M. Nakata

Use of a CT simulator in radiotherapy treatment planning for breast conserving therapy

A CT simulator (CT-S) is a real-time, CT-linked, 3-D treatment planning system, which consists of a CT scanner, a multi-image display, a treatment planning device with real-time visual optimization, and a laser beam projector. This system was clinically evaluated in 339 Stage 0, I and II breast cancers in 337 patients who received breast conserving therapy. Following quadrantectomy or wide excision with complete axillary dissection, a total of 50 Gy was delivered to the ipsilateral breast using 60Co gamma-rays. In patients with involved surgical margins, boost electron irradiation was also given. Treatment planning could be accomplished within 40-50 min using the CT-S. The parameters for the tangential portals could be optimized for each patient, and the wedge filters and the electron energy for boost irradiation could be appropriately selected. The incidence of moist desquamation and depigmentation at the areola was slightly decreased with the use of wedge filters. During the follow-up for 2-71 months (median, 22 months), local recurrence and symptomatic radiation pneumonitis occurred in one patient each. The CT-S appears to be useful for the individualized optimization of tangential irradiation fields in breast conserving therapy.

SU‐E‐T‐525: Dosimetric Validation of the Algorithm Based on Linear Boltzmann Transport Equations for Photon 4MV Dose Calculation

PURPOSE To evaluate a dosimetric accuracy of AcurosXB dose calculation algorithm for 4 MV photon beam. METHODS Four MV beam (Clinac-6EX) and AAA and AcurosXB algorithms (pre-release version 11.0.03.) were used in this study. The differences of the calculation with AAA (EAAA) and AcurosXB (EAXB) to the measurement were evaluated in the depth doses to 25 cm depth and dose profiles within the water and slab phantoms (water, lung and bone equivalent). In addition, the clinical cases, including three whole breast plans and three head and neck IMRT plans, were evaluated. First the AAA plans were calculated, then AcurosXB plans were recalculated with dose-to-medium with identical beam setup and monitor units as in the AAA plan. RESULTS In the water phantom study, the EAAA and EAXB were up to 2.2% and 1.5% in the depth doses for the open field (field size = 4 - 40cm square), respectively. Under the heterogeneity conditions, the EAAA and EAXB were less than 4.4% and 2.2% in lung region, and less than 12.5% and 6.3% in bone region, respectively. In the re-buildup region after passing through the lung phantom, the AAA overestimated the doses about 10%; however AcurosXB had good agreement with measurement within 3%. Dose profiles with AcurosXB were better agreement with measurement than AAA. In the clinical cases, the dose of the skin surface region with AcurosXB were higher than AAA by at least 10%, and the dose differences over 5% appeared in heterogeneous region. However, DVH shapes of each organ were similar between AAA and AcurosXB within 2%. CONCLUSIONS In phantom study, AcurosXB had better agreement to measurement than AAA, especially in heterogeneous region and re-buildup region. In the clinical cases, there were large differences between AcurosXB and AAA in the surface region. Evaluation Agreement of non-clinical versions of Acuros XB with　Varian Medical Systems.

PD-0531: Dosimetric investigation of rotational setup errors for spine stereotactic radiosurgery: a phantom study

glands were independently associated with Xer6m, with a correlation coefficient of 0.454 between the model predictions and Xer6m. No multivariable model with multiple significant independent predictors was found for Xer12wk. A significant correlation between Xer6m and Xer12wk was found with a correlation coefficient of 0.627 Conclusions: The volume and density of the parotid glands and the density of the submandibular glands are associated with xerostomia after radiotherapy. The changes of these CT image features over the course of treatment are more strongly related to xerostomia than the instantaneous feature values at baseline. Moreover, the feature changes in the first 12 weeks after start of treatment are equally or even more strongly related to xerostomia at 6 month than at 12 weeks. These associations suggest that, even early after treatment, the CT image feature changes may be used as objective biomarkers of the development of xerostomia.

