Development of a model to predict permanent symptomatic postradiosurgery injury for arteriovenous malformation patients. Arteriovenous Malformation Radiosurgery Study Group

Radiobiological analysis of tissue responses following radiosurgery

Stereotactic radiosurgery provided clinicians the ability to administer high single-doses of radiation to intracranial targets with relative safety. Analysis of clinical outcome from radiosurgery calls into question some of the radiobiological principles that have guided conventional fractionated radiotherapy in the past. The response of the tumor or target tissue being irradiated, as well as the response of the surrounding normal tissue, seems predominantly determined by the tumor or target vasculature. Injury to the tumor or target vasculature appears to contribute to the probability of developing normal tissue complications. Estimations of alpha/beta values from radiosurgical dose-response data consistently yield values that conflict with values from fractionated radiotherapy and with linear-quadratic theory. This indicates that projections from high-dose single-fractions to fractionated dose-equivalents made using the linear-quadratic formula are unreliable. Radiobiological analysis of clinical data from radiosurgery provides models for guiding treatment to achieve desired effects and provide reliable estimates of complications.

[Intracerebral radiotherapy under stereotaxic conditions]

Stereotactic radiosurgery is used for treating several brain diseases. Radiosurgery is a non-invasive alternative to surgery for brain metastases, and randomized trials are on going to assess the role of radiosurgery. Radiosurgery has been advocated for patients with small benign meningioma or with vestibular schwannoma, but there is no proof of efficacy and safety of radiosurgery in comparison with other treatments. Radiosurgery can obliterate 80-90% of small arteriovenous malformations, but no information exists on the survival of treated compared with untreated patients. The limited information available suggests that radiosurgery should be fully evaluated in well-designed prospective studies.

Skin fibroblasts in vitro radiosensitivity can predict for late complications following AVM radiosurgery

BACKGROUND AND PURPOSE

A small proportion of patients undergoing radiotherapy display heightened normal tissue reactions. We have set out to determine whether this sensitivity is genetic in nature and can be assessed using an in vitro skin fibroblast assay in order to predict and avoid excessive normal tissue complications.

PATIENTS AND METHODS

In this study we compared five arteriovenous malformation (AVM) patients who were treated with radiotherapy and showed severe normal tissue reactions (necrosis) to two AVM patients who showed normal reactions. Fibroblasts taken from patients were cultured in vitro and irradiated.

RESULTS

The results showed that the fibroblasts from the sensitive patients were also more radiosensitive in vitro than the cells from the normally responding patients.

CONCLUSIONS

The results suggest underlying genetic radiosensitivity and that such an assay may be used for prediction of severe radiosensitivity in AVM patients.

An analysis of the dose-response for arteriovenous malformation radiosurgery and other factors affecting obliteration

PURPOSE

The aim of this study was to better understand arteriovenous malformation (AVM) obliteration rates after radiosurgery.

METHODS AND MATERIALS

We studied obliteration after Gamma knife radiosurgery in 351 AVM patients with 3-11 years of follow-up imaging. The median marginal dose was 20 Gy (range: 12-30) and median treatment volume was 5.7 cm(3) (range: 0.26-24). Stereotactic targeting was with angiography alone in 250 AVMs, and additional magnetic resonance (MR) imaging in 101 AVMs.

RESULTS

We documented obliteration by angiography in 193/264 (73%) AVM, and by MR alone in 75/87 (86%) AVM for a 75% corrected obliteration rate. We identified persistent out-of-field nidus in 18% of embolized vs. 5% of non-embolized patients, (P = 0.006). Multivariate analysis correlated in-field obliteration with marginal dose (P < 0.0001) and sex (P < or = 0.026, but not for overall obliteration P = 0.19). A mathematical dose-response model for overall obliteration was constructed to generate a dose-response curve for AVM obliteration with a maximum overall obliteration rate of 88% and minimal improvement above 25 Gy. We could not define the value of alpha/beta for AVM obliteration to a level of statistical significance.

CONCLUSION

The rate of AVM obliteration from radiosurgery depends on the marginal dose administered with a dose-response curve that reaches a maximum of approximately 88%. The dose-response plateau reflects problems with target definition which is made more difficult by prior embolization.

