A framework for automated and streamlined kV cone beam computed tomography image quality assurance: a multi-institutional study.

The purpose of this study was to develop and evaluate a framework to support automated standardized testing and analysis of Cone Beam Computed Tomography (CBCT) image quality QA across multiple institutions. A survey was conducted among the participating institutions to understand the variability of the CBCT QA practices. A commercial, automated software platform was validated by seven institutions participating in a consortium dedicated to automated quality assurance. The CBCT image analysis framework was used to compare periodic QA results among 23 linear accelerators (linacs) from seven institutions. The CBCT image quality metrics (geometric distortion, spatial resolution, contrast, HU constancy, uniformity and noise) data are plotted as a function of means with the upper and lower control limits compared to the linac acceptance criteria and AAPM recommendations. For example, mean geometric distortion and HU constancy metrics were found to be 0.13 mm (TG142 recommendation: ≤2 mm) and 13.4 respectively (manufacturer acceptance specification: ≤±50).Image upload and analysis process was fully automated using a MATLAB-based platform. This analysis enabled a quantitative, longitudinal assessment of the performance of quality metrics which were also compared across 23 linacs. For key CBCT parameters such as uniformity, contrast, and HU constancy, all seven institutions used stricter goals than what would be recommended based on the analysis of the upper and lower control limits. These institutional goals were also found to be stricter than that found in AAPM published guidance. This work provides a reference that could be used to machine-specific optimized tolerance of CBCT image maintenance via control charts to monitor performance we well as the sensitivity of different tests in support of a broader quality assurance program. To ensure the daily image quality needed for patient care, the optimized statistical QA metrics recommended to using along with risk-based QA.

Delivery of Radiation at the Lowest Dose Rate by a Modern Linear Accelerator is Most Effective in Inhibiting Prostate Cancer Growth.

PURPOSE: External beam radiotherapy is one of the treatment options for organ-confined prostate cancer. A total dose of 70 to 81 Gray (Gy) is given daily (1.8-2.5 Gy/d), with a dose rate of 3 to 6 Gy/min over 28 to 45 treatments during 8 to 9 weeks. We applied the latest technological development in linear accelerators for enabling a wide range of dose rates (from 0.2-21 Gy/min) to test the effect of different delivery dose rates on prostate tumor growth in an animal xenograft model. MATERIALS AND METHODS: A prostate cancer xenograft model was established in CD1/nude mice by means of PC-3 and CL-1 cells. The animals were radiated by a TrueBeam linear accelerator that delivered 4 dose rates ranging from 0.6 to 14 Gy/min, and reaching a total dose of 20 Gy. The mice were weighed and monitored for tumor development twice weekly. A 2-way analysis of variance was used to compare statistical differences between the groups. RESULTS: Tumor growth was inhibited by radiation at all 4 dose rates in the 20 study mice compared to no radiation (n = 5, controls). The most significant reduction in tumor volumes was observed when the same dose of radiation was delivered at a rate of 0.6 Gy/min (P < .01). The animals' weights were not affected by any dose rate. CONCLUSIONS: Delivery of radiation with a TrueBeam linear accelerator at the lowest possible rate was most effective in prostate cancer growth inhibition and might be considered a preferential treatment mode for localized prostate cancer.

Personalized radiation dosimetry for PRRT-how many scans are really required?

