Clinical significance of 3D reconstruction of arteriovenous malformation using digital subtraction angiography and its modification with CT information in stereotactic radiosurgery

PURPOSE: A three-dimensional (3D) reconstruction method of arteriovenous malformation (AVM) nidus from digital subtraction angiography (DSA) in combination with CT and/or MRI was developed, and its usefulness was evaluated in this study. MATERIALS AND METHODS: The contour of the AVM nidus was delineated on two orthogonal projected DSA images. First, the volume and center of the AVM nidus were calculated in a classic DSA plan using three maximal lengths of the nidus in three perpendicular directions, assuming that the nidus had a prolate ellipsoid shape. Second, in the 3D-DSA plan, the contours of the AVM nidus on the two orthogonal projected DSA images were segmented to be compatible with the slice thickness of the CT image. Assuming that each segment of the nidus has an ellipsoid pillar shape, the volume and center of each segment were calculated. The volume and 3D shape of the nidus were calculated by 3D reconstruction in the 3D-DSA plan. Third, in the CT-DSA plan, the contour based on the segmented DSA was superimposed on the corresponding transaxial CT image slice by slice. The cylindrical shape of the nidus in the transaxial image was modified using the enhanced CT images in the CT-DSA plan. These three planning methods were compared using dose-volume statistics from real patients' data. Eighteen patients with intracranial AVMs in different brain locations who had been treated by radiosurgery were the subjects of this study. To examine the visibility (validity) of the nidus on the CT image, the "nidus" was delineated on an enhanced CT image without DSA superposition in the CT plan and compared with the CT-DSA plan. RESULTS: The variance in the distance between coordinates determined by the CT plan and those determined by the classic DSA plan was significantly larger than the variance in the CT-DSA plan (p < 0.0001 for lateral, AP, and craniocaudal directions). The difference in the variance was not reduced by the addition of MRI (p < 0.0001 for each direction). The mean volume +/- SD of the nidus calculated was 5.9 +/- 8.0 cm(3) in the classic DSA plan, 4.0 +/- 5.6 cm(3) in the 3D-DSA plan, and 3.6 +/- 5.2 cm(3) in the CT-DSA plan. The 3D-DSA plan significantly reduced the mean nidus volume 31.8% +/- 12.7% from the classic DSA plan (p = 0.0054). The CT-DSA plan further significantly reduced the volume 9.8% +/- 8.8% from the 3D-DSA plan (p = 0.0021). The mean overlapping volume of the nidus between the CT plan and CT-DSA plan was 2.6 +/- 4.3 cm(3) (range 0.17-18.9), corresponding to 63.7% +/- 19.2% (range 11.4-85.3%) of the volume in the CT-DSA plan. CONCLUSIONS: The superposition of the segmented DSA information on CT was shown to be an important tool to determine the precise shape of the nidus and is suggested to be useful to reduce partial occlusion of the AVM or radiation complications in radiosurgery.     

Linac-based stereotactic radiosurgery for brain arteriovenous malformations

Aims

Most papers dealing with radiosurgery for cerebral arteriovenous malformations (AVMs) present the results of gamma-knife treatment, whereas linac radiosurgery is becoming increasingly popular. Moreover, there is still much uncertainty about the rationale of combined endovascular and radiosurgical treatment. The aims of this study were to evaluate obliteration and rebleeding rates, and to determine factors influencing obliteration and adverse effects after linac-based stereotactic radiosurgery for cerebral AVMs.

Materials and methods

Records of 62 consecutive patients were analysed. Thirty-one had partial embolisation, five surgery, 29 had no prior treatment. The mean follow-up was 28.4 months. The mean volume treated was 11.7 cm³ and included embolised portions of AVMs. Actuarial obliteration rates and annual bleeding hazard rates after radiosurgery were calculated using Kaplan-Meier survival and life table analyses.

Results

Actuarial obliteration rates after 1, 2 and 3 years of follow-up were 17, 36 and 40%, respectively. Prior embolisation did not influence the obliteration rate. In 77.3% of patients, obliteration occurred during the first 2 years of follow-up. Annual bleeding hazard rates after stereotactic radiosurgery were 3.4 and 1.1% during the first and second year of follow-up, respectively. Non-symptomatic imaging abnormalities were detected in 33.9% of patients after a median time of 8.8 months. The Spetzler-Martin grade, AVM score, radiation dose, volume and AVM nidus < 3 cm significantly influenced the probability of obliteration. A dose less than 15 Gy significantly reduced the probability of obliteration.

