M.F. McAleer

SU‐GG‐J‐74: Congruence of Cone‐Beam CT Image Center with the Radiation Isocenter of a Linear Accelerator

Purpose: To develop a quantitative method to evaluate the cone‐beam CT(CBCT)image center congruence with the linac radiation isocenter. Method and Materials: A ball‐bearing (BB) was placed in the proximity of the isocenter of a Varian Trilogy linac. The center of BB served as a static reference point in the space. The BB was imaged with square megavolt (MV) radiation beams at gantry angles of 0, 90, 180, and 270 degrees. The radiation isocenter was localized relative to the BB center based on the four MV EPIDimages. Then the BB was imaged with the CBCT. The CBCTimage center was localized relative to the BB center. Finally, the 3D displacement between the CBCTimage center and the radiation isocenter was computed. Results: The CBCTimage center was found to have excellent short‐term positional reproducibility, i.e., <0.1 mm wobbling in each of the x(lateral), y(vertical), and z(longitudinal) directions in 10 consecutive acquisitions. Measured in a 7‐month period, the CBCTimage center deviated from the radiation isocenter by 0.40 ± 0.12 mm (x), 0.43 ± 0.04 mm (y), and 0.34 ± 0.14 mm (z). Small yet systematic discrepancies were found between the CBCTimage center, 2D MV or kV portal image center, and the radiation isocenter. For the linac studied, we detected a 0.8 mm discrepancy between the CBCTimage center and the MV EPIDimage center in the anterior‐posterior direction. This discrepancy was demonstrated in a clinical case study where the patient was positioned with 3D CBCTimaging followed by 2D MV portal imaging verification. Conclusion: The CBCTimage center was localized relative to the radiation isocenter using a simple QA procedure. Computerized analysis revealed that the misalignment of CBCTimage center was highly reproducible in short term and long term and could be quantified at sub‐millimeter scale.

SU-F-T-392: Superior Brainstem and Cochlea Sparing with VMAT for Glioblastoma Multiforme

PURPOSEnVolumetric arc therapy (VMAT) can provide similar target coverage and normal tissue sparing as IMRT but with shorter treatment times. At our institution VMAT was adopted for the treatment glioblastoma multiforme (GBM) after a small number of test plans demonstrated its non-inferiority. In this study, we compare actual clinical treatment plans for a larger cohort of patients treated with either VMAT or IMRT.nnnMETHODSn90 GBM patients were included in this study, 45 treated with IMRT and 45 with VMAT. All planning target volumes (PTVs) were prescribed a dose of 50 Gy, with a simultaneous integrated boost to 60 Gy. Most IMRT plans used 5 non-coplanar beams, while most VMAT plans used 2 coplanar beams. Statistical analysis was performed using Fishers exact test or the Wilcoxon-Mann-Whitney rank sum test. Included in the analysis were patient and treatment characteristics as well as the doses to the target volumes and organs at risk.nnnRESULTSnTreatment times for the VMAT plans were reduced by 5 minutes compared with IMRT. The PTV coverage was similar, with at least 95% covered for all plans, while the median boost PTV dose differed by 0.1 Gy between the IMRT and VMAT cohorts. The doses to the brain, optic chiasm, optic nerves and eyes were not significantly different. The mean dose to the brainstem, however, was 9.4 Gy less with VMAT (p<0.001). The dose to the ipsilateral and contralateral cochleae were respectively 19.7 and 9.5 Gy less (p<0.001).nnnCONCLUSIONnComparison of clinical treatment plans for separate IMRT and VMAT cohorts demonstrates that VMAT can save substantial treatment time while providing similar target coverage and superior sparing of the brainstem and cochleae. To our knowledge this is the first study to demonstrate this benefit of VMAT in the management of GBM.

