S Alani

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.

A Novel Device for Protecting Rectum During Prostate Cancer Irradiation: In Vivo Data on a Large Mammal Model

PURPOSE Hypofractionation schemes and associated higher rectal doses have evoked the need for improved protection of the rectum during prostate cancer irradiation. MATERIALS AND METHODS An implantable, biodegradable, inflatable, preshaped triangular balloon of commercially used poly(L-lactide-co-epsilon-caprolactone) co-polymer material was developed to provide separation between prostate and rectum. Biocompatibility and degradability of the balloon implanted subcutaneously or perineally, and in the context of transperineal implantation and local irradiation were evaluated in several in vivo studies. RESULTS The device was found to be biocompatible in subcutaneously implanted rabbits up to 42 days, in a transperineally implanted dog up to 12 months and in 8 transperineally implanted pigs up to 6 months. Upon inflation in situ the balloon separated the tissues, remained inflated for several months and subsequently biodegraded. No systemic or local toxicity was noted, as shown by histopathology. Device insertion into the perineal area using a dedicated introductory kit was convenient and feasible. Three-month followup in irradiated pigs that received 15 Gy in 3 fractions 1 week apart showed a stable balloon position with no local or systemic side effects. CONCLUSIONS This novel device was safe and effective for its intended use of separating tissues for a desired duration. A clinical study will commence to evaluate the safety and efficacy of this device during irradiation in patients with prostate cancer.

SU‐E‐E‐18: Stereotactic Treatment of Multiple Targets Using Sngle Isocenter: Planning, Dosimetric and Delivery Advantages

Purpose: To evaluate the relative plan quality of single‐isocenter vs. multi‐isocenter for radiosurgical treatment of multiple brain metastases Methods: Ten patients referred to stereotactic radiosurgery treatment for 2–3 lesions in the brain. Two stereotactic radiosurgery plans were generated for each patient, First Plan using static beams and arcs for multi isocenter treatment plan and a second plan with one isocenter covering all lesions using static beams All plans were generated using ergo++ software on Elekta synergy‐s with beam modulator 16×21 cm with 4mm interdigitating leaves. Plans were normalized to deliver a prescription dose to the 80% isodose‐line Results: All plans were judged clinically acceptable, and no significant differences for OAR were observed in the dosimetry parameters. Nevertheless patient with different size of lesions and proximity to OAR had a different prescription dose which lead to much higher dose at the center of the lesion in the single iso plan compare to the multi isocenter plan and still kept very tight dose cover and gradient, in some cases the maximum dose was higher by 20% and average machine on time was 43.6± 9.58% higher, respectively Conclusions: Our initial results suggest that single‐isocenter plans can be utilized to deliver conformity equivalent to that of multiple isocenter techniques. Single isocenter radiosurgery for multiple targets can be efficiently delivered, and requiring less than one‐half the beam time required for multiple isocenter set ups.

SU‐FF‐T‐333: Onyx Embolization Effect On Diagnostic Images for Radiosurgical in AVM Patients

Background:Treatment of arterio‐venous malformations (AVM) of the brain can be a challenge due to the complexity of location, size and their proximity to the cerebral vascular circulation. stereotactic radiosurgery(SRS) recent advanced in catheter technique and new embolization materials, in particular Onyx have increased the success rate of total and near‐total obliteration. The use of Onyx cause distortion of the MRI and CTimages and there for has to be considering in any radiation treatment planning. Methods and Material: Between 12/2006 and 12/2008 we treated 13 AVM patients after Onyx immobilization with SRS. A bottle with 1.5ml of onyx been irradiate to find and define the absorption of it per mm of thickness, after doing that we contor the onyx and the projected area in the planning system and apply a homeginity correction to virtual organ that been draw. a pinpoint chamber been used for relative measurement because of the size of the bottle. Results: In particular larger Onyx embolized AVMs showed significant imaging artefacts especially on CT, which rendered this imaging modality useless for planning purposes. The absorption of the onyx with thickness of about 15mm was up to 5.6% higher compare to a 15mm of bolous without the onyx.Conclusion: The use of Onyx caused significant image artefact on MR and more so on CT. this results shows the importance of the correction that need to apply manually in the planning system. For patient with a large area and volume of onyx, this results shows the crucial of the correction that must be applied, if not a dose calculation can be completely wrong especially for radiosurgery patients, who prescribed high dose in single fraction to a completely healthy brain.

SU‐G‐BRC‐14: Multi‐Lesion, Multi‐Rx, Brain Radiosurgery with Novel Single Isocenter Technique

