Evaluation of Intrafractional Motion for Frameless Intracranial Treatments

Proceedings of the 52nd Annual ASTRO Meeting Conclusions: Although intra-fractional variation remains a concern, our preliminary analysis suggests a margin of 2 to 2.5 mm may be adequate to compensate for potential movements during treatment. Although preliminary data failed to show an association between pain scale or treatment duration and intra-fractional variations, a more robust and heterogeneous data set may be required. Careful immobilization and optimal pharmacologic management of pain may lessen the margins required for treatment planning. Author Disclosure: K. Nguyen, None; A. Liu, None; J. Wong, None.

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Comparison of Whole vs. Partial Vertebral Body Radiosurgery for Spinal Metastases: The Experience of a Single Institution

V. Patel1, R. E. Wegner1, D. E. Heron1, J. C. Flickinger1, A. H. Mintz2, S. A. Burton1, P. Gerszten2, M. Huq1 1 University of Pittsburgh Cancer Institute, Dept of Radiation Oncology, Pittsburgh, PA, 2University of Pittsburgh Cancer Institute, Dept of Neurological Surgery, Pittsburgh, PA

Purpose/Objective(s): To evaluate the difference in clinical outcomes for patients with metastatic spine disease treated with stereotactic body radiotherapy (SBRT) using a whole vs. partial vertebral body contouring approach. Materials/Methods: A retrospective study was performed for the clinical outcomes of 152 metastatic lesions to the spine in 117 patients treated with SBRT using the Cyberknife Robotic Radiosurgical System. Each patient was treated with a single session of radiosurgery using either a whole vertebral body (WB) or a partial vertebral body contour approach (PB). The primary endpoint was re-treatment rate and the secondary endpoints were pain status, neurologic status, toxicity, tumor control, and overall survival. Results: Re-treatment rates were lower in the WB group in comparison to the PB group (11% (WB) vs. 18% (PB)) but did not reach statistical significance, (p = 0.212). Mean survival times were 496 days for the WB group and 447 days for the PB group. Two years following treatment, the overall survival rate for the WB group was 25.7% vs. 20.9% in the PB group, (p = 0.538). 100% of the patients in the PB group and 97% in the WB group had pain relief following SBRT (p = 0.270). The PB group showed relatively better control of tumor progression in comparison to the WB group (35% (PB) vs. 21% (WB)) after two years of post-radiation follow-up (p = 0.070). The PB group also showed relatively better improvement of neurologic symptoms in comparison to the WB group (14% (PB) vs. 6% (WB)) after 2 years of follow-up (p = 0.081). Conclusions: Contouring the whole vertebral body for radiosurgical treatment of patients with metastatic spine disease shows potential benefits by reducing the re-treatment rate, providing symptomatic relief and local tumor control. Further investigation is warranted with a randomized, prospective trial evaluating both contour plans. Author Disclosure: V. Patel, None; R.E. Wegner, None; D.E. Heron, None; J.C. Flickinger, None; A.H. Mintz, None; S.A. Burton, None; P. Gerszten, None; M. Huq, None.

3331

Effect of Titanium Rod on Small Spinal Stereotactic Radiosurgery (SSRS) Dosimetry