Arteriovenous malformations

Arteriovenous malformations of the brain are congenital vascular lesions that affect 0.01-0.50% of the population, and are generally present in patients aged 20-40 years. The usual clinical presentations are haemorrhage, seizures, progressive neurological deficit, or headache. Results of natural history studies have shown a yearly haemorrhage rate of 1-4%. Frequency of rebleeding has increased over the years, and several factors that increase risk of haemorrhage have been identified. Although substantial, the morbidity associated with haemorrhages could be less than previously thought. Over the past decade, great advances have been made in application of endovascular embolisation techniques, stereotactic radiosurgery, and microsurgery, allowing effective multidisciplinary treatment of arteriovenous malformations, including those previously deemed to be untreatable. Increasing attention has been paid to management of flow-related aneurysms associated with these malformations. Finally, many reports of recurrent arteriovenous malformations have coincided with new theories regarding the embryogenesis of these disorders and laboratory work suggesting their proliferative potential.

A proposed radiosurgery-based grading system for arteriovenous malformations

OBJECT

Radiosurgery is an effective treatment strategy for properly selected patients harboring arteriovenous malformations (AVMs). Grading scales that are currently used to predict patient outcomes after AVM resection are unreliable tools for the prediction of the results of AVM radiosurgery.

METHODS

A grading system was developed to predict outcomes following AVM radiosurgery, based on the multivariate analysis of data obtained in 220 patients treated between 1987 and 1991 (Group 1). The dependent variable in all analyses was excellent patient outcome (complete AVM obliteration without any new neurological deficit). The grading scale was tested on a separate set of 136 patients with AVMs treated between 1990 and 1996 at a different center (Group 2). One hundred twenty-one (55%) of 220 Group 1 patients had excellent outcomes. Multivariate analysis identified five variables related to excellent patient outcomes: AVM volume (p = 0.001), patient age (p < 0.001), AVM location (p < 0.001), previous embolization (p = 0.02), and number of draining veins (p < 0.001). Regression analysis modeling permitted removal of two significant variables (previous embolization and number of draining veins) and resulted in the following equation to predict patient outcomes after AVM radiosurgery: AVM score = (0.1)(AVM volume in cm3) + (0.02)(patient age in years) + (0.3)(location of lesion: frontal or temporal) = 0; parietal, occipital, intraventricular, corpus callosum, cerebellar = 1; or basal ganglia, thalamic, or brainstem = 2). Seventy-nine (58%) of 136 Group 2 patients had excellent outcomes. All variables in the model remained significant for the Group 2 patients: AVM volume (p = 0.01), patient age (p = 0.01), and AVM location (p < 0.001). Testing of the entire model on the Group 2 patients demonstrated that the AVM score could be used to predict patient outcomes after radiosurgery (p < 0.0001). All patients with an AVM score of 1 or lower had an excellent outcome compared with only 39% of patients with an AVM score higher than 2. The Spetzler-Martin grade (p = 0.13), the K index (p = 0.26), and the obliteration prediction index (p = 0.21) did not correlate with excellent patient outcomes.

CONCLUSIONS

Despite significant differences in preoperative patient characteristics and dose prescription guidelines at the two centers, the proposed AVM grading system strongly correlated with patient outcomes after single-session radiosurgery for both patient groups. Although further testing of this model by independent centers using prospective methodology is still required, this system allows a more accurate prediction of outcomes from radiosurgery to guide choices between surgical and radiosurgical management for individual patients with AVMs.

Management of cysts arising after radiosurgery to treat intracranial arteriovenous malformations

OBJECTIVE

The proper treatment for patients with cyst formation after arteriovenous malformation radiosurgery is unknown.

METHODS

The treatment of six patients who developed cysts after arteriovenous malformation radiosurgery is described. Four patients had undergone gamma knife radiosurgery (two patients developed cysts after repeat procedures), and two patients had undergone linear accelerator-based radiosurgery. The median prescription isodose volume at the time of the first radiosurgical procedure was 13.2 cm3 (range, 8.0-28.7 cm3).

RESULTS

The cysts were discovered a median of 48 months (range, 24-89 mo) after radiosurgery. Three patients were originally without symptoms, and observation with serial imaging was performed; two of those patients developed symptoms 13 and 40 months later, whereas one patient has remained without symptoms for 51 months. Initial treatments for patients with symptomatic cysts included cyst aspiration (n = 3) and placement of a cystoperitoneal shunt (CPS) (n = 2). The median cyst volume was 14 cm3 (range, 4-63 cm3). Cyst recurrence occurred within 2 months for patients who underwent aspiration alone, necessitating placement of a CPS. Shunt placement eliminated the cysts for four patients, at a median follow-up time of 16 months (range, 9-27 mo). One patient's cyst persisted despite CPS placement, and cyst excision was performed. No morbidity occurred with any of the cyst treatments.

CONCLUSION

Cyst formation after arteriovenous malformation radiosurgery may occur many years after the procedure. Although most symptomatic cysts can be effectively treated with CPSs, cyst excision may be necessary if the mass effect is not relieved with the less invasive approach.

