The radiation response of the cervical spinal cord of the pig: effects of changing the irradiated volume

Spinal cord tolerance to single-session uniform irradiation in pigs: implications for a dose-volume effect

BACKGROUND AND PURPOSE

This study was performed to test the hypothesis that spinal cord radiosensitivity is significantly modified by uniform versus laterally non-uniform dose distributions.

MATERIALS AND METHODS

A uniform dose distribution was delivered to a 4.5-7.0 cm length of cervical spinal cord in 22 mature Yucatan minipigs for comparison with a companion study in which a laterally non-uniform dose was given [1]. Pigs were allocated into four dose groups with mean maximum spinal cord doses of 17.5 ± 0.1 Gy (n=7), 19.5 ± 0.2 Gy (n=6), 22.0 ± 0.1 Gy (n=5), and 24.1 ± 0.2 Gy (n=4). The study endpoint was motor neurologic deficit determined by a change in gait within one year. Spinal cord sections were stained with a Luxol fast blue/periodic acid Schiff combination.

RESULTS

Dose-response curves for uniform versus non-uniform spinal cord irradiation were nearly identical with ED(50)'s (95% confidence interval) of 20.2 Gy (19.1-25.8) and 20.0 Gy (18.3-21.7), respectively. No neurologic change was observed for either dose distribution when the maximum spinal cord dose was ≤ 17.8 Gy while all animals experienced deficits at doses ≥ 21.8 Gy.

CONCLUSION

No dose-volume effect was observed in pigs for the dose distributions studied and the endpoint of motor neurologic deficit; however, partial spinal cord irradiation resulted in less debilitating neurologic morbidity and histopathology.

Biological mechanisms of normal tissue damage: importance for the design of NTCP models

The normal tissue complication probability (NTCP) models that are currently being proposed for estimation of risk of harm following radiotherapy are mainly based on simplified empirical models, consisting of dose distribution parameters, possibly combined with clinical or other treatment-related factors. These are fitted to data from retrospective or prospective clinical studies. Although these models sometimes provide useful guidance for clinical practice, their predictive power on individuals seems to be limited. This paper examines the radiobiological mechanisms underlying the most important complications induced by radiotherapy, with the aim of identifying the essential parameters and functional relationships needed for effective predictive NTCP models. The clinical features of the complications are identified and reduced as much as possible into component parts. In a second step, experimental and clinical data are considered in order to identify the gross anatomical structures involved, and which dose distributions lead to these complications. Finally, the pathogenic pathways and cellular and more specific anatomical parameters that have to be considered in this pathway are determined. This analysis is carried out for some of the most critical organs and sites in radiotherapy, i.e. spinal cord, lung, rectum, oropharynx and heart. Signs and symptoms of severe late normal tissue complications present a very variable picture in the different organs at risk. Only in rare instances is the entire organ the critical target which elicits the particular complication. Moreover, the biological mechanisms that are involved in the pathogenesis differ between the different complications, even in the same organ. Different mechanisms are likely to be related to different shapes of dose effect relationships and different relationships between dose per fraction, dose rate, and overall treatment time and effects. There is good reason to conclude that each type of late complication after radiotherapy depends on its own specific mechanism which is triggered by the radiation exposure of particular structures or sub-volumes of (or related to) the respective organ at risk. Hence each complication will need the development of an NTCP model designed to accommodate this structure.

ICRP publication 118: ICRP statement on tissue reactions and early and late effects of radiation in normal tissues and organs--threshold doses for tissue reactions in a radiation protection context