X. Wang, J. Yang, R. Taylor, S. Kry, O. Vassillev M. D. Anderson Cancer Center, Houston, TX Purpose/Objective(s): Frequently we encounter patients with spinal hardware, such as Titanium rod in spine SBRS. Small beams down to 2 cm2 are often used in the IMRT planning and delivery. Dose perturbations from the hardware within the small beams are expected but not quantified. The purpose of this study is to evaluate the effect of Titanium rod on the dosimetry of small field in water phantom using Monte Carlo simulation and to evaluate the accuracy of a clinical treatment planning system for such calculations. Materials/Methods: Monte Carlo (MC) simulation of a Varian 2100 series linear accelerator with 6 MV beam was conducted using BEAMnrc/DOSXYZnrc code. The treatment head was modeled using BEAMnrc to generate the phase space data at a plan right below the lower jaw. The phase space file was then used as input for DOSXYZnrc phantom dose calculations. The incident electron beam parameters were determined using measured broad beam dose data. The accuracy of the MC simulation was validated with experimental measurements of percent depth dose (PDD) and cross profiles (at depth of 1.5, 5, 10 and 20 cm) for various field sizes of 1x1, 2x2, and 10x10cm. The measurements were performed using CC01 ionization chambers with a 100 cm SSD setup in water. After the validation, simulation was done in water phantom for a 2x2cm field with 100 cm SSD and with/without a 5 mm diameter Titanium rod at 5 cm depth. For the same setup, doses were calculated using the clinical Pinnacle treatment planning software that has a CT-density lookup table with maximum density at 1.75 g/cm3. An experimental CT-density lookup table that assigns Titanium with a density 4.5 g/cm3 was also created for the Pinnacle calculation. The quality of the Pinnacle calculations was compared with the MC simulation. Results: The MC modeling was validated to match the measurements of PDD and cross profiles of various field sizes. For the 2x2 cm field, the MC simulations with/without a 5 mm diameter Titanium rod at 5 cm depth showed the rod decreased the dose by 7% at 6 cm depth and 4% at 10 cm depth. Pinnacle calculation showed to be within 1% of the MC simulation result when a density of 4.5 g/cm3 is used for Titanium. However, if a bone density such as 1.75 g/cm3 is used for the rode, minimum effect of rod was observed, which indicate the clinical treatment planning system is almost 4% higher than MC calculation. Conclusions: Presence of Titanium rod in the path of a small SSRS beam could cause significant changes in PDD and cross profiles pass the rod. It is appropriate to assign correct density in the CT lookup table for the planning system’s to account for the high density material. Author Disclosure: X. Wang, None; J. Yang, None; R. Taylor, None; S. Kry, None; O. Vassillev, None.

3332

Evaluation of Intrafractional Motion for Frameless Intracranial Treatments

B. Wang, P. Rassiah-Szegedi, H. Zhao, Y. J. Huang, V. Sarkar, M. Szegedi, R. L. Jensen, D. C. Shrieve, B. J. Salter Huntsman Cancer Institute, University of Utah, Salt Lake City, UT Purpose/Objective(s): With the recent growth of image guided approaches, a frameless setup process has been adopted by many institutions for intracranial stereotactic treatments. One advantage is the avoidance of invasive head-ring immobilization. However, intrafractional motion is a concern for mask-based immobilization due to the tight setup margins and high prescription doses

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Volume 78, Number 3, Supplement, 2010

associated with such treatments. In this study, we evaluated the intrafractional motion of mask immobilization using orthogonal kV image guidance data. Materials/Methods: Seventy-one intrafractional shifts (213 x, y, z data points) were analyzed for eleven multi-lesion patients treated on our Novalis machine (median = 3 lesions, range, 2-6). Nine patients received a single fraction SRS treatment, and two patients were treated with 10 SRT fractions each. The three-piece BrainLab mask system was used for immobilization (BrainLAB AG, Heimstetten, Germany). Following initial setup to the first lesion isocenter, a pair of orthogonal stereoscopic x-ray images were acquired and initial 6DOF localization shifts were determined. These shifts were then performed by the BrainLab 6DOF robotic couch. After treating the first lesion, the ExacTrac system was used to move the couch to the second lesion’s isocenter while maintaining the 6DOF image guidance shifts from the first isocenter. A pair of ExacTrac x-ray images was acquired for the second isocenter, and image guidance shifts were calculated. These image guidance shifts were interpreted as the intrafractional motion that occurred during the treatment of the first lesion. The same workflow was repeated to determine the intrafractional motion between subsequent lesions. Results: Intrafractional motion was analyzed for both single- and multiple-fraction patients. The mean total treatment times per lesion were 18 min (range, 15-28 min) and 10 min (range, 7-12 min) for the single- and multiple-fraction patients, respectively. These times include imaging and setup time. For the single-fraction patients, the intrafractional motion had a median value of 0.41 mm (range, 0.02-2.13 mm). Of the 25 intrafractional motion shifts for the single fraction patients only 15% were greater than 1 mm. For the two patients with 10 fractions, the intrafractional motion had a median value of 0.24 mm (range, 0.00-1.65 mm). Of the 46 intrafractional shifts for the multiple fraction patients only 4% were greater than 1 mm. Not surprisingly, the vast majority occurred toward the end of the total treatment time. Conclusions: The vast majority of individual-lesion intrafractional motions observed here were less than 1 mm. Larger intrafractional motions (up to 2.13 mm) were seen to be more likely towards the end of treatment, thus emphasizing the importance of maintaining patient compliance through efficient delivery times. Author Disclosure: B. Wang, None; P. Rassiah-Szegedi, None; H. Zhao, None; Y.J. Huang, None; V. Sarkar, None; M. Szegedi, None; R.L. Jensen, None; D.C. Shrieve, None; B.J. Salter, None.

