Volume effect in spinal cord

The influence of the α/β ratio on treatment time iso-effect relationships in the central nervous system

Purpose: To investigate the influence of changes in α/β ratio (range 1.5-3 Gy) on iso-effective doses, with varying treatment time, in spinal cord and central nervous system tissues with comparable radio-sensitivity. It is important to establish if an α/β ratio of 2 Gy, the accepted norm for neuro-oncology iso-effect estimations, can be used.Methods: The rat spinal cord irradiation data of Pop et al. provided ED₅₀ values for radiation myelopathy for treatment times that varied from minutes to ∼6 days. Analysis using biphasic repair kinetics, allowing for variable dose-rates, provided the best fit with repair half-times of 0.19 and 2.16 hr, each providing ∼50% of overall repair; with an α/β ratio 2.47 Gy (CI 1.5-3.95 Gy). Using the above data set, graphical methods were used to investigate changes in the repair parameters for differing fixed α/β ratios between 1.5 and 3.0 Gy. Two different intermittent dose delivery equations were used to evaluate the implications in a radiosurgery setting.Results: Changes in the α/β ratio (1.5-3.0 Gy) have a minor effect on equivalent doses for radiation myelopathy for treatment durations of a few hours. Changing the α/β value from 2 Gy to 2.47 Gy, modified equivalent single doses by < 1% when overall treatment times ranged from 0.1 to 5.0 hr. Significant changes were only found for treatment times longer than 5-10 hr. These two α/β ratios were also compared in a practical radiosurgery situation, using two different models for estimating BED, again there was no significant loss of accuracy.Conclusions: It is reasonable to use an α/β ratio of 2 Gy for CNS tissue, with the same repair half-times as published in the original publication by Pop et al., in situations where the assessment of the BED in radiosurgery is used with other form of radiotherapy. In radiosurgery, the variation in BED with treatment duration (for a fixed physical dose) is very similar, but absolute BED values depend on the α/β value. In radiosurgery, clinical recommendations obtained using BED calculations using the originally proposed α/β ratio of 2.47 Gy are still appropriate. For calculations involving a combination of radiosurgery and other modalities, such as fractionated radiotherapy, it would be appropriate in all cases to apply a value of 2 Gy, the accepted norm in neuro-oncology, without significant loss of accuracy in the radio-surgical component. This may have important applications in retreatment situations.

Consideration of dose limits for organs at risk of thoracic radiotherapy: atlas for lung, proximal bronchial tree, esophagus, spinal cord, ribs, and brachial plexus

PURPOSE

To review the dose limits and standardize the three-dimenional (3D) radiographic definition for the organs at risk (OARs) for thoracic radiotherapy (RT), including the lung, proximal bronchial tree, esophagus, spinal cord, ribs, and brachial plexus.

METHODS AND MATERIALS

The present study was performed by representatives from the Radiation Therapy Oncology Group, European Organization for Research and Treatment of Cancer, and Soutwestern Oncology Group lung cancer committees. The dosimetric constraints of major multicenter trials of 3D-conformal RT and stereotactic body RT were reviewed and the challenges of 3D delineation of these OARs described. Using knowledge of the human anatomy and 3D radiographic correlation, draft atlases were generated by a radiation oncologist, medical physicist, dosimetrist, and radiologist from the United States and reviewed by a radiation oncologist and medical physicist from Europe. The atlases were then critically reviewed, discussed, and edited by another 10 radiation oncologists.

RESULTS

Three-dimensional descriptions of the lung, proximal bronchial tree, esophagus, spinal cord, ribs, and brachial plexus are presented. Two computed tomography atlases were developed: one for the middle and lower thoracic OARs (except for the heart) and one focusing on the brachial plexus for a patient positioned supine with their arms up for thoracic RT. The dosimetric limits of the key OARs are discussed.

CONCLUSIONS

We believe these atlases will allow us to define OARs with less variation and generate dosimetric data in a more consistent manner. This could help us study the effect of radiation on these OARs and guide high-quality clinical trials and individualized practice in 3D-conformal RT and stereotactic body RT.