PURPOSE There is a strong trend to treat multiple brain metastases with radiosurgery rather than whole brain irradiation. This feasibility study investigates a novel planning technique for radio-surgical treatment of multiple brain lesions with differing dose prescriptions, a single isocenter, and dynamic conformal arcs. The novel technique will be compared to the well-established single-isocenter volumetric modulated arc therapy (VMAT) technique commonly used for treating brain lesions. METHODS Six patients with metastatic brain lesions were selected for a prospective treatment planning study to evaluate Interdigitating MLC Dynamic Conformal Arc (IMDCA) technique. Arcs were planned for simultaneous irradiation to maximize beam delivery efficiency. To accommodate varying PTV dose prescriptions, selected arcs were re-irradiated in reverse. Beam weights were adjusted until all prescription constraints were met. The number of lesions ranged between 2 to 4 (mode = 3). For comparison, SRS VMAT plans were generated utilizing an established single-isocenter, 3 arc planning template. All plans were compared by means of Paddick conformity index (PCI), RTOG Conformity Index (RCI), gradient index (GI), and the normal brain volume receiving 10% (V10) of the highest prescription dose. The monitor units and delivery time were tabulated for each plan. RESULTS IMDCA achieved conformal plans (PCI = 0.72±0.03, RCI = 1.33±0.03) with steep dose fall-off (GI = 3.79±0.03) on average for all of the plans evaluated. The VMAT plans had slightly better conformity (PCI = 0.85 ± 0.03, RCI = 1.13 ± 0.03) than IMDCA, but overall worse GI (4.29 ± 0.06). IMDCA plans had lower V10% values, required 50% fewer MUs, and had 34% shorter beam delivery time on average compared to VMAT plans. CONCLUSION IMDCA plans with varying dose prescriptions for multiple lesions, had comparable dosimetric coverage as VMAT plans, but were obtained with significantly lower integral dose, fewer monitor units, and quicker delivery time.

SU-F-J-185: Detection of Low Contrast Objects: Optimization of CT Simulation Reconstruction Protocol Parameters

PURPOSE CT simulation has become an integral component of modern RT planning and therefore must be continually optimized. We evaluated the detection of small and low-contrast regions in images obtained during CT simulation. METHODS A CT phantom containing a contrast detail modulus for detection of low-contrast structures was used to optimize the CT reconstruction protocol for abdomen. The parameters (A) Pitch, (B) Reconstruction Filter, and (C) Rotation Time type were varied for assessment of image quality.Three factors, three levels, and nine experiments were identified. According to the Taguchi approach an L9 orthogonal array was selected. The reconstruction parameters of the CT scanner (Brilliance Big Bore), Pitch, Reconstruction Filter type, and Rotation Time, were iteratively scanned according to the orthogonal array. A Catphan 604 CT phantom was used to characterize low-contrast resolution (CPT730 module). All CT scan images were analyzed by IMAGE-OWL software. The objective of the study was to identify parameters that maximize the low-contrast resolution of the images. The ANOVA and F-tests were used to analyze results using JMP statistical software. RESULTS The optimal settings and predicted optimal values for low-contrast resolution were determined. The ANOVA was used to determine the optimum combination of process parameters more accurately by investigating the relative importance of each process parameter. We determined that Pitch (61.4%) had the most significant influence on low contrast resolution, followed by the Reconstruction Filter type (29.7%). The optimal setting level is A1-B1-C3, 0.68 pitch, smooth filter, and gantry rotation time 1.5sec., respectively. Additional measurements were made to confirm the prediction error model is justified and the results are validated. CONCLUSION These experiments have several implications for CT imaging, especially for clinical detection of small, low-contrast lesions in liver or pancreas. In the phantom model of this study, optimal Contrast Detail Values were determined to be: 1% contrast, 2mm; 0.5% contrast, 4mm; 0.3% contrast, 7mm.

SU‐FF‐T‐611: Intensity‐Modulated Radiation Therapy (IMRT) Versus Stereotactic Radiosurgery (SRS) in Spinal Tumors ‐ a Head to Head Comparison Between the Most Advanced Planning Techniques

Background:Radiation treatment of spinal and paraspinal tumors has been limited by the tolerance of the spinal cord. With new treatment technologies, like IMRT and extracranial SRS, higher radiationdose can be delivered to the target with the capability of sparing sensitive normal structure. This study is comparing the most advanced radiation techniques (IMRT and SBRT) for spinal tumortreatment.Methods: 12 patients were treated for 13 spinal lesions; the treated lesions included metastases and primary spinal tumors in all spinal segments. Treatment indications were tumor control and pain palliation. All patients had been treated with SRS. We retrospectively compared the SRS and IMRTtreatment plan. SRS was planned using the 3Dline, (ERGO++, Elekta) for direct treatment planning. For IMRT planning we used CMS Xio software (CMS, St. Louis, MO). A dose‐volume histogram for the peripheral tissue and organ at risk around the target generated and evaluate. The IMRT plan had to meet the same tumor coverage as the srs. The SRS plan were transferred to the XiO planning system and recomputed in order to eliminate algorithm accuracy performance difference. Results: The median target volume was 8.1 cc (4.1–12.5cc). The SRS plan showed lower median target dose (5.3%, range 3–31%) and lower median dose (27.9%, range 6.6–27.2%) to critical structures (spinal cord, kidneys). The IMRT plan showed higher peripheral dose volume exposure for of 30.3%, 49.4% and 62.7% for the 10%, 20% and 30%, respectively, of overall exposed tissue volume. The planning and quality assurance duration for SRS was on average 75 minutes and for the IMRT plan 480 minutes. Conclusion:SRS technique showed a higher and faster gradient fall off reflected by a more conformal tumor coverage and less exposure to normal tissue.SRS was less time consuming for planning and quality assurance compared to the IMRT.