3333

Dosimetric Comparison of Radiosurgical Modalities using EDR Film: Gamma Knife vs. CyberKnife

1,2

S. Lee , S. Jang1,2, T. T. Sio2, J. Segala1, D. Allard1, B. H. Curran1,2, E. S. Sternick1,2, D. E. Wazer1,2 1

Rhode Island Hospital, Providence, RI, 2Brown University Medical School, Providence, RI

Purpose/Objective(s): There are several techniques available to the clinician for the delivery of radiosurgical treatments, but few side-by-side comparisons of different treatment modalities. In this study, dosimetric differences between the Gamma Knife (GK) and the CyberKnife (CK) are examined for a set of hypothetical brain metastases using EDR films. Materials/Methods: In our institution, both the GK (modified model C) and the CK (model G4) are available for radiosurgery. CT images of an anthropomorphic head phantom were obtained with and without a GK head frame. Using these images, three hypothetical, slightly ellipsoidal tumors were contoured in the supra-tentorial as well as the infra-tentorial regions for the GK and CK treatment planning systems. Lesion one was created in the right frontal region with diameters of approximately 2 cm and a volume of 4.5 cm3. Lesions two and three were created in the left frontal and occipital areas. Both tumors were equal in size, approximately 1 cm in diameter, and with a volume of 0.45 cm3. The center of the left frontal tumor was located 3 mm above the central plane of the left occipital tumor. Using the 6 cm diameter cone of the CK unit, an optical density curve was established for EDR2 film. A GK plan was prescribed to deliver 180 cGy to the 50% isodose line (IDL) for lesion one. Lesions two and three were prescribed to deliver 200 cGy to the 50% IDL. For the CK plan, 180 cGy was prescribed to the 64% IDL for lesion one. For the left occipital and frontal tumors, 200 cGy was prescribed using the 64% and 67% IDLs respectively. All tumors were fully covered by the prescription dose. For each plan, three deliveries were created; to lesion one alone, to lesions 2 and 3, and to all lesions. The film was placed such that the film plane encompassed the prescription IDL for lesion one, the left occipital tumor, and a portion of the left frontal tumor. In all six plans (3 each for GK and CK) were created, approved, and delivered in the same manner as would be done for a patient. Results: The irradiated films were processed, scanned using a Vidar Dosimetry Pro film scanner, and analyzed. Dose fall-off for the GK plans was superior to that of the CK plans in the peripheral area. For lesion one, GK doses were higher than CK, ranging from 8.4% (3 tumors) to 44.8% (single tumor). Peripheral doses for CK were generally higher than GK by as much as 65.8% depending on the beam path, increasing with multiple target deliveries. Conclusions: Dosimetric differences between GK and CK were compared for single and multi-target cases. The tumor dose to GK was higher than CK due to lower IDL prescription but this effect diminished as the number of treatment targets increased. Peripheral doses were higher for CK than GK for most cases. Author Disclosure: S. Lee, None; S. Jang, None; T.T. Sio, None; J. Segala, None; D. Allard, None; B.H. Curran, None; E.S. Sternick, None; D.E. Wazer, None.

3334

A Database of Online Registry for Dose Tolerance Limits in Hypofractionated SBRT

J. Xue1, T. LaCouture1, M. Chew2, N. Pahlajani1, L. Hughes1, N. Kramer1, S. Asbell1, J. Grimm1 1

Cooper University Hospital, Camden, NJ, 2Lehigh University, Bethlehem, PA

Purpose/Objective(s): To compile an online database of up-to-date published information on dose tolerance limits for SBRT clinical use and research. Materials/Methods: Based on an extensive literature review and our clinical experience with Gamma Knife and CyberKnife, a database of the dose tolerance limits in hypofractionated SBRT has been developed in our institution. It includes many anatomical structures from the head to the body, and is applicable to SBRT treatments in one to five fractionated sessions. The goal is to make the database available online, to enable other SBRT clinics to contribute to the work and to gain quick access to the information. Dose tolerance limits in hypofractionated SBRT are still evolving and far from consensus. The proposed online database can provide a useful resource for easy access to and better understanding of SBRT dose tolerance limits. The information is organized by each critical structure, followed with the dose limits of specified format (volumes, percentage and maximum, etc.) as well as complication probability and reference source. The newly published QUANTEC report refines the dose tolerance limits for most