PURPOSE: Over recent years, peptide receptor radiotherapy (PRRT) has been recognized as an effective treatment for patients with metastatic neuroendocrine tumors (NETs). Personalized dosimetry can contribute to improve the outcome of peptide receptor radiotherapy (PRRT) in patients with metastatic NETs. Dosimetry can aid treatment planning, ensuring that absorbed dose to vulnerable normal organs (kidneys and bone marrow) does not exceed safe limits over serial treatments, and that absorbed dose to tumor is sufficient. Absorbed dose is estimated from a series of post-treatment SPECT/CT images. Total self-dose is proportional to the integral under the time activity concentration curve (TACC). Method dependence of image-based absorbed dose calculations has been previously investigated, and we set out here to extend previous work by examining implications of number of data points in the TACC and the numerical integration methods used in estimating absorbed dose. METHODS: In this retrospective study, absorbed dose estimates and effective half-lives were calculated by fitting curves to TACCs for normal organs and tumors in 30 consecutive patients who underwent a series of 4 post-treatment SPECT/CT scans at 4 h, 24 h, 4-5 days, and 1 week following 177Lu-DOTATATE PRRT. We examined the effects of including only 2 or 3 rather than all 4 data points in the TACC, and the effect of numerical integration method (mono-exponential alone or in combination with trapezoidal rule) on the absorbed dose and half-life estimates. Our current method is the combination of trapezoidal rule over the first 24 h, with mono-exponential fit thereafter extrapolated to infinity. The other methods were compared to this current method. RESULTS: Differences in absorbed dose and effective half-life between the current method and estimates based only on the second, third, and fourth scans were very small (mean differences < 2.5%), whereas differences between the current method and 4-point mono-exponential fit were higher (mean differences < 5%) with a larger range. It appears that in a 4-point mono-exponential fit the early (4 h) time point may skew results, causing some large errors. Differences between the current method and values based on only 2 time points were relatively small (mean differences < 3.5%) when the 24 h and 1 week scans were used, but when the 24 h and 4-5 days scans, or the 4-5 days and 1 week scans were used, differences were greater. CONCLUSION: This study indicates that for 177Lu-DOTATATE PRRT, accurate estimates of absorbed dose for organs and tumors may be estimated from scans at 24 h, 72 h, and 1 week post-treatment without an earlier scan. It may even be possible to cut out the 72 h scan, though the uncertainty increases. However, further work on more patients is required to validate this.

Volumetric Modulated Arc Therapy as an Example of the Advanced Capability of Modern Radiotherapy.

BACKGROUND: Family physicians and internal medicine specialists play an essential role in treating cancer patients. Modern technological advances in radiotherapy are not widely appreciated by primary care physicians. Bone metastases are a frequent complication of cancer. Palliative radiation therapy, as a component of modern advances in radiation treatments, should not subject normal bodily structures to excessive doses of irradiation. The sacrum is a common destination site for bone metastases, yet its concave shape along with its proximity to the rectum, intestines, and femoral heads creates treatment-planning challenges. OBJECTIVES: To investigated whether the volumetric modulated arc therapy (VMAT) technique is preferable to more conventional radiation strategies. METHODS: The study comprised 22 patients with sacral metastases who were consecutively treated between 2013 and 2014. Two plans were generated for the comparison: three-dimensional (3D) and VMAT. RESULTS: The planning target volume (PTV) coverage of the sacrum was identical in VMAT and 3D planning. The median values for the rectal dose for 3D and VMAT were 11.34 ± 5.14 Gy and 7.7 ± 2.76 Gy, respectively. Distal sacral involvement (S4 and S5) was observed in only 2 of 22 cases, while the upper pole of the rectum ended at the level above S3 in just 3 cases. CONCLUSIONS: Radiation therapy continues to be an integral component of the palliative armamentarium against painful metastases. Radiation oncologist, in conjunction with referral physicians, can tailor treatment plans to reflect the needs of a given patient.

Combined acetyl-11-keto-ß-boswellic acid and radiation treatment inhibited glioblastoma tumor cells.

Glioblastoma multiforme (GBM) is the most common and most aggressive subtype of malignant gliomas. The current standard of care for newly diagnosed GBM patients involves maximal surgical debulking, followed by radiation therapy and temozolomide chemotherapy. Despite the advances in GBM therapy, its outcome remains poor with a median survival of less than two years. This poor outcome is partly due to the ability of GBM tumors to acquire adaptive resistance to therapy and in particular to radiation. One of the mechanisms contributing to GBM tumor progression and resistance is an aberrant activation of NF-ĸB, a family of inducible transcription factors that play a pivotal role in regulation of many immune, inflammatory and carcinogenic responses. Acetyl-11-keto-ß-boswellic acid (AKBA) is a pentacyclic terpenoid extracted from the gum Ayurvedic therapeutic plant Boswellia serrata. AKBA is anti-inflammatory agent that exhibits potent cytotoxic activities against various types of tumors including GBM. One of the mechanisms underlying AKBA anti-tumor activity is its ability to modulate the NF-ĸB signaling pathway. The present study investigated in vitro and in vivo the effect of combining AKBA with ionizing radiation in the treatment of GBM and assessed AKBA anti-tumor activity and radio-enhancing potential. The effect of AKBA and/or radiation on the survival of cultured glioblastoma cancer cells was evaluated by XTT assay. The mode of interaction of treatments tested was calculated using CalcuSyn software. Inducing of apoptosis following AKBA treatment was evaluated using flow cytometry. The effect of combined treatment on the expression of PARP protein was analysed by Western blot assay. Ectopic (subcutaneous) GBM model in nude mice was used for the evaluation of the effect of combined treatment on tumor growth. Immunohistochemical analysis of formalin-fixed paraffin-embedded tumor sections was used to assess treatment-related changes in Ki-67, CD31, p53, Bcl-2 and NF-ĸB-inhibitor IĸB-α. AKBA treatment was found to inhibit the survival of all four tested cell lines in a dose dependent manner. The combined treatment resulted in a more significant inhibitory effect compared to the effect of treatment with radiation alone. A synergistic effect was detected in some of the tested cell lines. Flow cytometric analysis with Annexin V-FITC/PI double staining of AKBA treated cells indicated induction of apoptosis. AKBA apoptotic activity was also confirmed by PARP cleavage detected by Western blot analysis. The combined treatment suppressed tumor growth in vivo compared to no treatment and each treatment alone. Immunohistochemical analysis showed anti-angiogenic and anti-proliferative activity of AKBA in vivo. It also demonstrated a decrease in p53 nuclear staining and in Bcl-2 staining and an increase in IĸB-α staining following AKBA treatment both alone and in combination with radiotherapy. In this study, we demonstrated that AKBA exerts potent anti-proliferative and apoptotic activity, and significantly inhibits both the survival of glioblastoma cells in vitro and the growth of tumors generated by these cells. Combination of AKBA with radiotherapy was found to inhibit factors which involved in cell death regulation, tumor progression and radioresistence, therefore it may serve as a novel approach for GBM patients.