Organizational response of normal tissues to irradiation

A central tenet in the treatment of cancer patients with radiation has been that normal tissue complications were related to the volume of the tissue irradiated, although the mechanisms underlying this phenomenon were poorly understood. The advent of new treatment techniques, such as three-dimensional (3-D) conformal treatments, drove the developers of models to evaluate the resultant complex dose distribution plans, particularly in terms of predicting normal tissue complications. However, a lack of experimental data on the effects of changing volume on normal tissue responses made it difficult to substantiate these models. Consequently, radiobiology research on normal tissue dose volume effects in experimental animal models was initiated, providing considerable insight into the effect of changing volume on normal tissue response for a large number of tissues. This paper summarizes these data and the potential impact of new concepts and data in molecular radiation biology on dose volume effects in normal tissues.

Morbidity related to axillary irradiation in the treatment of breast cancer

Some of the most debilitating morbidity after surgery and radiotherapy for breast cancer is related to treatment of the axilla. This includes persistent arm lymphoedema, impaired shoulder mobility and brachial plexopathy. Considerable research efforts have been carried out on the radiation pathogenesis and the clinical radiobiology of these clinical endpoints, which has enabled their severity and incidence to be minimized. It is clear that the radiation dose-response relationships for these late endpoints are very steep. In other words, even small changes in the exact dose fractionation and physical dose distribution can cause major changes in toxicity. In particular, in many treatment schedules dose fractions larger than 2 Gy have been used without a sufficient reduction in total dose to avoid increased late effects. This is important, as much of the available literature reports side effects after suboptimal dose-fractionation schedules and inferior radiotherapy techniques. Such reports are not representative of what can be achieved using modern radiotherapy. An interesting parallelism to the problems encountered in reviewing historical experience is found in the British breast litigation, the current status of which is presented in this article. Furthermore, morbidity after radiotherapy is strongly influenced by concomitant surgery and/or chemotherapy, and this should be allowed for when designing the overall treatment. Apart from other therapeutic modalities, it has been suggested that other exogenous factors have an influence on the risk of radiotherapy-related morbidity. However, patients' age and, in the case of lymphoedema, also obesity are the only factors that have been established with some certainty. Routine adjustment of radiotherapy dose in these cases is not recommended. Two current developments may strengthen the role of radiotherapy in the treatment of breast cancer. Sentinel node biopsy may allow nodal staging without major surgical excision of axillary nodes and this opens the possibility for a more optimal combination of radiotherapy and surgery in the management of the axilla. With more cancers now being detected by systematic screening programmes, this will also increase the possibilities for conservative management, which in most cases involves radiotherapy. In conclusion, the improved understanding of the clinical radiobiology of late sequelae after radiotherapy allows treatment schedules and techniques to be devised that are therapeutically effective while maintaining a minimal risk of serious, late morbidity.

Gamma surgery for vestibular schwannoma

OBJECT

The goal of this study was to assess the results of gamma surgery (GS) for vestibular schwannoma (VS) in 200 cases treated over the last 10 years and to review the role of this neurosurgical procedure in the management of VS.

METHODS

Follow-up reviews ranging from 1 to 10 years were available in 153 of these patients. Follow-up images in these cases were analyzed using computer software that we developed to obtain volume measurements for the tumors, and the clinical condition of the patients was assessed using questionnaires. Gamma surgery was the primary treatment modality in 96 cases and followed microsurgery in 57 cases. Tumors ranged in volume from 0.02 to 18.3 cm(3). In the group in which GS was the primary treatment, a decrease in volume was observed in 78 cases (81%), no change in 12 (12%), and an increase in volume in six cases (6%). The decrease was more than 75% in seven cases. In the group treated following microsurgery, a decrease in volume was observed in 37 cases (65%), no change in 14 (25%), and an increase in volume in six (11%). The decrease was more than 75% in eight cases. Five patients experienced trigeminal dysfunction; in three cases this was transient and in the other two it was persistent, although there has been improvement. Three patients had facial paresis (in one case this was transient, lasting 6 weeks; in one case there was 80% recovery at 18 months posttreatment; and in one case surgery was performed after the onset of facial paresis for presumed increase in tumor size). Over a 6-year period, hearing deteriorated in 60% of the patients. Three patients showed an improvement in hearing. No hearing deterioration was observed during the first 2 years of follow-up review.

CONCLUSIONS

Gamma surgery should be used to treat postoperative residual tumors as well as tumors in patients with medical conditions that preclude surgery. Microsurgery should be performed whenever a surgeon is confident of extirpating the tumor with a risk-benefit ratio superior to that presented in this study.