This report provides a review of early and late effects of radiation in normal tissues and organs with respect to radiation protection. It was instigated following a recommendation in Publication 103 (ICRP, 2007), and it provides updated estimates of 'practical' threshold doses for tissue injury defined at the level of 1% incidence. Estimates are given for morbidity and mortality endpoints in all organ systems following acute, fractionated, or chronic exposure. The organ systems comprise the haematopoietic, immune, reproductive, circulatory, respiratory, musculoskeletal, endocrine, and nervous systems; the digestive and urinary tracts; the skin; and the eye. Particular attention is paid to circulatory disease and cataracts because of recent evidence of higher incidences of injury than expected after lower doses; hence, threshold doses appear to be lower than previously considered. This is largely because of the increasing incidences with increasing times after exposure. In the context of protection, it is the threshold doses for very long follow-up times that are the most relevant for workers and the public; for example, the atomic bomb survivors with 40-50years of follow-up. Radiotherapy data generally apply for shorter follow-up times because of competing causes of death in cancer patients, and hence the risks of radiation-induced circulatory disease at those earlier times are lower. A variety of biological response modifiers have been used to help reduce late reactions in many tissues. These include antioxidants, radical scavengers, inhibitors of apoptosis, anti-inflammatory drugs, angiotensin-converting enzyme inhibitors, growth factors, and cytokines. In many cases, these give dose modification factors of 1.1-1.2, and in a few cases 1.5-2, indicating the potential for increasing threshold doses in known exposure cases. In contrast, there are agents that enhance radiation responses, notably other cytotoxic agents such as antimetabolites, alkylating agents, anti-angiogenic drugs, and antibiotics, as well as genetic and comorbidity factors. Most tissues show a sparing effect of dose fractionation, so that total doses for a given endpoint are higher if the dose is fractionated rather than when given as a single dose. However, for reactions manifesting very late after low total doses, particularly for cataracts and circulatory disease, it appears that the rate of dose delivery does not modify the low incidence. This implies that the injury in these cases and at these low dose levels is caused by single-hit irreparable-type events. For these two tissues, a threshold dose of 0.5Gy is proposed herein for practical purposes, irrespective of the rate of dose delivery, and future studies may elucidate this judgement further.

Spinal cord tolerance to reirradiation with single-fraction radiosurgery: a swine model

PURPOSE

This study was performed to determine swine spinal cord tolerance to single-fraction, partial-volume irradiation 1 year after receiving uniform irradiation to 30 Gy in 10 fractions.

METHODS AND MATERIALS

A 10-cm length of spinal cord (C3-T1) was uniformly irradiated to 30 Gy in 10 consecutive fractions and reirradiated 1 year later with a single radiosurgery dose centered within the previously irradiated segment. Radiosurgery was delivered to a cylindrical volume approximately 5 cm in length and 2 cm in diameter, which was positioned laterally to the cervical spinal cord, resulting in a dose distribution with the 90%, 50%, and 10% isodose lines traversing the ipsilateral, central, and contralateral spinal cord, respectively. Twenty-three pigs were stratified into six dose groups with mean maximum spinal cord doses of 14.9 ± 0.1 Gy (n = 2), 17.1 ± 0.3 Gy (n = 3), 19.0 ± 0.1 Gy (n = 5), 21.2 ± 0.1 Gy (n = 5), 23.4 ± 0.2 Gy (n = 5), and 25.4 ± 0.4 Gy (n = 3). The mean percentage of spinal cord volumes receiving ≥10 Gy for the same groups were 34% ± 1%, 40% ± 1%, 46% ± 3%, 52% ± 1%, 56 ± 3%, and 57% ± 1%. The study endpoint was motor neurologic deficit as determined by a change in gait during a 1- year follow-up period.

RESULTS

A steep dose-response curve was observed with a 50% incidence of paralysis (ED(50)) for the maximum point dose of 19.7 Gy (95% confidence interval, 17.4-21.4). With two exceptions, histology was unremarkable in animals with normal neurologic status, while all animals with motor deficits showed some degree of demyelination and focal white matter necrosis on the irradiated side, with relative sparing of gray matter. Histologic comparison with a companion study of de novo irradiated animals revealed that retreatment responders had more extensive tissue damage, including infarction of gray matter, only at prescription doses >20 Gy.

CONCLUSION

Pigs receiving spinal radiosurgery 1 year after receiving 30 Gy in 10 fractions were not at significantly higher risk of developing motor deficits than pigs that received radiosurgery alone.

Changes in cell cycle, apoptosis and necrosis following the establishment of a (125)I brachytherapy model in the spinal cord in Banna mini-pigs

Brachytherapy is regarded as the most effective method in the treatment of metastatic spinal tumors since little damage is caused to surrounding healthy tissue. However, this method may cause radiation myelopathy if an overdose occurs. In the present study, we established a Banna mini-pig (125)I spinal cord implantation model to provide a tool for the study of how to reduce these types of side effects. Cell cycle alteration, apoptosis and necrosis of spinal cord neurons in the presence of various doses and durations of (125)I brachytherapy were also investigated. The pigs were randomly divided into four groups, A, B, C and D. In group A, four (125)I seeds (total radioactivity, 4.0 mCi) were implanted into the dura mater of the spinal canal at the level of T13. In groups B and C, eight (125)I sources (total radioactivity, 8.0 mCi) were inserted at the same location. Groups A and C were raised for up to 8 months and group B for only 2 months. Neurons from the swine spinal cord at the T13 level were collected and cell cycle analysis was performed. Apoptosis and necrosis were tested by a terminal deoxynucleotidyl transferase dUTP nick end-labeling (TUNEL) assay. The Banna mini-pig brachytherapy model was successfully established. Radiation myelopathy was closely associated with radiation dose and duration, more neurons were blocked in the G2 and S phases as dose and time increased, and an increase in apoptosis and necrosis was detected. Ratios of apoptosis and necrosis were reduced as lower doses and shorter durations of radiation were applied. Our results demonstrate that the Banna mini-pig is an ideal animal to study (125)I brachytherapy. Low-dose and short-term brachytherapy may effectively decrease apoptosis and necrosis in spinal cord cells in Banna mini-pigs.

