED50 level for white matter necrosis. To simulate concomitant boost protocols, these top-up doses were given either preceding (initial top-up) or following (final top-up) a b.i.d. schedule of 1 Gy/F delivered at 0, 1, 2, 4, 8 or 24 h interfraction intervals. The end point was forelimb paralysis secondary to white matter necrosis.

RESULTS

For interfraction intervals of 0, 1, 2, 4, 8 and 24 h, the initial top-up schedules yielded ED50 values of 18.2, 19.2, 23.7, 21.3, 27.2 and 29.7 Gy, respectively; the corresponding ED50s from the final top-up schedules were 17.5, 19.0, 20.7, 21.2, 26.9 and 30.3 Gy, respectively. A 10% reduction in the ED50 value from pooled data was observed when the interfraction interval was reduced from 24 (ED50 = 30.3 Gy) to 8 h (ED50 = 27.1 Gy). Fitting the incomplete repair (IR) version of the LQ model with mono-exponential repair kinetics gave alpha/beta values of 1.4 and 1.5 Gy, and similar repair half-times of 4.3 and 5.0 h for the initial and final top-up experiments, respectively. The IR model with bi-exponential repair kinetics did not provide a better fit to the data.

CONCLUSIONS

We conclude that the sequence of top-up doses has no apparent influence on radiation sensitivity or repair kinetics in the rat spinal cord. The clinical implication is that the interfraction interval but not the timing of the boost is a critical determinant of spinal cord tolerance in concomitant boost protocols.

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.

The extent, time course, and fraction size dependence of mouse spinal cord recovery from radiation injury

PURPOSE

This experiment was designed to assess: (a) the influence of fraction size and time interval between fractions on the tolerance of the spinal cord to high cumulative doses of radiation; and (b) the influence of the long-term recovery process on the tolerance of the spinal cord to reirradiation.

METHODS AND MATERIALS

The T10-L2 level of the spinal cord of C3Hf mice was irradiated using a conventionally fractionated regimen of 2.0 Gy once daily, a prolonged fractionated regimen of 1.2 Gy once daily, a hyperfractionated regimen of 1.2 Gy twice daily, or a single dose of 12 Gy followed 0-190 days later by a second dose of 5-20 Gy. Mice in the multifractionated regimen groups were given a single 15 Gy top-up dose 24 h after reaching a cumulative fractionated dose of 24-70 Gy. Hind limb strength was measured weekly for 2 years after the completion of irradiation.

RESULTS

Paralysis occurred in a bimodal time distribution, with peaks at 5-10 months and 15-23 months after the completion of irradiation. The cumulative radiation dose was directly associated with the incidence of paralysis in each radiation schedule (p < 0.0001) and inversely associated with the time to onset of paralysis in the 1.2 Gy b.i.d. (p = 0.0001) and 2.0 Gy q.d. schedules (p = 0.03). The median latency of paralysis in each group was inversely associated with the incidence of paralysis in that group (p < 0.001). Decreasing the fraction size from 2.0 to 1.2 Gy once daily markedly increased the radiation tolerance of the spinal cord (p < 0.0001), consistent with a very small alpha-beta value of -0.30 Gy (approximately 95% confidence interval -0.72, +0.18) in the linear-quadratic model. Decreasing the time interval from 24 h to alternating 8 and 16 h periods produced an offsetting diminuation in cord tolerance (p < 0.0001). The 1.2 Gy once daily schedule resulted in ED20 and ED50 values that were approximately double those of the 2.0 Gy once daily and the 1.2 Gy twice daily schedules and a relative risk of paralysis from a given dose that was 0.03 times the risk associated with the other two regimens (p < 0.0001). There was no significant difference between the 2.0 Gy once daily and the 1.2 Gy twice daily dose-paralysis curves (p = 0.86). The residual from a single 12 Gy radiation dose was 17% after 190 days, leaving the retreatment ED50 only 10% below the ED50 of previously unirradiated spinal cord. The relative risk of paralysis after 12 Gy plus a second radiation dose decreased from 1.00 with no time interval between doses to 0.51-0.73 with a 0.25, 1 or 3 day interval, 0.32 with a 7 day interval, 0.11 with a 30 day interval, and 0.06 with a 190 day interval.

CONCLUSION

The increased radiation tolerance of the murine spinal cord produced by decreasing the fraction size from 2.0 to 1.2 Gy was offset by the diminished tolerance produced by decreasing the time interval between fractions from 24 to 8-16 h, resulting in no significant difference in the dose-paralysis curves of conventional and hyperfractionated radiation schedules. The rodent spinal cord eliminates the majority of the occult radiation injury produced by a radiation dose equal to half the ED50 during the months following irradiation. This permits retreatment of previously irradiated spinal cord to high doses without the induction of myelopathy.

