High dose rate (HDR) and low dose rate (LDR) interstitial irradiation (IRT) of the rat spinal cord

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.

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.

Characterization of late radiation effects in the rat thoracolumbar spinal cord by MR imaging using USPIO

The aim of this study was to detect late radiation effects in the rat spinal cord using MR imaging with ultra-small particles of iron oxide (USPIO) contrast agent to better understand the development of late radiation damage with emphasis on the period preceding neurological signs. Additionally, the role of an inflammatory reaction was assessed by measuring macrophages that internalized USPIO. T2-weighted spin echo MR measurements were performed at 7T in six rats before paresis was expected (130-150 days post-irradiation, early group), and in six paretic rats (150-190 days post-irradiation, late group). Measurements were performed before, directly after and, only in the early group, 40 h after USPIO administration and compared with histology. In the early group, MR images showed focal regions in grey matter (GM) and white matter (WM) with signal intensity reduction after USPIO injection. Larger lesions with contrast enhancement were located in and around edematous GM of three animals of the early group and five of the late group. Forty hours after injection, additional lesions in WM, GM and nerve roots appeared in animals with GM edema. In the late paretic group, MR imaging showed WM necrosis adjacent to areas with large contrast enhancement. In conclusion, detection of early focal lesions was improved by contrast administration. In the animals with extended radiation damage, large hypo-intense regions appeared due to USPIO, which might be attributed to blood spinal cord barrier breakdown, but the involvement of blood-derived iron-loaded macrophages could not be excluded.

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.

Loss of biological effect in prolonged fraction delivery

PURPOSE

The decrease of biologic effect if delivery of dose fractions takes more than a few minutes has been occasionally recognized in the literature but has been insufficiently studied. It has been recognized as a problem in the long exposures necessary for stereotactic radiotherapy and is also a potential problem in some applications of IMRT. Modeling repair rates is a complex function of dose per fraction, dose rate, half-times of repair, and nature of the tissue of interest (the alpha/beta ratio of intrinsic radiosensitivity to repair capacity). In this article, we model repair rates for a range of doses per fraction and draw conclusions.

METHODS AND MATERIALS

We review the data on half-times of repair in tissues in situ in animals and human patients and conclude that a single first-order (exponential) repair rate is no longer an appropriate assumption for most tissues. At least 2 half-times of repair, and perhaps a distribution of half-times, are required. The faster components have a median half-time of 0.3 h (range, 0.08-1.2 h), and the longer components have a median of 4 h (range, 2.4->6 h). Modeling repair rates by a two-component model is the simplest approach. We have used two models of repair to represent these ranges, one with equal proportions of 0.2 h + 4.0 h half-times, the other with 0.4 h + 4.0 h half-times of repair. Data are also reviewed on the few experiments that have been reported with cell culture that investigate this problem.

RESULTS

Computations indicate that any fraction delivery that lasts more than half an hour might experience a clinically significant loss of cell-sterilizing effect. We suggest that a loss of more than 10% in biologically effective dose should be compensated for and show modeled doses and fraction durations for which this situation seems to be likely. It will be dose, tissue, and system dependent and will require more investigation at the clinical level.

CONCLUSION

It is suggested that any radiotherapy schedule that requires more than half an hour for the delivery of 1 fraction should have careful records made and reported, to look for a possible decrease of biologic effect with fraction duration.

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.

Repair between dose fractions: a simpler method of analyzing and reporting apparently biexponential repair

Increasing numbers of animal experiments in situ are reporting that repair of sublethal radiation damage in vivo slows down with time, usually described as two components of (monoexponential) repair. For repair of DNA strand breaks, plotting the reciprocal of proportion unrepaired as a function of time yielded straight lines. Two processes have been suggested as causing this: (1) a second-order process (bimolecular) instead of first-order (exponential) and (2) a skewed distribution of monoexponential rates. The present paper investigates whether such plots of hyperbolic or reciprocal repair are relevant for laboratory animal tissue results. Published repair data were reanalyzed from laboratory animal experiments that employed split doses or two fractions per day. Graphs are presented of the reciprocal proportion of damage remaining as a function of the interval between the two doses. If the reciprocal model applies, the graphs would be straight lines. Different animal data showed no inconsistency with straight reciprocal plots. These reciprocal plots describe well with one parameter tau, the first half-time, repair curves previously thought to be "biexponential", and to require three parameters. Straight reciprocal plots mean that in a constant time interval tau the unrepaired damage falls from 1 to (1/2), then from (1/2) to (1/3), then (1/3) to (1/4), etc. A much larger proportion of damage would therefore remain unrepaired at several half-times than is estimated by current mono- or biexponential models. The practical implications for clinical radiotherapy are important.