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.

Spinal cord tolerance to single-fraction partial-volume irradiation: a swine model

PURPOSE

To determine the spinal cord tolerance to single-fraction, partial-volume irradiation in swine.

METHODS AND MATERIALS

A 5-cm-long cervical segment was irradiated in 38-47-week-old Yucatan minipigs using a dedicated, image-guided radiosurgery linear accelerator. The radiation was delivered to a cylindrical volume approximately 5 cm in length and 2 cm in diameter that was positioned lateral to the cervical spinal cord, resulting in a dose distribution with the 90%, 50%, and 10% isodose lines traversing the ipsilateral, central, and contralateral spinal cord, respectively. The dose was prescribed to the 90% isodose line. A total of 26 pigs were stratified into eight dose groups of 12-47 Gy. The mean maximum spinal cord dose was 16.9 ± 0.1, 18.9 ± 0.1, 21.0 ± 0.1, 23.0 ± 0.2, and 25.3 ± 0.3 Gy in the 16-, 18-, 20-, 22-, and 24-Gy dose groups, respectively. The mean percentage of spinal cord volumes receiving ≥ 10 Gy for the same groups were 43% ± 3%, 48% ± 4%, 51% ± 2%, 57% ± 2%, and 59% ± 4%. The study endpoint was motor neurologic deficit determined by a change in gait during a 1-year follow-up period.

RESULTS

A steep dose-response curve was observed with a median effective dose for the maximum dose point of 20.0 Gy (95% confidence interval, 18.3-21.7). Excellent agreement was observed between the occurrence of neurologic change and the presence of histologic change. All the minipigs with motor deficits showed some degree of demyelination and focal white matter necrosis on the irradiated side, with relative sparing of the gray matter. The histologic findings were unremarkable in the minipigs with normal neurologic status.

CONCLUSIONS

Our results have indicated that for a dose distribution with a steep lateral gradient, the pigs had a lower median effective dose for paralysis than has been observed in rats and more closely resembles that for rats, mice, and guinea pigs receiving uniform spinal cord irradiation.

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.

Dose-volume effects in rat thoracolumbar spinal cord: the effects of nonuniform dose distribution

PURPOSE

To investigate dose-volume effects in rat spinal cord irradiated with nonuniform dose distributions and to assess regional differences in radiosensitivity.

METHODS AND MATERIALS

A total of 106 rats divided into three groups were irradiated with (192)Ir gamma-rays at a high dose rate. The groups were irradiated with one, two, or six catheters distributed around the thoracolumbar spinal cord to create different dose distributions. After irradiation, the animals were tested for motor function for 9 months. The response was defined as motor dysfunction and WM or nerve root necrosis. Dose-response data were analyzed with a probit analysis as function of the dose level at a percentage of the volume (D(%)) and with different normal tissue complication probability models. Additionally, the histologic responses of the individual dose voxels were analyzed after registration with the histologic sections.

RESULTS

The probit analysis at D(24) (24% of the volume) gave the best fit results. In addition, the Lyman Kutcher Burman model and the relative seriality model showed acceptable fits, with volume parameters of 0.17 and 0.53, respectively. The histology-based analysis revealed a lower radiosensitivity for the dorsal (50% isoeffective dose [ED(50)] = 32.3) and lateral WM (ED(50) = 33.7 Gy) compared with the dorsal (ED(50) = 25.9 Gy) and ventral nerve roots (ED(50) = 24.1 Gy).

CONCLUSIONS

For this nonuniform irradiation, the spinal cord did not show typical serial behavior. No migration terms were needed for an acceptable fit of the dose-response curves. A higher radiosensitivity for the lumbar nerve roots than for the thoracic WM was found.

