Coordination of Cone beam CT and X-ray imaging in conjunction with Optical Image Guidance in Stereotactic Body Radiation therapy

Proceedings of the 53rd Annual ASTRO Meeting a BED equivalent to 2 Gy x 30. First, we used a BED equivalent to the BED for 12.5 Gy x 4 for various tumor a/b ratios (20, 10, 5.15, 3 and 1.5 Gy) and fractionation schemes (1 to 10 fractions). Second, using an a/b ratio of 10 Gy, various 3-fraction schedules (with fractional doses of 30, 28.7, 20, 15 and 10 Gy) were plotted along with their corresponding BEDs from 1 - 10 fractions. Red Shell volumes were individually calculated to analyze the fractionation effect for different a/b ratios and prescription doses. Results: In the first set of calculations with a BED equivalent to that of 12.5 Gy x 4, an increase in fractionation was most beneficial in reducing the Red Shell volume when the a/b ratio was high. The greatest benefit was seen within the first few fractions. When the a/b ratio was low, an increase in fractionation was detrimental. With a/b ratio of 5.15 Gy, all fractionation schemes had the same Red Shell volume. In the second set of calculations, the greatest Red Shell volume reduction with increased fractionation was seen with the lowest BED equivalent to 10 Gy x 3; the least volume reduction was seen with the highest BED. There was a detrimental effect to fractionation with the highest BED at 30 Gy x 3. For a/b ratios resulting in a BED equivalent to 28.7 Gy x 3, all fractionation schemes had the same Red Shell volume. Conclusions: Multiple interrelated factors, particularly a/b ratio and BED, affect Red Shell volume in our calculations. Increased fractionation was not always beneficial, and was detrimental when the a/b ratio was low or if the BED was very high . Differences in normal tissue radiation sensitivity were not included in our calculations, and will be a focus of our future efforts. Author Disclosure: J. Yang: None. J. Lamond: None. J. Fowler: None. R. Lanciano: None. J. Feng: None. L. Brady: None.

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Coordination of Cone beam CT and X-ray imaging in conjunction with Optical Image Guidance in Stereotactic Body Radiation therapy

Y. Liu, J. Campbell INTEGRIS Cancer Institute of Oklahoma, Oklahoma City, OK Purpose/Objective(s): Stereotactic body radiation therapy (SBRT) requires high precision of patient position and target localization. For SBRT patients positioning, cone beam CT imaging has been widely used, generally with zero couch rotation. The purpose of this study is to implement Stereotactic radiation surgery (SRS) patient positioning technology to SBRT by expanding patient positioning with couch rotation. Materials/Methods: Currently we are using Varian Novlis Tx for treating SBRT patients implementing CBCT. BrainLAB X-ray imaging system in conjunction with optical guidance is primarily used for SRS patients. CBCT and X-ray imaging system were independently calibrated with 1.0 mm accuracy. For daily imaging QA, we set up a Penta imaging phantom with infrared markers. The imaging phantom has two image isocenters for CBCT and X-ray imaging respectively with CBCT isocenter offset from the X-ray imaging isocenter. The X-ray imaging system was implemented through BrainLAB ExacTrac system with CBCT localized position at the initial zero position for the X-ray imaging system. For the other couch positions, X-ray images were fused with patient DRRs for positioning. Results: For daily imaging QA, the longitudinal, vertical and lateral coordination between CBCT and X-ray imaging average 0.4 ± 0.6, 0.1 ± 0.6 and 0.6 ± 0.7 mm. The shift from the CBCT imaging isocenter to the X-ray imaging isocenter is 0.7 ± 0.5 mm accuracy for a 6-month period of tracking. Patient position accuracy: After initially localizing the patient with CBCT at the zero couch position, we then position the patient with the X-ray imaging system. The computed translational and rotational shift accuracy are 0.6 ± 0.6 mm and 0.5 ± 0.3 degree respectively, based on 39 SBRT patients couch rotations. Conclusions: Accurate coordination of CBCT and X-ray imaging in conjunction with optical imaging guidance can be expanded to patient positioning with couch rotation. The X-ray imaging capability at rotated-couch positions improved the physician confidence level during SBRT treatment. Author Disclosure: Y. Liu: None. J. Campbell: None.

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Noncoplanar RapidArc for Stereotactic Radiotherapy (SBRT) for Peripheral Lung Tumors

M. Ding, W. Zollinger, D. Heard, R. Ebeling Northeast Louisiana Cancer Center, Monroe, LA Purpose/Objective(s): RapidArc, a fast volumetric modulated arc therapy, has been applied to SBRT for treating early stage, non-small cell lung cancer (NSCLC). In this study, we focus on using noncoplanar RapidArc SBRT for peripheral lung lesions in our clinic. The treatment process and quality assurance (QA) are presented. The dosimetric accuracy and clinical outcomes are also discussed. Materials/Methods: Three peripheral NSCLC patients were treated. A Body Pro-Loc system (CIVCO) was used in all phases of the treatment process (from simulation to beam delivery). To avoid collisions in image guidance and to achieve higher target dose conformity in planning, two isocenters were applied for peripheral lung lesion SBRT. A setup isocenter CBCT rotating iso located in the body center, was used for tumor localization. A treatment isocenter RapidArc iso located in the tumor center, was shifted from the setup isocenter in both vertical and lateral directions and double checked with AP and Lateral portal images. Three couch positions were selected in planning for spreading out the dose to chest wall and lung. The treatment plans were evaluated with dose conformity parameters (R100%: the ratio of prescription isodose volume to the PTV; R50%: the ratio of 50% prescription isodose volume to the PTV) and OAR doses to the ribs and the whole lung. Patient-specific plan QA was performed with a Delta4 phantom (ScandiDos) to check dosimetric accuracy and potential collision. Results: The dose prescription of 45Gy (9Gy/fx) was for PTV1 (6.63cc) and PTV2 (8.44cc), located in the RUL and LUL nodules. 60Gy (12Gy/fx) was for PTV3 (16.87cc) in the LLL nodule. The conformities (R100% = 1.10, 1.10 and 1.05; R50% = 6.0, 4.9 and 4.4) met recent SBRT lung RTOG criteria for all the plans. The OAR doses to the whole lung (D1500cc = 0.4, 0.4 and 1.2Gy; D1000cc = 0.8, 0.8 and 2.2Gy) and to the ribs (D\1cc = 19.5, 28.7 and 40.3Gy) were below the constraints detailed in AAPM TG 101 SBRT report. It should be noted that the shortest distances between the targets and the ribs were 7.1, 2.5, and 1.7mm respectively. PTV3 did include some chest wall. For the patient-specific plan QAs, the g-indices (dose deviation: ± 3.0%; special deviation: ± 3.0mm) were 0.948, 0.944 and 0.977, which were above the generally acceptable QA criteria g-index of 0.9 (an ideal

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