Is there a role for fractionated radiotherapy in the treatment of arteriovenous malformations?

Single fraction radiotherapy for the treatment of arteriovenous malformations (AVMs) is appropriate when brain tolerance is not a limiting factor, but when tolerance is a concern, there is a potential for therapeutic gain with fractionated treatment. alpha/beta values for AVM obliteration have been derived and found to be higher than previously assumed and are likely to be approximately 10.0 Gy or more compared with approximately 1.5 Gy for the tolerance of small volumes of brain. Models are derived to describe, qualitatively, the potential gain that can be achieved with fractionation. Past experience has identified an important volume effect that has limited the use of stereotactic treatments to small volumes. Volume-dependent, dose-volume relationships are described, including a tissue-specific volume exponent (phi) which was found to apply across a relatively wide range of target volumes, thereby permitting the derivation of tolerance guidelines to volumes larger than previously available. More data to define parameter values more precisely are desirable.

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.

A mathematical model of the volume effect which postulates cell migration from unirradiated tissues

PURPOSE

In order to simulate the large variation in tolerance doses for very small treatment volumes, we introduce a model which assumes the presence of cells which have migrated from unirradiated tissues.

METHODS AND MATERIALS

In order to represent serial architecture, the new model adds a new parameter to the familiar expression for serial architecture. Data derived from the model is fitted to the dose-response data developed by Hopewell et al. (Hopewell, J.W., Morris, A.D. and Dixon-Brown, A. The influence of field size on the late tolerance of the rat spinal cord to single doses of X rays. Br. J. Radiol. 60: 1099-1108, 1987) using white matter necrosis of rat spinal cord.

RESULTS

The new model with a cell-migration term more accurately describes the large differences in threshold doses for a very small treatment volume than a model without a cell-migration term.

CONCLUSION

Although these results do not prove that cell migration is the mechanism behind the volume effect for very small volume, they do suggest that the probability of normal tissue complication is more accurately predicted by the new model.

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

PURPOSE

An investigation of the field size effect for the cervical spinal cord of the pig after single doses of gamma-rays. In this study, clinically relevant volumes of the spinal cord were irradiated.

METHODS AND MATERIALS

The effects of the local irradiation of different lengths of the spinal cord (2.5 cm, 5.0 cm, and 10.0 cm) have been evaluated in mature pigs (37-43 weeks). Single doses of 25-31 Gy were given using a 60Co gamma-source, at a dose rate of 0.21-0.30 Gy/min. The incidence of radiation-induced paralysis was used as the endpoint. The data were analyzed using probit analysis and a normal tissue complication probability (NTCP)-model.

RESULTS

Twenty-five animals out of a total of 53 developed paralysis, with histological evidence of parenchymal and vascular changes in their white matter. The slope of the dose-response curves decreased with the decrease in field size; however, there was no significant difference at the radiation dose associated with a 50% incidence of paralysis (ED50) irrespective of the method of analysis. The ED50 values +/- standard errors (+/- SE) were 27.02 +/- 0.36 Gy, 27.68 +/- 0.57 Gy, and 28.28 +/- 0.78 Gy for field lengths of 10, 5, and 2.5 cm, respectively. Analysis of the data with a normal tissue complication probability (NCTP) model gave similar results. The latent period for paralysis was 7.5-16.5 weeks with no significant differences between dose and field size.

CONCLUSION

No significant field size-related differences in response were detectable in the cervical spinal cord of mature pigs after single dose irradiations, specifically at a clinically relevant level of effect (< ED10).

Dose-volume effects in the spinal cord

Experimental data on dose-volume relationships for the spinal cord are now available for a variety of animal models (monkey, dog, pig, rat). Most studies show a marginal volume effect for cord lengths longer than 1 cm, but a steep increase in tolerance doses for irradiated lengths of less than 1 cm. From a comparison of several theoretical models with available clinical/experimental data it can be concluded that there is at present no need for a further expansion of models, but a great need of reliable data.