Conclusion

At least a 3 year follow-up is required to accurately assess the outcome. The best effects of the treatment are achieved for small (<3 cm), low-grade lesions with a low AVM score. The bleeding risk after stereotactic radiosurgery gradually decreases.     

Combining stereotactic angiography and 3D time-of-flight magnetic resonance angiography in treatment planning for arteriovenous malformation radiosurgery

Purpose: This study was initiated to evaluate the advantages of using three-dimensional time-of-flight magnetic resonance angiography (3D TOF MRA), as an adjuvant to conventional stereotactic angiography, in obtaining three-dimensional information about an arteriovenous malformation (AVM) nidus and in optimizing radiosurgical treatment plans.

Methods and Materials: Following angiography, contrast-enhanced MRI and MRA studies were obtained in 22 consecutive patients undergoing Gamma Knife radiosurgery for AVM. A treatment plan was designed, based on the angiograms and modified as necessary, using the information provided by MRA. The quantitative analysis involved calculation of the ratio of the treated volume to the MRA nidus volume (the tissue volume ratio [TVR]) for the initial and final treatment plans.

Results: In 12 cases (55%), the initial treatment plans were modified after including the MRA information in the treatment planning process. The mean TVR for the angiogram-based plans was 1.63 (range 1.17–2.17). The mean coverage of the MRA nidus by the angiogram-based plans was 93% (range 73–99%). The mean MRA nidus volume was 2.4 cc (range 0.6–5.3 cc). The MRA-based modifications resulted in increased conformity with the mean TVR of 1.46 (range 1.20–1.74). These modifications were caused by MRA revealing irregular nidi and/or vascular components superimposed on the angiographic projections of the nidi. In a number of cases, the information from MRA was essential in defining the nidus when the projections of the angiographic outlines showed different superior and/or inferior extent of the nidus. In two cases, MRA revealed irregular nidi, correlating well with the angiograms and showed that the angiographically acceptable plans undertreated 27% of the MRA nidus in one case and 18% of the nidus in the other case. In the remaining 10 cases (45%), both MRI and MRA failed to detect the nidus due to surgical clip artifacts and the presence of embolizing glue.

Conclusions: The 3D TOF MRA provided information on irregular AVM shape, which was not visualized by angiography alone, and it was superior to MRI for defining the AVM nidus. However, when imaging artifacts obscured the AVM nidus on MRI and MRA, angiography permitted detection of AVM. Utilizing MRA as a complementary imaging modality to angiography increased accuracy of the AVM radiosurgery and allowed for optimal dose planning.     

Stereotactic radiosurgery for intracranial arteriovenous malformations using a standard linear accelerator

We have previously described the development of a technique which utilizes a standard linear accelerator to provide stereotactic, limited field radiation. The radiation is delivered using a modified and carefully calibrated 6 MV linear accelerator. Precise target localization and patient immobilization is achieved using a Brown-Roberts-Wells (BRW) stereotactic head frame which is in place during angiography, CT scanning, and treatment. Seventeen arteriovenous malformations (AVMs) have been treated in 16 patients from February 1986 to July 1988. Single doses of 1500-2500 cGy were delivered using multiple non-coplanar arcs with small, sharp edged x-ray beams to lesions less than 2.7 cm in greatest diameter. The dose distribution from this technique has a very rapid dropoff of dose beyond the target volume. Doses were prescribed at the periphery of the AVMs, typically to the 80-90% isodose line. Eleven of 16 patients have been followed by repeat angiography at least 1 year following treatment. Five of 11 have had complete obliteration of their AVM in 1 year and an additional three patients have achieved complete obliteration by 24 months. There have been no incidences of rebleeding or serious complications in any patient. We conclude that stereotactic radiosurgery using a standard linear accelerator is an effective and safe technique in the treatment of intracranial AVMs and the results compare favorably to the more expensive and elaborate systems that are currently available for stereotactic treatments.     