Techniques for precision irradiation of the lateral half of the rat cervical spinal cord using 150 MeV protons [corrected]

Techniques for high precision irradiation experiments with protons, to investigate the volume dependence of the tolerance dose of the rat cervical spinal cord are described. In the present study, 50% of the lateral cross section of the spinal cord was irradiated. The diameter of the cross section of this part of the rat spinal cord is at maximum 3.5 mm. Therefore, a dedicated procedure was developed to comply with the needs for a very high positioning accuracy and high spatial resolution dosimetry. By using 150 MeV protons a steep dose gradient (20-80% = 1 mm) in the centre of the spinal cord was achieved. This yields a good dose contrast between the left and right halves of the cord. A home-made digital x-ray imager with a pixel resolution of 0.18 mm/pixel was used for position verification of the spinal cord. A positioning accuracy of 0.09 mm was obtained by using information of multiple pixels. The average position stability during the irradiation was found to be 0.08 mm (1 SD) without significant systematic deviations. Profiles of the dose distribution were measured with a 2D dosimetry system consisting of a scintillating screen and a CCD camera. Dose volume histograms of the whole spinal cord as well as separately of the white and grey matters were calculated using MRI imaging of the cross section of the rat cervical spinal cord. From the irradiation of 20 animals a dose-response curve has been established. MRI showed radiation-induced damage at the high dose side of the spinal cord. Analysis of the preliminary dose-response data shows a significant dose-volume effect. With the described procedure and equipment it is possible to perform high precision irradiations on selected parts of the spinal cord.

Radiation tolerance of rat spinal cord to pulsed dose rate (PDR-) brachytherapy: the impact of differences in temporal dose distribution

PURPOSE

To investigate the impact of a time-variable dose rate during a high dose rate (HDR-) or pulsed dose rate (PDR-) brachytherapy fraction with the HDR-microSelectron and to compare this with the biological effect of a constant dose rate treatment with the same average dose rate (as in the case of (192)Ir-wires). Moreover, the kinetics of repair in rat spinal cord are investigated using a wide spectrum of temporal dose distributions.

MATERIALS AND METHODS

Two parallel catheters are inserted on each side of the vertebral bodies of the rat spinal column (Th(10)-L(4)) and connected to the HDR-microSelectron. Interstitial irradiation (IRT) is performed with a stepping (192)Ir-point source, varying the activity of the point source between 0.3 and 6.5 Ci. Three different groups of experiments are defined, varying the overall treatment time and average dose rates in the range of 3-8, 28-53 and 82-182 min and 312-489 Gy/h, 32-56 Gy/h and 13-15 Gy/h, respectively. Difference in temporal dose distribution (dose rate variation) during almost the same overall treatment time is obtained by varying the number of pulses per dwell position in either one or ten runs through the implant. For reasons of comparison, previously reported results of continuous irradiation at a constant dose rate obtained with two (192)Ir-wires in a fixed position are reanalyzed. Paralysis of the hindlegs after 5-6 months and histopathological examination of the spinal cord of each animal are used as experimental endpoints.

RESULTS

During one run of the (192)Ir-point source, the peak dose rate is at least 25 times higher as compared with the minimum local dose rate and almost four times higher as compared with the average dose rate. For the three different groups of varying overall treatment times and average dose rates there is a significant difference in biological effect, with an ED(50)-value of 23.1-23.6 Gy (average dose rate 312-489 Gy/h), 25.4-27.9 Gy (average dose rate 312-489 Gy/h) and 29.3-33 Gy (average dose rate 13-15 Gy/h). For these range of single doses, difference in temporal dose distribution with either one or ten runs is only significant for treatment times less then 1 h. For the prolonged treatment times at lower average dose rates, the difference between one or ten run is no longer significant. However, the results with the (192)Ir-point source at an average dose rate/run of 13-15 Gy/h are significantly different from the ED(50)-value of 33 Gy using (192)Ir-wires at the same but constant dose rate. Using different types of analysis to estimate the repair parameters, the best fit of the data is obtained assuming biphasic repair kinetics and a variable dose rate (geometrically dependent) for the (192)Ir-point source. On the basis of the incomplete repair LQ model, two repair processes with an alpha/beta ratio=2.47 Gy and repair halftimes of 0.19 and 2.16 h are detected. The partition coefficient for the longer repair process is 0.98. This results in the proportion of total damage associated with the longer repair halftime being 0.495 for short sharp fractions with complete repair in between.