Radiation myelopathy following single courses of radiotherapy and retreatment

PURPOSE

To assess the latent time, survival and dose-fractionation factors associated with permanent radiation myelopathy following single and multiple courses of radiotherapy to the spinal cord.

METHODS AND MATERIALS

A retrospective analysis was undertaken of all patients who were registered at the Princess Margaret Hospital between 1955 and 1985, and who developed permanent radiation myelopathy. There were 22 males and 13 females with ages ranging from 30 to 72 years. Twenty-four patients developed permanent myelopathy after one course of radiation therapy and 11 patients following retreatment. Seven patients had histological confirmation of radiation myelopathy at autopsy.

RESULTS

The actuarial survival was 14% at 5 years (median: 8.3 months) from the date of diagnosis of radiation myelopathy. Latent times for myelopathy following a single course of treatment (mean: 18.5 months, 7-57 months), were significantly longer than those after reirradiation (mean: 11.4 months, 4-25 months), p = 0.03. There was not a single incident of myelopathy in patients who received fractionated radiotherapy given once daily to an extrapolated response dose (ERD) of < or = 100 Gy2 (equivalent to 50 Gy in 25 daily fractions). Four patients who developed myelopathy after an ERD of < 100 Gy2 were all treated on accelerated fractionation protocols with multiple fractions given per day. Patients who were reirradiated received significantly higher doses (mean ERD of 148 Gy2) than those who had a single course of treatment (mean ERD of 121 Gy2), p = 0.001.

CONCLUSION

We conclude that the risk of radiation myelopathy following conventionally fractionated radiotherapy to the spinal cord is extremely small; giving multiple fractions per day reduces the spinal cord tolerance; latent time to myelopathy decreases following retreatment; and there is possible long-term recovery of radiation damage in the human spinal cord.

Loss of reirradiation tolerance in the kidney with increasing time after single or fractionated partial tolerance doses

The aim was to determine the influence of initial dose and dose per fractionation on retreatment tolerance of the kidney. Mouse kidney was bilaterally irradiated with various single or fractionated X-ray doses equivalent to about 12-70% of a defined response dose. The mice were retreated with a range of single doses after 2 or 26 weeks. The development of functional kidney damage was followed by monthly testing of clearance of 51CrEDTA until the animals expressed overt renal dysfunction (maximum follow-up 70 weeks after retreatment). Reirradiation tolerance was assessed by probit analysis and Kaplan-Meier actuarial estimates of the incidence of a defined level of renal damage at 40 weeks after retreatment. Doses required to give a 50% incidence of damage (RD50) were compared for animals that had received previous single dose or fractionated irradiations, or that were previously unirradiated. Multivariate analysis of time to expression of renal damage (latency) was also done using the Cox Proportional Hazards model. Results demonstrated that previous irradiation always compromised retreatment tolerance, even for intervals of 26 weeks after initial treatments with < 20% full response dose. Reirradiation tolerance was inversely related to the initial dose and tolerance decreased significantly with increasing interval between treatments, suggesting progression rather than recovery from the initial damage. Linear-quadratic analysis of the data for reirradiation at 26 weeks after partial-response doses gave an alpha/beta = 1.4 Gy. This was significantly lower than the alpha/beta = 3.3 Gy obtained for initial treatments alone (no retreatment), indicating a larger fractionation-sparing effect for the retreatment situation.

LQ-based model for biological radiotherapy planning

Despite the increasing accumulation of radiobiological data, radiotherapy planning does not take into account the alpha/beta values of normal irradiated tissues. The importance of the fact that the dose per fraction outside the 100% isodose area is not at all identical to the one inside the tumor area is also underestimated. Altered fractionation regimens further complicate the reliability of the conventional radiotherapy plans. In this study we report a theoretical application of the Macejewski's concept of "normalised total dose" that could make feasible the integration of alpha/beta and dose per fraction values in everyday radiotherapy practice. The concept of cumulative normalised total dose permits the preparing of radiotherapy plans with normalised isodose areas (the cumulative isodose area maps) for chosen ranges of radiation normalised doses. The effect of overall treatment time and interfraction interval is also taken into account. The present study suggests some guidelines that could be of value for the elaboration of a computer program for biological radiotherapy planning.

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)

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.