Repeat linear accelerator radiosurgery for cerebral arteriovenous malformations

PURPOSE: To evaluate repeat radiosurgery (RS2) for cerebral arteriovenous malformations (AVMs) after failure of initial radiosurgery (RS1). METHODS AND MATERIALS: Between 1986 and 2000, 41 patients underwent RS2. Nine patients were not assessable: seven had insufficient follow-up (RS2 in 1999 and 2000) and two had no recent control angiography data. Thus, 32 (78%) of 41 patients were assessed. Most lesions (29 [90%] of 32) were supratentorial: 22 (69%) on the left, 8 (25%) on the right, and 2 on the midline (6%). The patients had Spetzler-Martin Grade 1-5 (median Grade 3). The symptoms before RS1 included hemorrhage in 20 (63%), epilepsy in 10 (31%), progressive neurologic deficits in 2 (6%), and headaches in 6 (19%). Five patients had two or more symptoms. Twenty-two patients (69%) had received other treatment before RS1, including neurosurgery in 3 patients (9%) and one to six embolizations in 19 patients. At RS1, the median largest nidus dimension was 2.7 cm (range 0.8-5). The median volume was 2.7 cm(3) (range 1.2-9.9). The median time from RS1 to RS2 was 52 months (range 12-126). Between RS1 and RS2, 7 (22%) of 32 patients experienced bleeding. The same irradiation technique was used for RS1 and RS2, except for 2 patients who underwent RS2 at another institution. Circular 15-MV X-ray minibeams (range 6-20 mm) and coronal arcs were used. RS1 was monoisocentric in 75% of cases and multiisocentric in 25%. At RS2, the median largest nidus dimension was 3 cm (range 1.4-5). The median volume was 4.2 cm(3) (range 0.8-13.4). RS2 was monocentric in 72% of cases and multiisocentric in 28%. After RS2, the median follow-up was 19.5 months (range 0-79; mean 25.3). RESULTS: After RS2, the obliteration rate was 59.3% (19 of 32). The median time to arteriographic obliteration was 21 months (range 12-96). The survival rate was 97% (31 of 32). Five of the 13 patients with a nonobliterated nidus experienced complications; 3 had bleeding (9%) and 2 without prior neurologic deficits developed partially regressive neurologic deficits. One patient with a previously existing deficit developed an additional new partially regressive neurologic deficit after an episode of bleeding. Thus, 3 (9%) of 32 patients had neurologic complications. Moderate-grade parenchymal changes at MRI increased after RS2 (88.2% vs. 57.7% after RS1; p = 0.10, not significant). However, necrosis-like changes did not significantly increase. After RS1 failure, salvage may be attempted by embolization, neurosurgery, or RS2. RS2 should be considered after the second successive annual angiogram if reduction of the nidus is <25%. CONCLUSION: The results after RS2 are encouraging. A multidisciplinary approach is mandatory to reduce the initial failure rate and to choose the modality and timing of salvage treatment.     

Clinical outcome after repeated radiosurgery for brain arteriovenous malformations

Introduction

We assessed the clinical and radiological outcome after repeated radiosurgery for brain arteriovenous malformations (bAVMs) after failure of initial radiosurgery.

Materials and methods

Fifteen patients underwent repeated radiosurgery. The mean bAVM volume at first radiosurgery (S1) was 4.6 ± 4.3 ml and that at second radiosurgery (S2) was 2.1 ± 2.5 ml. The median marginal dose was 18 Gy at S1, and 21 Gy at S2. Modified Rankin Scale (MRS) score was determined in all patients at last follow-up (FU).

Results

Complete obliteration was reached in nine patients (60%). Median time to obliteration was 50 months after S2. An excellent outcome (no new neurologic deficiencies, complete obliteration) was reached in seven patients (47%). Eleven patients (73%) showed a MRS ? 1. Radiation-induced complications occurred in 20%, of which 13% occurred after S2. Radiological complications included cyst formation (n = 1), radiation-related edema (n = 4), and radiation necrosis (n = 1), resulting in an increasing mean MRS of 0.5 at S1, 0.6 at S2, to 0.8 at FU. No (re-)bleedings were encountered during 137-patient years at risk.

Discussion

Repeated radiosurgery is a viable option for the treatment of small remnant bAVMs. We report 20% permanent radiation-induced complications. Such complications were mainly seen in relatively large, and therefore difficult to treat, bAVMs.     

Dynamic CT angiography for cyberknife radiosurgery planning of intracranial arteriovenous malformations: a technical/feasibility report

Background.

Successful radiosurgery for arteriovenous malformations (AVMs) requires accurate nidus delineation in the 3D treatment planning system (TPS). The catheter biplane digital subtraction angiogram (DSA) has traditionally been the gold standard for evaluation of the AVM nidus, but its 2D nature limits its value for contouring and it cannot be imported into the Cyberknife TPS. We describe a technique for acquisition and integration of 3D dynamic CT angiograms (dCTA) into the Cyberknife TPS for intracranial AVMs and review the feasibility of using this technique in the first patient cohort.