The influence of methotrexate on radiation-induced damage to different lengths of the rat spinal cord

An experimental model in the rat has been used to assess the possible enhancement of damage to the spinal cord when radiation is given in the presence of methotrexate (MTX). The dose of MTX used, 4 mg/kg, was the maximum dose that could be given to the rat, when administered into the cerebral spinal fluid circulation, without risk of serious neurological effects. Lengths of 4, 8 and 16 mm of the cervical spine were irradiated with single doses of X rays. For animals that developed paralysis within 30 weeks, caused predominantly by the presence of white matter necrosis, there was no evidence to indicate that MTX enhanced the radiation response of the rat spinal cord, at least at a more clinically relevant level of effect i.e. a low incidence of paralysis. However, for the doses associated with the 50% level of effect (ED50) to an 8 mm long field a significant (p less than 0.005) enhancement of the response was seen, suggesting a dose modification factor of 1.19 +/- 0.07. This was interpreted in terms of an hypothesis to explain the well documented volume effect for very small fields in the rat spinal cord which is based on the migration of viable cells in the periphery of the irradiated site. The apparent smaller effect seen when only 4 mm of the spine was irradiated might be related to the nature of the lesion induced at doses required to produce paralysis in these small fields; the lesions were not restricted to white matter but were more severe and also involved the grey matter and nerve roots after a slightly shorter latent period.

The effect of single doses of radiation on mouse spinal cord

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

Lack of late skin necrosis in man after high-dose irradiation using small field sizes: experiences of grid therapy

Out of a total of 437 patients with superior vena caval syndrome or advanced malignancy, given single-dose grid radiotherapy, four survived to 7 years. The dose to the skin under each of the 77 holes in the grid was approximately 58 Gy. The lack of skin necrosis in the total of 308 skin circles of 1 cm diameter among these survivors, compared with known necrosis rates in larger irradiated areas, implies that there is a marked field-size effect for late necrosis in small areas of irradiated skin.

Dose-volume correlation in radiation-related late small-bowel complications: a clinical study

The effects of the volume of irradiated small bowel on late small-bowel tolerance was studied, taking into account the equivalent total dose and type of pre-irradiation surgical procedure. A method was developed to estimate small-bowel volumes in the high-dose region of the radiation treatment using CT-scans in the treatment position. Using this method small-bowel volumes were measured for three-field and AP-PA pelvic treatments (165 cm3 and 400 cm3, respectively), extended AP-PA pelvic treatment (790 cm3), AP-PA treatment of para-aortic nodes (550 cm3) and AP-PA treatment of para-aortic and iliac nodes (1000 cm3). In a retrospective study of 111 patients irradiated after surgery for rectal or recto-sigmoid cancer to a dose of 45-50 Gy in 5 weeks, extended AP-PA pelvic treatment (n = 27) resulted in a high incidence of severe small-bowel complications (37%), whereas for limited (three-field) pelvic treatment (n = 84) the complication rate was 6%. These complication data together with data from the literature on postoperative radiation-related small-bowel complications were analysed using the maximum likelihood method to fit the data to the logistic form of the dose-response relation, taking the volume effect into account by a power law. The analysis indicated that the incidence of radiation-related small-bowel complications was higher after rectal surgery than after other types of surgery, which might be explained by the development of more adhesions. For both types of surgery a volume exponent of the power-law of 0.26 +/- 0.05 was established. This means that if the small-bowel volume is increased by a factor of 2, the total dose has to be reduced by 17% for the same incidence of small-bowel complications.

The biological effect of inhomogeneous dose distributions in fractionated radiotherapy

The linear quadratic (LQ) model is applied to an organ receiving a fractionated course of radiotherapy with an inhomogeneous dose distribution. It is shown that the gradient in the extrapolated response dose (ERD) will be steeper than the gradient in the physical dose. This effect will be greatest for an organ with a small alpha/beta ratio treated with large dose fractions. Clinical implications are discussed with an emphasis on radiation myelitis.

Radiosurgery and the double logistic product formula

The double logistic product formula is proposed as a method for predicting the probability of developing brain necrosis after high dose irradiation of small target volumes as used in stereotactic radiosurgery. Dose-response data observed for the production of localized radiation necrosis for treating intractable pain with the original Leksell gamma unit were used to choose the best fitting parameters for the double logistic product formula. This model can be used with either exponential or linear quadratic formulas to account for the effects of dose, fractionation and time in addition to volume. Dose-response predictions for stereotactic radiosurgery with different sized collimators are presented.