CONCLUSIONS

Even in the range of high dose rates of 15-500 Gy/h, spinal cord radiation tolerance is significantly increased by a reduction in dose rate. For larger doses per fraction in PDR-brachytherapy dose rate variation is important, especially for tissues with very short repair half times (components). In rat spinal cord the repair of sublethal damage (SLD) is governed by a biphasic repair process with repair halftimes of 0.19 and 2.16 h.

The clinical radiobiology of brachytherapy

The unique geometrical features of brachytherapy, together with the wide variety of temporal patterns of dose delivery, result in important interactions between physics and radiobiology. These interactions exert a major influence on the way in which brachytherapy treatments should be evaluated, both in absolute and comparative terms. This article reviews the main physical and radiobiological aspects of brachytherapy and considers examples of their influence on specific types of treatment. The issues relating to the optimization of high dose rate brachytherapy are presented, together with the implications of multiphasic repair kinetics for low dose-rate and pulsed high dose rate brachytherapy. The opportunities for application of radiobiological principles to improve various brachytherapy techniques, together with the integration of brachytherapy with teletherapy, are also outlined. Equations for the numerical evaluation of brachytherapy treatments are presented in the Appendices.

Tolerance of rat spinal cord to continuous interstitial irradiation

PURPOSE

To study the kinetics of repair in rat spinal cord during continuous interstitial irradiation at different dose rates and to investigate the impact of a rapid dose fall off over the spinal cord thickness.

MATERIAL AND METHODS

Two parallel catheters were inserted on each side of the vertebral bodies from the level of T10 to L4. These catheters were afterloaded with two 192Ir- wires of 4 cm length each (activity 1-10 mCi/cm) or connected to the HDR- microSelectron. Experiments have been carried out to obtain complete dose response curves at 7 different dose rates: 0.53, 0.90, 1.64, 2.56, 4.4, 9.9 and 120 Gy/h. Paralysis of the hindlegs after 5 - 6 months and histopathological examination of the spinal cord of each animal were used as experimental endpoints.

RESULTS

The distribution of the histological damage was a good reflection of the rapid dose fall - off over the spinal cord, with white matter necrosis or demyelination predominantly seen in the dorsal tracts of the spinal cord or dorsal roots. With each reduction of the dose rate, spinal cord tolerance was significantly increased, with a maximum dose rate factor of 4.3 if the dose rate was reduced from 120 Gy/h to 0.53 Gy/h (ED50 of 17.3 Gy and 75.0 Gy, respectively). Estimates of the repair parameters using different types of analysis are presented. For the direct analysis the best fit of the data was obtained if a biexponential function for repair was used. For the 100% dose prescribed at the ventral side of the spinal cord the alpha/beta ratio is 1.8 Gy (0.8 - 2.8) and two components of repair are observed: a slow component of repair of 2.44 h (1.18 - infinity) and a fast component of 0.15 h (0.02 - infinity). The proportion of the damage repaired with the slow component is 0.59 (0.18 - 1). For the maximum of 150% of the prescribed dose at the dorsal side of the spinal cord the alpha/beta ratio is 2.7 Gy (1.5 - 4.4); the two components for the kinetics of repair remain the same.

CONCLUSIONS

Spinal cord radiation tolerance is significantly increased by a reduction in dose rate. Depending on the dose prescription, the alpha/beta ratio is 1.8 or 2.7 Gy for the 100% and 150% of the reference dose (rate), respectively; for the kinetics of repair a biphasic pattern is observed, with a slow component of 2.44 hours and a fast component of 0.15 hours, which is independent of the dose prescription.