SU-F-T-642: Sub Millimeter Accurate Setup of More Than Three Vertebrae in Spinal SBRT with 6D Couch

PURPOSEnTo assess the initial setup accuracy in treating more than 3 vertebral body levels in spinal SBRT using a 6D couch.nnnMETHODSnWe retrospectively analyzed last 20 spinal SBRT patients (4 cervical, 9 thoracic, 7 lumbar/sacrum) treated in our clinic. These patients in customized immobilization device were treated in 1 or 3 fractions. Initial setup used ExacTrac and Brainlab 6D couch to align target within 1 mm and 1 degree, following by a cone beam CT (CBCT) for verification. Our current standard practice allows treating a maximum of three continuous vertebrae. Here we assess the possibility to achieve sub millimeter setup accuracy for more than three vertebrae by examining the residual error in every slice of CBCT. The CBCT had a range of 17.5 cm, which covered 5 to 9 continuous vertebrae depending on the patient and target location. In the study, CBCT from the 1st fraction treatment was rigidly registered with the planning CT in Pinnacle. The residual setup error of a vertebra was determined by expanding the vertebra contour on the planning CT to be large enough to enclose the corresponding vertebra on CBCT. The margin of the expansion was considered as setup error.nnnRESULTSnOut of the 20 patients analyzed, initial setup accuracy can be achieved within 1 mm for a span of 5 or more vertebrae starting from T2 vertebra to inferior vertebra levels. 2 cervical and 2 upper thoracic patients showed the cervical spine was difficult to achieve sub millimeter accuracy for multi levels without a customized immobilization headrest.nnnCONCLUSIONnIf the curvature of spinal columns can be reproduced in customized immobilization device during treatment as simulation, multiple continuous vertebrae can be setup within 1 mm with the use of a 6D couch.

Proton Versus Conventional Radiation Therapy for Pediatric Salivary Gland Tumors: Acute Toxicity and Dosimetric Characteristics

in regular fixed jaw delivery mode with FW of 2.5 cm for nasopharyngeal carcinoma (NPC) treatments. Materials/Methods: Eight patients with NPC previously treated with HT in fixed jaw delivery mode with FWof 2.5 cm were replanned by using HT dynamic jaw delivery mode with preset FW of 5 cm. As in the previous plans, all nasopharynx, left, and right nodal planning target volumes (PTVNP, PTVLtN, PTVRtN) were aimed to achieve a volume of 95% covered by the prescribed dose. The maximum dose was limited to less than 115% of the prescribed dose. To evaluate the plan quality, PTV coverage, dose conformity and homogeneity were reported by the dose received by 95% of the target volumes (D95), conformity number (CN), and homogeneity index (HI), respectively. For organs at risk (OARs), the maximum dose (Dmax) and near-maximum dose (D2) of brainstem, spinal cord, optic chiasm, left and right lens, and optic nerves were reported while mean dose (Dmean) of pituitary, left and right parotid glands were also reported according to ICRU 83. The number of monitor units (MU) required was used to evaluate the delivery efficiency. Wilcoxon signed tank test was used to investigate significance of differences in the results between the plans created by the two techniques. A 2-tailed P<.05 was considered statistically significant. Results: For PTVNP, PTVLtN, PTVRtN, there was no statistical significant difference in all PTV coverage, dose conformity, and homogeneity between the two treatment modes. With regard to critical organ sparing, the Dmax and D2 of optic chiasm and Dmean of pituitary, left and right parotid glands were statistical significantly lower using dynamic jaw delivery mode, with a mean decrease of 51.1% 22.0%, 51.1% 22.4%, 40.8% 23.9%, 7.0% 6.9%, 7.2% 6.6%, respectively (PZ.008, .008, .008, .039, and .039, respectively). However, there was no statistical significant difference in Dmax and D2 of brainstem, spinal cord, left and right lens, and optic nerves between the 2 planning techniques. The number of MUs required was significantly lower for dynamic jaw delivery mode, with a mean decrease of 47.9% 2.3% (PZ.008). Conclusion: Our results showed that HT planned with dynamic jaw delivery mode with preset FW of 5 cm provided statistically no difference on plan quality, but with better sparing of optic chiasm, pituitary and parotid glands compared with HT in fixed jaw delivery mode with FW of 2.5 cm. A significant better delivery efficiency was also achieved. Author Disclosure: W. Lam: None. H. Geng: None. C. Kong: None. Y. Ho: None. W. Wong: None. B. Yang: None. K. Cheung: None. S. Yu: None.