Patients and methods.

Dynamic continuous whole brain CT images were acquired in a Toshiba 320 volume CT scanner with data reconstruction every 0.5 sec. This multi-time-point acquisition enabled us to choose the CT data-set with the clearest nidus without significant enhancement of surrounding blood vessels. This was imported to the Cyberknife TPS and co-registered with planning CT and T2 MRI (2D DSA adjacent for reference). The feasibility of using dCTA was evaluated in the first thirteen patients with outcome evaluation from patient records.

Results.

dCTA data was accurately co-registered in the Cyberknife TPS and appeared to assist in nidus contouring for all patients. Imaging modalities were complementary. 85% of patients had complete (6/13) or continuing partial nidus obliteration (5/13) at 37 months median follow-up.

Conclusions.

dCTA is a promising imaging technique that can be successfully imported into the Cyberknife TPS and appears to assist in radiosurgery nidus definition. Further study to validate its role is warranted.     

Linac radiosurgery for cerebral arteriovenous malformations: results in 169 patients

Purpose: To present the SALT group results using Linac radiosurgery (RS) for AVM in 169 evaluable patients treated from January 1990 thru December 1993.

Methods and Materials: Median age was 33 years (range 6–68 years). Irradiation was the only treatment in 55% patients. Other treatment modalities had been used prior to RS in 45%: one or more embolizations in 36%, surgery in 6%, and embolization and surgery in 3% patients. Nidus were supratentorial in 94% patients, infratentorial in 6% patients. Circular 15 MV x-ray minibeams (6–20 mm) were delivered in coronal arcs by a GE-CGR Saturne 43 Linac. Patient set-up included a Betti arm-chair, a Talairach frame. Prescribed peripheral dose was 25 Gy on the 60%–70% isodose (max dose 100%). Arteriographic results were reassessed in December 1997 at 48 to 96 months follow-up.

Results: The overall obliteration rate (OR) was 64% (108/169). AVM volumes ranged from 280 to 19,920 mm³, median 2460 mm³. OR was 70% for AVM ≤ 4200 mm³ vs. 51% for AVM > 4200 mm³ (p = 0.02). The largest nidus dimension ranged from 8 to 51 mm, median 22 mm. OR was 70% for nidus ≤ 25 mm vs. 54% for nidus > 25 mm (p = 0.04). OR was 71%, in the absence of embolization, vs. 54% for previously embolized nidus (p = 0.03). OR was 71% for monocentric RS vs. 54% for multi-isocenters (p = 0.03). Peripheral doses (Pd) ranged from 15 to 28Gy, median 25 Gy, OR was 52% in patients receiving Pd ≤ 24.1 Gy or > 25.9 Gy and 70% for 25 Gy (NS). Peripheral isodoses ranged from 50%–90%, median 70%: OR was 67% for peripheral isodoses of 65% and 70% vs. 61% for peripheral isodoses of 50%–60% (NS). The mean lesion doses (MLd) ranged from 14 to 36 Gy, median 29 Gy: OR was 72% for MLd > 28 Gy vs. 55% for values ≤ 28 Gy (p = 0.02). The mean lesion isodoses (MLi) ranged from 53 to 90 Gy, median 79%: OR was 75% for MLi > 79% vs. 57% for lower values (p = 0.03). The minimum lesion doses (mLd) ranged from 3.6 to 23, median 16 Gy: OR was 69% for mLd > 17 Gy, vs. 59% for mLd ≤ 16 Gy (p = 0.05). The minimum lesion iodoses (mLi) ranged from 9%–65%, median 45%: OR was 71% for mLi > 40%, vs. 54% for mLi ≤ 40% (p = 0.05). The coverage ratio (CR) ranged from 33%–100%, median 85%: OR was 68% for CR > 85% vs. 60% for CR ≤ 84% (NS). For patients treated according to our protocol, i.e., 24–26 Gy on the 60%–70% isodoses, OR was higher (68%) than for other patients (47%) (p = 0.02). After multivariate analysis, absence of previous embolization and mono isocentric-irradiation were independent factors predicting obliteration. Complications were: recurrent hemorrhage, 4 patients (1 patient died); brain necrosis on MRI, 2 patients; subsequent epilepsy, 4 patients; other subsequent neurologic deficits, 3 patients.