Toward improving the therapeutic ratio in stereotactic radiosurgery: selective modulation of the radiation responses of both normal tissues and tumor

✓A review of the radiobiological factors that influence the response of the brain to radiation is provided in relation to stereotactic radiosurgery (SRS). The prospects for intervention after radiation treatment to selectively modulate the expression of late central nervous system (CNS) injury is considered, as well as an account of recent interest in the use of radiation enhancers to selectively increase the response of tumors to radiation.

Brain necrosis in humans, after conventional irradiation, indicates that the risk of necrosis increases rapidly after an equivalent single dose of 12 or 13 Gy. When single-dose treatments are extended due to 60Co decay or planned extension of treatment times, account should be taken of the effects of the repair of sublethal radiation damage to DNA on the efficacy of treatment. Both repair capacity and repair kinetics will also influence tumor control, but parameters to quantify this effect have not yet been established.

The volume of CNS tissue that has been irradiated affects the tissue response, but this effect is only significant for volumes less than 0.05 cm3. The gain obtained from irradiation of small volumes is reduced, however, when focal irradiation is given within a wider field of irradiation.

Based on a vascular hypothesis explaining the pathogenesis of late CNS damage, approaches designed to selectively modulate the frequency of late CNS damage have been validated. Given the high intrinsic radioresistance of some tumors, as opposed to the presence of hypoxia, an interest has developed in the use of selective radiation enhancers in the treatment of tumors. The compound presently available has proved to be disappointing clinically due to toxicity at effective doses, when repeated administration is required. However, when given at high single doses it is less toxic and may be more effective. Less toxic radiation enhancers need to be developed.

Long-term survival and functional status of patients with low-grade astrocytoma of spinal cord

PURPOSE

To determine survival and changes in neurologic function and Karnofsky performance status (KPS) in a series of patients treated for low-grade astrocytoma of the spinal cord during the past two decades.

METHODS

This study consisted of 14 patients with pathologically confirmed low-grade astrocytoma of the spinal cord who were treated between 1980 and 2003. All patients underwent decompressive laminectomy followed by biopsy (n = 7), subtotal resection (n = 6), or gross total resection (n = 1). Ten patients underwent postoperative radiotherapy (median total dose 50 Gy in 28 fractions). The overall survival, progression-free survival, and changes in neurologic function and KPS were measured.

RESULTS

The overall survival rate at 5, 10, and 20 years was 100%, 75%, and 60%, respectively. The progression-free survival rate at 5, 10, and 20 years was 93%, 80%, and 60%, respectively. Neither overall survival nor progression-free survival was clearly correlated with any patient, tumor, or treatment factors. Neurologic function and KPS worsened after surgery in 8 (57%) of 14 and 9 (69%) of 13 patients, respectively. At a mean follow-up of 10.2 years, neurologic function had stabilized or improved in 8 (73%) of 11 remaining patients, but the KPS had worsened in 5 (50%) of 10. Most patients who were employed before surgery were working at last follow-up.

CONCLUSION

Patients who undergo gross total resection of their tumor may be followed closely. Patients who undergo limited resection should continue to receive postoperative RT (50.4 Gy in 1.8-Gy fractions). The functional measures should be routinely evaluated to appreciate the treatment outcomes.

Dose-volume effects in rat thoracolumbar spinal cord: an evaluation of NTCP models

PURPOSE

To evaluate models for normal-tissue-complication probability (NTCP) on describing the dose-volume effect in rat thoracolumbar spinal cord.

METHODS AND MATERIALS

Single-dose irradiation of four field lengths (4, 1.5, 1.0, and 0.5 cm) was evaluated by the endpoints paresis and white-matter necrosis. The resulting dose-response data were used to rank phenomenological and tissue architecture NTCP models.

RESULTS

The 0.5-cm field length showed a steep increase in radiation tolerance. Statistical analysis of the model fits, which included evaluation of goodness of fit (GOF) and confidence intervals, resulted in the rejection of all the models considered. Excluding the smallest field length, the Schultheiss (D(50) = 21.5 Gy, k = 26.5), the relative seriality (D(50) = 21.4 Gy, s = 1.6, gamma(50) = 6.3), and the critical element (D(50,FSU) = 26.6 Gy, gamma(50,FSU) = 2.3, n = 1.3) model gave the best fit.

CONCLUSION

A thorough statistical analysis resulted in a serial or critical-element behavior for the field lengths of 1.0 cm and greater. Including the 0.5-cm field length, the radiation response markedly diverged from serial properties, but none of the models applied acceptably described this dose-response relationship. This study suggests that the commonly assumed serial behavior of the spinal cord might be valid for daily use in external- beam irradiation.