Partial irradiation of the brain

Late radiation injury is the main dose-limiting factor for radiotherapy of tumors of the central nervous system (CNS). Clinical experience as well as analyses of complication data, both for brain necrosis and for changes in neuroimaging after radiosurgery, suggest a pronounced volume effect in the brain. However, the relationships of dose and volume to complications after irradiation of lesions in the brain have yet to be quantitatively assessed. The quantification of volume effects and the modeling of normal tissue response to partial organ irradiation of the brain are particularly demanding because of the highly differentiated and complex structure of the brain and the variety of endpoints after radiotherapy for CNS diseases. This article summarizes the existing clinical data that demonstrate a volume effect in the brain and the current state of knowledge regarding the modeling of complications following partial irradiation of the brain.

Reporting terminology for brain arteriovenous malformation clinical and radiographic features for use in clinical trials

“If you wish to converse with me,” said Voltaire, “define your terms.” How many a debate would have been deflated into a paragraph if the disputants had dared to define their terms!

Will Durant: The Story of Philosophy

Treatment outcome of single or hypofractionated single-isocentric stereotactic irradiation (STI) using a linear accelerator for intracranial arteriovenous malformation

BACKGROUND AND PURPOSE

We investigated the use of hypofractionated stereotactic radiotherapy (HFSR) to reduce adverse radiation effects in comparison to single-fraction stereotactic radiosurgery (SRS) for intracranial arteriovenous malformations (AVMs).

MATERIALS AND METHODS

This study includes 53 intracranial AVMs treated between 1991-1998. HFSR was selected for 26 AVMs with a maximum diameter > or 2.5 cm or at eloquent area. Twenty-seven patients were treated with SRS (18 AVMs < 2.5 cm at non-eloquent area, nine patients who were unfit for prolonged ring-wearing). The most frequent minimum dose (Dmin) was 20 Gy for SRS and 28 Gy for HFSR in four fractions. The mean follow-up duration was 34.6 months for SRS and 35.4 months for HFSR.

RESULTS

As a whole, the 3 and 5-year actuarial obliteration rates were 64 and 92%. Age <20 years old (P=0.02) and a maximum diameter <2 cm were favorable factors (P=0.05). A difference in the distribution of patients was observed in size (> or =2.5 cm or not) (P<0.001) and location (eloquent or not) (P<0.001) between SRS and HFSR due to the treatment selection. However, no significant differences were observed in the actuarial rates of obliteration and transient increased signals with T2-weighted MR images between SRS and HFSR. Radiation necrosis occurred in two patients treated with SRS and in none with HFSR. Intracranial hemorrhage after treatment happened in two treated with SRS and three with HFSR.

CONCLUSIONS

HFSR appears to be at least as effective as SRS in achieving complete obliteration of intracranial AVM, although its definitive role remains to be investigated.

Stereotactic radiosurgery for cavernous malformations

OBJECT

The use of stereotactic radiosurgery to treat cerebral cavernous malformations (CMs) is controversial. To evaluate the efficacy and safety of CM radiosurgery, the authors reviewed the experience at the Mayo Clinic during the past 10 years.

METHODS

Seventeen patients underwent radiosurgery for high-surgical-risk CMs in the following sites: thalamus/basal ganglia (four patients), brainstem (12 patients), and corpus callosum (one patient). All patients had experienced at least two documented hemorrhages before undergoing radiosurgery. Stereotactic magnetic resonance (MR) imaging was used for target localization in all cases. The median margin radiation dose was 18 Gy and the median maximum dose was 32 Gy. The median length of follow-up review following radiosurgery was 51 months. The annual hemorrhage rate during the 51 months preceding radiosurgery was 40.1%, compared with 8.8% in the first 2 years following radiosurgery and 2.9% thereafter. In 10 patients (59%) new neurological deficits developed that were associated with regions of increased signal on long-repetition time MR imaging performed a median of 8 months (range 5-16 months) after radiosurgery. Three patients recovered, giving the group a permanent radiation-related morbidity rate of 41%. Compared with 31 patients harboring arteriovenous malformations (AVMs) of sizes and in locations similar to those of the aforementioned CMs, who underwent radiosurgery during the same time period, the patients with CMs were more likely to experience radiation-related complications (any complication, 59% compared with 10%; p < 0.001; permanent complication, 41% compared with 10%; p = 0.02).

CONCLUSIONS

It is impossible to conclude that radiosurgery protects patients with CMs against future hemorrhage risk based on the available data, although it appears that some reduction in the bleeding rate occurs after a latency interval of several years. The risk of radiation-related complications after radiosurgery to treat CMs is greater than that found after radiosurgery in AVMs, even when adjusting for lesion size and location and for radiation dose.