Modified Citrus Pectin as a Potential Sensitizer for Radiotherapy in Prostate Cancer.

BACKGROUND: Radiotherapy is one of the primary therapies for localized prostatic carcinoma. Therefore, there is an emerging need to sensitize prostatic cancer cells to chemotherapy/radiotherapy. Modified citrus pectin (MCP) is an effective inhibitor of galectin-3 (Gal-3), which is correlated with tumor progression, proliferation, angiogenesis, and apoptosis. PURPOSE: This study was directed to evaluate the efficacy of combining ionizing radiation (IR) with MCP on PCa cells. STUDY DESIGN: Effects of treatments on PCa cells survival were evaluated using XTT assay, flow cytometry, and clonogenic survival assay. Expression of selected proteins was estimated using western blotting. Cell motility, migration, and invasion were determined. Contribution of reactive oxygen species production to treatment effects on cell viability was tested. RESULTS: Radiotherapy combined with MCP reduced viability and enhanced radiosensitivity associated with a decrease in Gal-3, cleavage of the precursor of caspase-3, increased expression of the pro-apoptotic protein Bax, and downregulation of DNA repair pathways, poly-ADP-ribose polymerase, and proliferating cell nuclear antigen. MCP significantly reduced the invasive and migratory potential of PCa cells. Combining sodium pyruvate with MCP and IR mitigated the effect on cell viability. CONCLUSION: Our findings demonstrated that MCP sensitized PCa cells to IR by downregulating anti-apoptotic Gal-3, modulating DNA repair pathways, and increasing ROS production. For the first time the correlation between MCP, radiotherapy, and Gal-3 for prostatic cancer treatment was found. In addition, MCP reduced the metastatic properties of PCa cells. These findings provide MCP as a radiosensitizing agent to enhance IR cytotoxicity, overcome radioresistance, and reduce clinical IR dose.

Combined treatment approach to cerebral arteriovenous malformation in pediatric patients: stereotactic radiosurgery to partially Onyx-embolized AVM.