Estimation of parameters of dose-volume models and their confidence limits

Predictions of the normal-tissue complication probability (NTCP) for the ranking of treatment plans are based on fits of dose-volume models to clinical and/or experimental data. In the literature several different fit methods are used. In this work frequently used methods and techniques to fit NTCP models to dose response data for establishing dose-volume effects, are discussed. The techniques are tested for their usability with dose-volume data and NTCP models. Different methods to estimate the confidence intervals of the model parameters are part of this study. From a critical-volume (CV) model with biologically realistic parameters a primary dataset was generated, serving as the reference for this study and describable by the NTCP model. The CV model was fitted to this dataset. From the resulting parameters and the CV model, 1000 secondary datasets were generated by Monte Carlo simulation. All secondary datasets were fitted to obtain 1000 parameter sets of the CV model. Thus the 'real' spread in fit results due to statistical spreading in the data is obtained and has been compared with estimates of the confidence intervals obtained by different methods applied to the primary dataset. The confidence limits of the parameters of one dataset were estimated using the methods, employing the covariance matrix, the jackknife method and directly from the likelihood landscape. These results were compared with the spread of the parameters, obtained from the secondary parameter sets. For the estimation of confidence intervals on NTCP predictions, three methods were tested. Firstly, propagation of errors using the covariance matrix was used. Secondly, the meaning of the width of a bundle of curves that resulted from parameters that were within the one standard deviation region in the likelihood space was investigated. Thirdly, many parameter sets and their likelihood were used to create a likelihood-weighted probability distribution of the NTCP. It is concluded that for the type of dose response data used here, only a full likelihood analysis will produce reliable results. The often-used approximations, such as the usage of the covariance matrix, produce inconsistent confidence limits on both the parameter sets and the resulting NTCP values.

Radiobiological indices that consider volume: a review

Understanding and predicting the impact of any radiotherapy treatment is critical if patients are to receive treatment with a high likelihood of eliminating the tumour and low likelihood of complications. One of the major contributing factors in determining these effects is the volume treated. This review assesses the current use and accuracy of a series of models which consider volume, building on a previous review which investigated the impact of fractionation particularly with respect to the linear quadratic model. Volume is particularly important in assessing the overall effect with respect to destroying the clonogenic cells and preventing damage to the normal tissues. Dose volume histograms are one of the simplest and most useful forms of representing volume information, however it is difficult to correlate plans based only on DVHs. For this reason various reduction schemes have been introduced and tumour control probability and normal tissues complication probability models adjusted to use this information. Many of these models have proved quite useful in the clinic although they are limited by the available radiobiological data.

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

PURPOSE

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

METHODS AND MATERIALS

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

RESULTS

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

CONCLUSIONS

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

The controversies and pitfalls in modeling normal tissue radiation injury/damage

Highly conformal fields have become achievable in routine clinical practice. The optimal shape of the resultant dose distributions depends on information that is not currently available. This missing information is the dose-volume response of the normal tissues at risk. These functions are now the subject of aggressive research. The research involves collecting the dose-response data, modeling the dose-response function, and fitting the models to the data. The controversies addressed here influence the selection of the biomathematical model that one might use to describe such a function. The form that the dose-volume response function takes depends on the nature of the volume effect. The nature of the volume effect for a given radiation response is the subject of considerable debate. Related to this debate, this report addresses the existence of the volume effect, the existence of a threshold volume, and the existence of functional subunits. The pitfalls relate to the problems in accurate determination and application of the dose-response functions.

Volume effects in radiobiology as applied to radiotherapy

PURPOSE

To explore the radiobiological evidence for a dependence of normal tissue complication probability on irradiated normal tissue volume.

MATERIALS AND METHODS

Data from experimental studies on the volume effect in different organs, using different criteria of structural or functional organ damage and in different animals, were evaluated to investigate the volume effects for structural radiation damage as opposed to functional radiation damage, and the importance of organ anatomy and dose distribution within the organ on the development of chronic radiation damage in the respective organ.

RESULTS

There is little or no volume effect for structural radiation damage, however, some very pronounced volume effects have been reported for functional damage. Volume, as such, is not the relevant criterion, since critical, radiosensitive structures are not homogeneously distributed within organs.

CONCLUSION

Volume effects in patients and experimental animals are more related to organ anatomy and organ physiology than to cellular radiobiology.