A quality factor to compare the dosimetry of gamma knife radiosurgery and intensity-modulated radiation therapy quantitatively as a function of target volume and shape

✓The authors have developed a quality factor (QF) to compare gamma knife radiosurgery, linear accelerator radiosurgery, and intensity-modulated radiation therapy (IMRT) dosimetry. This QF relates the percentage of target covered (PTC) by the prescription radiation isodose, target volume (VT), and enclosed tissue volume, which receives greater than a particular dose (VX): QFX = PTC×VT/VX. The authors investigated target shape independent of volume in predicting radiosurgical complication rates.

Plastic targets of a defined volume (0.2, 0.5, 1.5, and 10 cm3) and four increasingly complex shapes (spherical, ellipsoid, simulated arteriovenous malformation [AVM], and horseshoe) were created. Dosimetry was studied on the Leksell GammaPlan, Adac/Pinnacle, and Nomos Corvus workstations. The dosimetry of a new 4 mm × 10—mm IMRT collimator array (the Nomos Beak) not yet validated for use in our clinical practice was studied.

Particularly for larger targets, the gamma knife and IMRT Beak plans show similar conformality (QF assuming 15-Gy volume [QF15]). Particularly for small and round targets the gamma knife plan quality is significantly higher (QF assuming 12-Gy volume [QF12]). As VT and complexity increase, the IMRT Beak QF12 approaches that of the gamma knife.

The QF12 of gamma knife dosimetry has an inverse correlation with target shape complexity independent of VT.

At a prescription dose of 15 Gy to the target margin, the QF15 is a conformality index. The 12-Gy volume (volume enclosed by 12-Gy surface/volume receiving at least 12 Gy) estimates the radiosurgical normal tissue complication rate for AVMs. When the target is well covered, the QF12 is inversely proportional to the complication risk and is a measure of the plan quality.

Occlusive hyperemia: a radiosurgical phenomenon?

OBJECTIVE

Causes of neurological deficits after arteriovenous malformation (AVM) radiosurgery, including hemorrhage, radiation injury, and delayed cyst formation, are described.

CONCEPT

Occlusive hyperemia has been described as a reason for neurological deterioration after AVM resection. Thrombosis of draining veins or dural sinuses is thought to cause postoperative bleeding or neurological deficits secondary to venous hypertension. In a similar manner, local hemodynamic changes can occur in the brain adjacent to an AVM after radiosurgery if venous outflow is obstructed. Two patients are presented whose cases demonstrate this phenomenon.

CONCLUSION

Patients can experience clinical worsening after AVM radiosurgery from premature thrombosis of draining veins. Local hemodynamic changes could explain why imaging changes thought to be radiation related occur more frequently after radiosurgery of AVMs than of tumors.

The rationale and technique of staged-volume arteriovenous malformation radiosurgery

PURPOSE

Stereotactic radiosurgery is an effective management strategy for properly selected arteriovenous malformation (AVM) patients. However, the risk of postradiosurgical radiation-related injury generally limits this procedure to patients with AVMs of an average diameter of 3 cm or less. Radiosurgery of large AVMs in a planned staged fashion was undertaken to limit the radiation exposure to the surrounding normal brain.

METHODS AND MATERIALS

Between April 1997 and December 1999, 10 patients with a median AVM volume of 17.4 cm(3) (range, 7.4-53.3 cm(3)) underwent staged-volume radiosurgery (23 procedures). At the first radiosurgical procedure, the total volume of the AVM is estimated and a dose plan calculated that covers 10 cm(3)-15 cm(3), or one-half the nidus volume if the AVM is critically located (brainstem, thalamus, or basal ganglia). At 6-month intervals thereafter, radiosurgery was repeated to different portions of the AVM with the previous dose plan(s) being re-created utilizing intracranial landmarks to minimize radiation overlap. Radiosurgical procedures were continued until the entire malformation has been irradiated.

RESULTS

The radiation dosimetry of staged-volume AVM radiosurgery was compared to hypothetical single-session procedures for the 10 patients. Staged-volume radiosurgery decreased the 12-Gy volume by an average of 11.1% (range, 4.9-21%) (p < 0.001). The non-AVM 12-Gy volume was reduced by an average of 27.2% (range, 12.5-51.3%) (p < 0.001).

DISCUSSION

Staged-volume radiosurgery of large AVMs results in less radiation exposure to the adjacent brain. Further follow-up is needed to determine whether this technique provides a high rate of AVM obliteration while maintaining an acceptable rate of radiation-related complications.