SU-E-T-351: Verification of Monitor Unit Calculation for Lung Stereotactic Body Radiation Therapy Using a Secondary Independent Planning System

PURPOSE To compare isocenter (IC) dose between X-ray Voxel Monte Carlo (XVMC) and Acuros XB (AXB) as part of an independent verification of monitor unit (MU) calculation for lung stereotactic body radiation therapy (SBRT) using a secondary independent treatment planning system (TPS). METHODS Treatment plans of 110 lesions from 101 patients who underwent lung SBRT with Vero4DRT (Mitsubishi Heavy Industries, Ltd., Japan, and BrainLAB, Feldkirchen, Germany) were evaluated retrospectively. Dose distribution was calculated with X-ray Voxel Monte Carlo (XVMC) in iPlan 4.5.1 (BrainLAB, Feldkirchen, Germany) on averaged intensity projection images. A spatial resolution and mean variance were 2 mm and 2%, respectively. The clinical treatment plans were transferred from iPlan to Eclipse (Varian Medical Systems, Palo Alto, CA, USA), and doses were recalculated with well commissioned AXB ver. 11.0.31 while maintaining the XVMC-calculated MUs and beam arrangement. Dose calculations were made in the dose-to-medium dose reporting mode with the calculation grid size of 2.5 mm. The mean and standard deviation (SD) of the IC dose difference between XVMC and AXB were calculated. The tolerance level was defined as |mean|+2SD. Additionally, the relationship between IC dose difference and the size of planning target volume (PTV) or computed tomography (CT) value of internal target volume (ITV) was evaluated. RESULTS The mean±SD of the IC dose difference between XVMC and AXB was -0.32±0.73%. The tolerance level was 1.8%. Absolute IC dose differences exceeding the tolerance level were observed in 3 patients (2.8%). There were no strong correlations between IC dose difference and PTV size (R=-0.14) or CT value of ITV (R=-0.33). CONCLUSION The present study suggested that independent verification of MU calculation for lung SBRT using a secondary TPS is useful.

SU‐E‐T‐106: Evaluation of Sensitivity and Uniformity of New Radiochromic Film with Two Commercial Scanners

PURPOSE A newly introduced radiochromic film, the GAFCHROMIC EBT3, has been expected as much useful device for the IMRT dosimetry. The purpose of this study was to investigate the sensitivity and the uniformity of the films between an Epson ES-10000G flatbed scanner and a Vidar DosimetryPRO Advantage (Red) scanner. METHODS Doses ranging from 1 cGy to 1600 cGy with 15-MV photon beam was irradiated to the film in a solid water phantom, respectively. All of the films were then digitized after irradiation using both two scanners. Sensitivities, local fluctuations of the film with two scanners were evaluated. Local fluctuations were defined as the relative (percent) standard deviation of the film response in ROIs (3 cmx3 cm). RESULTS As to the Vidar scanner, the sensitivity of the film was higher for low dose range (below <400 cGy). While, as to the Epson scanner, the sensitivity using the red color channel was higher than others for low dose range. At high dose range (above >400 cGy), the green color channel had higher sensitivity than others. The Vidar scanner exhibited the lower local fluctuations than the Epson scanner for all dose ranges. For the Epson scanner, the red color channel had the lower local fluctuations than the green and blue color channel for all dose ranges. CONCLUSIONS This study shows the characteristics of the new EBT3 films, in conjunction with the Epson ES-10000G flatbed scanner and the Vidar DosimetryPRO Advantage (Red) scanner.

TU-EE-A3-06: Impact of Respiratory Velocity On Target Volume Using 4DCT

Purpose: To assess the impact of respiratory velocity on target volume using four‐dimensional computed tomography (4DCT). Method and Materials: A 20 mm diameter object in a QUASAR™ phantom sinusoidally moved with 10 mm amplitude along the longitudinal axis of the CT couch. The motion period was set in the range of 2–12 sec at 2 sec intervals. 4DCT data were acquired on a General Electric 4‐slice Lightspeed RT CT scanner in an axial cine mode. Respiratory motion was recorded by a Varian Real‐time Positioning Management system. A CT slice thickness and image acquisition time were 1.25 mm and 0.5 sec, respectively. The cine duration was set to the motion period plus 2 sec. The number of 10 images per each couch position was reconstructed.Measurement repeated 3 times for each pattern. The object was automatically segmented using threshold on CTimages. Volumetric analysis was performed to evaluate variations in the object size by different periods. Results: The maximum volume of the object was 6.35 ml at a maximum instantaneous velocity (V max) of 30.11 mm/sec, which was larger by 51.2% than true volume. While the probability that a difference between imaged volume and true volume was more than 5%was 37.3% at the velocity of ⩽ 10.68 mm/sec corresponding to the V max with the period of 5.87 sec, it increased to 96.3% at the velocity of >10.68 mm/sec. A significant difference was seen between the mean volume with the period of ⩽ 10.68 mm/sec and >10.68 mm/sec (P<0.01). Conclusion: Severe motion artifacts are more pronounced at higher respiratory velocity. Even if the respiratory period is slow, motion artifacts remain as long as the object moves during CTdata acquisition.