Conclusion: Overall OR was 64% (48–96 months follow-up). After monovariate analysis higher ORs were associated with smaller volumes ≤ 4200 mm³, smaller nidus size ≤ 25 mm, absence of prior embolization, monoisocentric RS, higher values for mean and minimum lesion doses and compliance to our protocol. Higher values for the peripheral dose and isodose tended to give better results. Multivariate analysis showed that the absence of prior embolization and monoisocentric irradiation were independent factors predicting successfull irradiation.     

Pediatric cerebral arteriovenous malformations: the role of stereotactic linac-based radiosurgery

PURPOSE: To evaluate retrospectively clinical outcome and obliteration rates after linac-based radiosurgery (RS) in children with cerebral arteriovenous malformations (AVM). METHODS AND MATERIALS: Between 1996 and 2002, 22 children with cerebral AVM were treated at our institution. Mean age at treatment was 11.8 years (range, 4.4-16.4 years). Classification according to Spetzler-Martin was 1 child grade I (4%), 7 grade II (32%), 12 grade III (56%), 1 grade IV (4%), and 1 grade V (4%). Median single dose was 18 Gy/80%-isodose. Median AVM volume was 4.2 mL (range, 0.4-26.5 mL). Median RS-based AVM-score was 1.07 (range, 0.61-3.55). Fifty-nine percent of children experienced intracranial hemorrhage before RS. Median follow-up was 3.1 years (range, 1.7-7.3 years). RESULTS: Actuarial complete obliteration rate (CO) was 54% after 3 years and 65% after 4 years, respectively. Median time interval to CO was 27.1 months. Intracranial hemorrhage after RS was seen in five children after median 13.9 months. Annual bleeding risk was 9.1% after 1 year and 13.6% after 2 years. Maximum diameter>or=3 cm and AVM-volume>or=6 mL were significant predictors for intracranial hemorrhage. Neurologic deficits were improved/completely dissolved in 58% of children and remained stable in 42%. No new onset of neurologic dysfunction was seen after RS. CONCLUSIONS: RS is safe and effective in pediatric cerebral AVM with high obliteration rates. Size and volume of AVM are significant predictors for intracranial bleeding. The same treatment guidelines as in adults should be applied. Careful long-term follow-up observation is required after RS from long life expectation.     

Digital radiography--clinical application of digital subtraction angiography

Injection technique, advantages, disadvantages and main indications of DSA were briefly described and discussed. Advantages of DSA: (1) relatively noninvasive, (2) can be performed on an outpatient basis, therefore "cost-effective", (3) real time observation possible. Disadvantages of DSA: (1) limited spatial resolution, (2) artefacts, (3) small visual field. Main indications of DSA: (1) diagnosis and follow up study of vascular diseases, (2) evaluation of patency of by-pass graft, (3) functional assessment (of cardiac output and blood flow etc.). Aortic aneurysms, occlusive changes of the carotid or renal arteries are particularly well demonstrated by DSA. Because of three drawbacks mentioned above, DSA is not a good technique for the diagnosis of neoplasms. Although it is possible to visualize hypervascular tumors by DSA, informations obtained by DSA in cases of neoplasms are limited.     

Anatomic landmarks versus fiducials for volume-staged gamma knife radiosurgery for large arteriovenous malformations

PURPOSE: The purpose of this investigation was to compare the accuracy of using internal anatomic landmarks instead of surgically implanted fiducials in the image registration process for volume-staged gamma knife (GK) radiosurgery for large arteriovenous malformations. METHODS AND MATERIALS: We studied 9 patients who had undergone 10 staged GK sessions for large arteriovenous malformations. Each patient had fiducials surgically implanted in the outer table of the skull at the first GK treatment. These markers were imaged on orthogonal radiographs, which were scanned into the GK planning system. For the same patients, 8-10 pairs of internal landmarks were retrospectively identified on the three-dimensional time-of-flight magnetic resonance imaging studies that had been obtained for treatment. The coordinate transformation between the stereotactic frame space for subsequent treatment sessions was then determined by point matching, using four surgically embedded fiducials and then using four pairs of internal anatomic landmarks. In both cases, the transformation was ascertained by minimizing the chi-square difference between the actual and the transformed coordinates. Both transformations were then evaluated using the remaining four to six pairs of internal landmarks as the test points. RESULTS: Averaged over all treatment sessions, the root mean square discrepancy between the coordinates of the transformed and actual test points was 1.2 +/- 0.2 mm using internal landmarks and 1.7 +/- 0.4 mm using the surgically implanted fiducials. CONCLUSION: The results of this study have shown that using internal landmarks to determine the coordinate transformation between subsequent magnetic resonance imaging scans for volume-staged GK arteriovenous malformation treatment sessions is as accurate as using surgically implanted fiducials and avoids an invasive procedure.     