Radiation therapy and the management of intramedullary spinal cord tumors

The use of radiation therapy in the management of intramedullary spinal cord tumors remains controversial. Several studies indicate that the use of postoperative radiation therapy modestly improves both local control and survival in spinal cord ependymomas and astrocytomas. Modern treatment planning and imaging allow more accurate target definition and respect for related normal tissue tolerances.

Effect of incomplete repair on normal tissue complication probability in the spinal cord

PURPOSE

To incorporate the effects of repair into a model for normal tissue complication probability (NTCP) in the spinal cord.

METHODS AND MATERIALS

We used an existing model of NTCP for the spinal cord, based on a critical volume concept, into which we incorporated an incomplete repair (IR) scheme. Values for the repair half time were taken from existing experimental data. Repair corrections were expanded to account for the possibility of biphasic repair, namely the existence of long and short components of repair.

RESULTS

We found that the model predicts complete repair to occur at approximately 15 hours, consistent with experimental data. The dependence of the model on the value of the dose per fraction was also studied. It was found that there is a sparing effect as the dose per fraction is decreased below 2 Gy. Surface plots of the NTCP as a function of both the interfraction interval (IFI) and the dose per fraction were generated. We investigated "iso-NTCP" curves, which may allow freedom in choice of treatment plans in terms of the optimal IFI and dose per fraction. As for biphasic repair, as the relative weights of the long and short components of repair were varied, the NTCP changed as well. The model showed little difference between mono- and bi-exponential repair in the time to complete repair, due to a dominance of the long component at long IFIs.

CONCLUSIONS

Incorporating IR into NTCP modeling of the spinal cord is consistent with current experimental data. The concept of iso-NTCP curves is an approach which may be clinically useful.

Volume effects in the irradiated canine spinal cord: do they exist when the probability of injury is low?

PURPOSE

The purpose of this study was to investigate volume effects in the irradiated canine spinal cord.

MATERIALS AND METHODS

Eighty-nine beagle dogs were given 44-84 Gy photons in 4 Gy fractions to 4 or 20 cm lengths of thoracic spinal cord. As controls, 36 dogs were given 60-84 Gy in 2 Gy fractions to a 20 cm length of spinal cord and six dogs were unirradiated. Dogs were evaluated for clinical signs, and after euthanasia, for occurrence of gross lesions, severe lesions of massive hemorrhage, white matter necrosis and/or parenchymal atrophy and mild lesions of focal fiber loss. White matter vacuoles, meningeal thickness and dorsal root ganglia lesions were quantified. Data were analyzed to test for an effect of volume on dose-response curves.

RESULTS

Significant volume effects were found between 4 and 20 cm lengths of irradiated spinal cord for gross lesions, severe lesions and mild lesions (8.3-15.0 Gy difference at the ED50 level). The ED50 in 4 Gy fractions for severe lesions was 56.9 Gy (95% CI 53.1-60.6) for 20 cm and 68.8 Gy (95% CI 64.5-75.1) for 4 cm fields. Significant improvements in the fit of data to dose-response curves resulted when using models with either parallel or non-parallel curves, but in either case an appreciable difference existed between curves at low probabilities of injury. Volume effects were present for meningeal thickness and slopes of dose-response curves were different. Clinical signs correlated well with severe lesions for 20 cm (ED50 = 54.0 Gy), but not for 4 cm fields (ED50 = 77.6 Gy).

CONCLUSIONS

Volume effects exist for the occurrence of pathologic lesions in irradiated canine spinal cord. Clinical compensation for pathologic lesions occur at small, but not large irradiated volumes. There is insufficient data to support a decreased slope of dose-response curves with decreased volume. Volume effects estimated at the 50% level of spinal cord injury could also hold at low probabilities of injury characteristic of the clinic.

The volume effect in radiotherapy--its biological significance

Volume-related effects are an important consideration in clinical radiotherapy. The early rationale for the need to consider treatment volume has become clouded by the lack of clarity and a misinterpretation of some early clinical findings. In particular, there is a need to separate our understanding of biologically iso-effective radiation responses from the clinical concept of normal tissue tolerance, as they relate to changes in treatment volume. Animal data, including those for large animals, are reviewed. These animal studies indicate the need for caution in extrapolating retrospective clinical data to new treatment situations, since the conclusions reached may have been dictated by dogma and not by careful consideration of different factors that may have been involved. These include anatomical and physiological factors, and variations in the dose distribution pattern to a specific organ or tissue. These biological factors could limit the general applicability of simple approaches based on mathematical models to the volume effect relationship in radiotherapy.