PURPOSE: Treatment of cerebral arteriovenous malformations (AVM)-the most common cause of stroke in the pediatric population-can be challenging due to the complexity of size, morphology, and location. There is a significant risk in comparison to AVM treatment among adults. Thus, AVM treatment in the pediatric population imposes unique challenges. Recent improvements include optimized catheter techniques and better embolization materials, such as Onyx, a non-adhesive liquid embolic agent used in the adult population. These improvements have increased the success rate of total and near-total obliteration of cerebral AVM. However, the use of Onyx causes significant distortion of the MR and CT images, which must be accounted for in any radiation treatment planning predicated on CT and MRI. These image distortions impact on the actual delivered dose to the nidus and behoove heterogeneity correction. Our group has previously shared a solution for heterogeneity correction in the adult population. The purpose of this study is to show our experience in this unique group of pediatric patients. METHODS: This is a retrospective review of pediatric patients, who were undergoing combined endovascular embolization followed by SRS. The cohort consists of 14 patients undergoing SRS treatment in our institute between November 2006 and December 2012 with a mean follow-up of 49.9 months. Within this cohort, we retrospectively reviewed 12 consecutive pediatric patients who underwent a combined endovascular and SRS approach with a mean follow-up of 52.1 months and two patients receiving SRS-only treatment were excluded. RESULTS: In our cohort of 14 patients, 7 (50%) were male, with a mean age of 17.3 years (12.0-22.9) at the time of radiosurgery treatment. Mean age of beginning the combined modality treatment was 15.3 years (8.4-20). The median time from diagnosis to SRS was 24.3 months (11.1-64.4 months) in the complete cohort and 25.6 months (11.1-64.4) in the multimodality group. The overall median follow-up period was 49.9 months (range 12.8-118.8 months) in the complete cohort and 52.1 months (range 12.8-118.8 months) in the multimodality group. Eleven (78.6%) patients had at least one episode of hemorrhage prior to treatment. Spezler-Martin grades at baseline ranged from 2 to 5 (mean 3.2). Fifty percent had grade IV and V. Patients underwent a median of 2 (range 1-5) embolization procedures. The radiosurgical treatment dose to the margin of the angiography-based nidus: median prescription dose of 21.49 Gy (14.39-27.51) with a median max dose of 27.77 Gy (18.93-32.52). The median treatment volume was 0.6 cm3 (0.1-7.3 cm3). The Onyx embolization reduced the nidus target volume by a median of 66.7% (12.0-92.7%). We confirmed 10/14 (71%) complete closures. In 2/14 (14.2%) additional patients, a significant flow reduction was noted. In 1/14 (7.1%) patients, no significant change was noted during the observation period and two (14.2%) patients were without follow-up information. In two patients, post-treatment edema was noted; however, none was clinically significant and resolved without additional intervention or treatment. CONCLUSIONS: This cohort comprises the largest combined Onyx-SRS pediatric experience in the literature. In conjunction with our adult group study, we show that the use of Onyx reduces the SRS treatment target volume significantly. Importantly, we implemented the heterogeneity correction to avoid increased radiation exposure to normal surrounding brain tissue. The combined approach appears to be safe provided that the above-mentioned corrections are implemented.

Impact of Onyx Embolization on Radiosurgical Management of Cerebral Arteriovenous Malformations: Treatment and Outcome.

BACKGROUND: Stereotactic radiosurgery (SRS) is a well-established treatment modality for cerebral arteriovenous malformations (AVMs). The main limiting factor in the radiosurgical treatment of AVMs is the volume of the nidus, with high-grade lesions often requiring combined treatment to reduce the SRS target volume. To overcome this limitation, we have been using a combined treatment approach consisting of endovascular embolization with Onyx followed by SRS. OBJECTIVE: To evaluate our clinical experience for safety and feasibility of this multimodality treatment approach. METHODS: This is a retrospective review of all adult patients with cerebral AVMs who received SRS treatment to their AVM after endovascular embolization with Onyx between June 2007 and June 2014. RESULTS: Thirty-five consecutive patients were identified. The mean follow-up period was 52.4 ± 22.6 months (range 18-97 months). We confirmed 18 (51.4%) complete nidus closures at a median time of 49.5 months (range 6.5-81 months) from SRS. High-resolution Magnetic resonance imaging/magnetic resonance angiography was performed routinely in all patients until closure of the nidus. Digital subtraction angiography was performed to confirm complete obliteration in 5 of the patients (28%); 13 patients are either planned for digital subtraction angiography or have refused it. In 6 patients (17%) a significant flow reduction was noted after a mean of 32 ± 16 months. No significant improvement was observed in 9 patients (26%) during the follow-up period. Two patients were lost to follow-up. CONCLUSIONS: The multimodality treatment of cerebral AVMs using embolization with Onyx followed by SRS is feasible and safe. The use of Onyx significantly reduced the SRS treatment target volume.

Single-institution Experience of SBRT for Lung Metastases in Sarcoma Patients.