Diagnosis of hepatocellular carcinoma using digital subtraction imaging with the contrast agent,Levovist: comparison with helical CT,digital subtraction angiography,and US angiography

Digital subtraction imaging was compared to helical CT, DSA, and US angiography to assess its usefulness in the evaluation of vascularity in hepatocellular carcinoma (HCC). Digital subtraction imaging using Levovist as the contrast agent was performed in 41 patients with 43 nodules (22 men and 19 women, aged 50 to 83 years; mean age, 65 years; mean maximum tumor diameter, 27.8+/-17.1 mm). Digital subtraction imaging showed hypervascular enhancement in 39 of the 43 nodules (91%). Helical CT showed areas of high attenuation in 40 of the 43 nodules (93%), while DSA and US angiography showed positive enhancement in 38 and 43 of the 43 nodules (88% and 100%), respectively. Digital subtraction imaging is useful for evaluating vascularity in HCC when the tumor can be visualized with non-enhanced US angiography.     

Five year results of linac radiosurgery for arteriovenous malformations: outcome for large AVMS

Purpose: For radiosurgery of large arteriovenous malformations (AVMs), the optimal relationship of dose and volume to obliteration, complications, and hemorrhage is not well defined. Multivariate analysis was performed to assess the relationship of multiple AVM and treatment factors to the outcome of AVMs significantly larger than previously reported in the literature.

Methods and Materials: 73 patients with intracranial AVMs underwent LINAC radiosurgery. Over 50% of the AVMs were larger than 3 cm in diameter and the median and mean treatment volumes were 8.4 cc and 15.3 cc, respectively (range 0.4–143.4 cc). Minimum AVM treatment doses varied between 1000–2200 cGy (median: 1600 cGy).

Results: The obliteration rates for treatment volumes < 4 cc, 4–13.9 cc, and ≥ 14 cc were 67%, 58%, and 23%, respectively. AVM obliteration was significantly associated with higher minimum treatment dose and negatively associated with a history of prior embolization with particulate materials. No AVM receiving < 1400 cGy was obliterated. The incidence of post-radiosurgical imaging abnormalities and clinical complications rose with increasing treatment volume. For treatment volumes > 14 cc receiving ≥ 1600 cGy, the incidence of post-radiosurgical MRI T2 abnormalities was 72% and the incidence of radiation necrosis requiring resection was 22%. The rate of post-radiosurgical hemorrhage was 2.7% per person-year for AVMs with treatment volumes < 14 cc and 7.5% per person-year for AVMs ≥ 14 cc.

Conclusion: As AVM size increases, the dose-volume range for the optimal balance between successful obliteration and the risk of complications and post-radiosurgical hemorrhage narrows.     

Risk for hemorrhage during the 2-year latency period following gamma knife radiosurgery for arteriovenous malformations

PURPOSE: Radiosurgery does not immediately obliterate an arteriovenous malformation (AVM), and the risk for hemorrhage still persists until the AVM is occluded. There is controversy about whether this risk is altered after as compared to before radiosurgery. The aim of this paper is to study this topic further and to suggest a model to predict the risk for posttreatment hemorrhage. METHODS AND MATERIALS: The incidence of hemorrhages within the first 24 months following Gamma Knife radiosurgery was studied retrospectively among 1593 AVM patients, and was related to patient, AVM, and treatment parameters. RESULTS: Fifty-six patients experienced a hemorrhage in the latency period, representing an average annual incidence of 1.8%. The incidence of posttreatment hemorrhage was related to the patient's age, AVM volume, minimum dose, and average dose delivered to the AVM nidus. Based on these observations, an equation was defined that could quantify the probability for a posttreatment hemorrhage to occur. CONCLUSION: A model that can predict the probability for a hemorrhage within the first 24 months after radiosurgery is presented. The risk is higher for larger AVMs and for older patients, and it is lower when higher doses of radiation are used.