OBJECTIVES: Lung metastasectomy is regarded as the standard procedure for improving the prognosis of patients with metastatic sarcoma. Few reports are available in the literature describing the value of stereotactic body radiation therapy (SBRT) of lung metastases from primary sarcoma as an alternative to surgical treatment. We therefore sought to expand the evidence base for this modality. MATERIALS AND METHODS: Twenty-two patients with metastatic sarcoma to lung were treated by SBRT. The retrospective analysis of overall survival, toxicity, and local control of 53 treated lesions is presented in the study. Lung lesions were grouped into 2 categories for follow-up: <10 mm or ≥10 mm diameter. RESULTS: Of 34 lesions <10 mm, 24 achieved complete response, 3 partial response, and 7 stable disease. The results of 18 lesions measuring >10 mm were as follows: 5 complete response, 5 progressive disease, and 8 stable disease. No progressive disease of all SBRT treated lesions was found at a median follow-up of 95 months (SD 32). Five-year overall survival of the entire group was 62% from the time of diagnosis and 50% from start of treatment. The treatment was well tolerated with minimal, mainly skin toxicity. CONCLUSION: SBRT is an effective tool that might be used as an alternative to operative treatment of lung metastases in sarcoma patients.

Stereotactic radiosurgery (SRS) in high-grade glioma: judicious selection of small target volumes improves results.

We present a retrospective review of 55 Stereotactic Radiosurgery (SRS) procedures performed in 47 consecutive patients with high-grade glioma (HGG). Thirty-three (70.2%) patients were diagnosed with glioblastoma and 14 (29.8%) with grade III glioma. The indications for SRS were small (up to 30 mm in diameter) locally progressing lesions in 32/47 (68%) or new distant lesions in 15/47 (32%) patients. The median target volume was 2.2 cc (0.2-9.5 cc) and the median prescription dose was 18 Gy (14-24 Gy). Three patients (5.5% incidence in 55 treatments) developed radiation necrosis. In eight cases (17%) patients received a second salvage SRS treatment to nine new lesions detected during follow-up. In 22/55 SRS treatments (40.0%) patients received concurrent chemo- or biological therapy, including temozolamide (TMZ) (15 patients), bevacizumab (BVZ) (6 patients) and carboplatin in one patient. Median time to progression after SRS was 5.0 months (1.0-96.4). Median survival time after SRS was 15.9 months (2.3-109.3) overall median survival (since diagnosis) was 37.4 months (9.6-193.6 months). Long-lasting responses (>12 months) after SRS were observed in 25/46 (54.3%) patients. We compared a matched (histology, age, KPS) cohort of patients with recurrent HGG treated with BVZ alone with the current study group. Median survival was significantly longer for SRS treated patients compared to the BVZ only cohort (12.6 vs. 7.3 months, p = 0.0102). SRS may be considered an effective salvage procedure for selected patients with small volume, recurrent high-grade gliomas. Long-term radiological control was observed in more than 50% of the patients.

Patterns of Failure after Stereotactic Radiosurgery of the Resection Cavity Following Surgical Removal of Brain Metastases.

BACKGROUND: Whole brain radiation treatment (WBRT) is considered standard treatment for BM. However, exposing large volumes of normal brain tissue to irradiation can cause neurotoxicity. This study describes our experience with 100 consecutive patients with brain metastases who were managed with surgical extirpation followed by stereotactic radiosurgery (SRS) to the resection cavity. METHODS: Patients with 1-3 brain metastases (BM), who underwent resection of 1-2 BM between June 2005 and December 2013, were treated with SRS directed to the tumor cavity and for any synchronous BM. Local and distant treatment failures were determined based on neuroimaging. Kaplan-Meier curves were generated for local and distant failure rates and overall survival. RESULTS: One hundred and two resection cavities were treated with SRS in 100 consecutive patients. Thirty-two additional synchronous metastases were treated in 27 patients during the same session. The median overall survival was 18.9 months. Local control rate at 1 year was 84%. Longer delays between surgery and SRS were associated with increased risk of local failure (hazard ratio, -1.46; P = 0.02). Distant progression occurred in 44% of the patients at a mean of 8.8 ± 6.6 months after SRS treatment. Ten cases of leptomeningeal spread occurred around the resection cavities (9.8%). Central nervous system failure was not significantly associated with survival. Multivariate Cox regression analysis showed that recursive partitioning analysis and active systemic disease were significantly associated with survival. CONCLUSION: The strategy described provides acceptable local disease control when compared with WBRT following surgery. This approach can delay and even annul WBRT in the majority of selected BM patients, especially recursive partitioning analysis class I patients. SRS should be scheduled as soon as possible after surgery.

Flashes of light-radiation therapy to the brain.

We present a series of three patients who received therapeutic external beam radiation to the brain and experienced a phenomenon of the sensation of flashes of bright or blue light, simultaneous with radiation delivery. We relate this benign phenomenon to low-dose exposure to the eye fields and postulate that the occurrence is underreported in this treated population.

The advantage of 3D conformal treatment of lumbar spine metastases in comparison to traditional PA or AP-PA techniques: restoring an intermediate niche of therapeutic sophistication.

BACKGROUND: To evaluate the effect of the 3D radiation field design on normal tissues compared with commonly used appositional fields in patients with lumbar spine metastases. METHODS AND MATERIALS: Ten comparative treatment plans for radiation of lumbar spine metastases were compared for posterior and anterior- posterior fields with 3D plans. RESULTS: The PTV coverage in all comparative plans was similar. V 15 of the bowel in 3D, AP-PA and PA plans was 6.7 Gy (SD 6.47), 39.8 Gy (SD 11.4) and 37.3 Gy (SD15.7), respectively (p < 0.0001). The mean dose to both kidneys was 9.6 Gy (SD 4.8), 4.1 Gy (SD 3.9) and 4.6 Gy (SD 4.4) for appropriate plans (p = 0.002). Maximal dose to the spinal cord was 30.6 Gy (SD 2.1), 33.1 Gy (SD 9.8) and 37.7 Gy (SD 2) for 3D, AP-PA and PA plans. CONCLUSION: 3D conformal treatment planning of lumbar vertebral metastases was significantly better in term of bowel and spinal cord exposure compared to AP-PA and PA techniques. The exposure of the kidneys in 3D plans, while greater than in the comparative plans, did not violate accepted dose-volume thresholds.

Synergy-S stereotactic radiosurgery for spinal tumors.

BACKGROUND: Radiation treatment of spinal and paraspinal tumors has been limited by the tolerance of the spinal cord. As such, therapeutic options are restricted to surgically accessible lesions or the use of suboptimal dosing of external beam irradiation. OBJECTIVES: To evaluate the safety and applicability of the Elekta Synergy-S radiation unit for the treatment of spinal tumors. METHODS: We retrospectively reviewed all patients treated with stereotactic radiosurgery for spinal tumors between November 2007 and June 2011. RESULTS: Thirty-four patients were treated for 41 lesions. Treatment indications were local tumor control and pain palliation. The mean follow-up was 10.8 +/- 11.6 months (range 0.5-38 months). No acute radiation toxicity or new neurological deficits occurred during the follow-up period. Local tumor control was achieved in 21 of the 24 lesions (87.5%) available for radiological follow-up at a median of 9.8 months (range 3-32 months). Good analgesia was achieved in 24/30 lesions (80%) that presented with intractable pain. CONCLUSIONS: The safety and feasibility of delivering single and multiple-fraction stereotactic spinal irradiation was demonstrated and became a standard treatment option in our institution.

AP-PA field orientation followed by IMRT reduces lung exposure in comparison to conventional 3D conformal and sole IMRT in centrally located lung tumors.

Little attention has been paid to the fact that intensity modulated radiation therapy (IMRT) techniques do not easily enable treatment with opposed beams. Three treatment plans (3 D conformal, IMRT, and combined (anterior-posterior-posterio-anterior (AP-PA) + IMRT) of 7 patients with centrally-located lung cancer were compared for exposure of lung, spinal cord and esophagus. Combined IMRT and AP-PA techniques offer better lung tissue sparing compared to plans predicated solely on IMRT for centrally-located lung tumors.

Radiosurgical treatment planning of AVM following embolization with Onyx: possible dosage error in treatment planning can be averted.

Treatment of arteriovenous malformations (AVM) of the brain is challenging due to the size and location of the nidus-proper and its proximity to the cerebrovascular circulation. Recent advances in catheter techniques and new embolization materials such as Onyx (a liquid agent that is less adhesive and slowly polymerizing) have increased the probability of achieving obliteration. When planning radiosurgical cases following such embolization, however, one must be cognizant of the distortions introduced by this novel substance on imaging studies. A sample of Onyx was irradiated to define the attenuation per mm thickness. The difference in attenuation compared to water was determined. Dose calculations were performed using 3 methods of inhomogeneity corrections. Homogeneous calculations were compared to "standard" heterogeneity corrections and to "modified" heterogeneity corrections by assigning individual electron densities to the normal brain and the Onyx. The difference between the attenuation of water in comparison to the Onyx was approximately 3% for beam energy of 6 MV. Best calculation results were achieved when using the modified inhomogeneity corrections which were based on the actual attenuation of the Onyx. The use of Onyx caused significant image artifact on MR and especially CT. As such, a correction must be manually introduced into the planning system to account for this potential error. Otherwise, dose calculation may be unreliable and could have dire consequences for patients receiving high doses of radiotherapy.