Breast Cavity Boost Imaging: Comparison of kV Imaging versus kV Cone Beam Computed Tomography Techniques in Deep Seated Tumor Cavity

E694

International Journal of Radiation Oncology  Biology  Physics

University School of Public Health, Piscataway, NJ, 5Summit Medical Center, Summit, NJ, 6Rutgers Cancer Institute of New Jersey, New Brunswick, NJ, United States, 7Rutgers Cancer Institute of New Jersey & Rutgers Robert Wood Johnson Medical School, New Brunswick, NJ, 8 Rutgers Cancer Institute of New Jersey Department of Radiation Oncology, New Brunswick, NJ, 9Rutgers Cancer Institute of New Jersey Department of Neurosurgery, New Brunswick, NJ, 10Rutgers Cancer Institute of New Jersey, New Brunswick, NJ, 11Rutgers - Cancer Institute of New Jersey, New Brunswick, NJ

accurate dose calculations when compared with the current algorithms that are in clinical use. This study investigates clinical pencil beam scanning (PBS) planned dose differences between PB and MC in lung cancer patients. Materials/Methods: With IRB approval, a comparative dosimetric analysis between RaystationTM Monte Carlo v4.0 (pending FDA 510 clearance) and RaystationTM pencil beam v4.0 dose algorithms was performed on 10 patient plans. PBS gantry plans were generated using single-field optimization technique to maintain target coverage under 3% range and 3 mm setup uncertainty. Dose prescriptions ranged from 60 Gy(RBE) to 66.6 Gy(RBE). Dose differences between PBA and MC were recorded for the following plan metrics: CTV V95, CTV homogeneity index (HI), total lung V20, total lung VRX (relative volume of lung receiving the prescribed dose or higher), and global maximum dose. Non-parametric Wilcoxon signed-rank testing was carried out to evaluate pairwise differences in planned dose and dose-volume parameters between PBA and MC. Results: In this 10 patient sample, CTV V95 ranged from 99% to 100% with a median of 100% for PBA. When recalculated with MC, CTV V95 ranged from 77% to 94% with a median of 90% (pZ0.002). MC treatment plans showed a reduction of dose homogeneity within the target volume. The median CTV HI (D95/D5) was 0.98 for PBA and 0.91 for MC (p Z 0.002). Total lung V20 distributions were statistically identical to within 1% between dose calculation algorithms (pZ1.0). Median lung VRX was 7% and 1% for PBA and MC, respectively (p Z 0.002), indicating a reduction in lung volume receiving high dose when using MC. Lastly, the global maximum patient dose increased from a median value of 106% for PBA (range 101-113%) to 109% for MC (range 104-122%, p Z 0.002). Conclusion: A retrospective analysis of 10 lung cancer PBS plans revealed a planned dose reduction in coverage to the CTV when recalculated using MC. This suggests that the use of PBA dose calculation in this disease site may lead to underdosing of the target volume and may support the clinical integration of MC dose calculation during treatment planning.

Purpose/Objective(s): Leptomeningeal carcinomatosis (LMC) is a rare manifestation of metastatic cancer and occurs in 5-10% of patients with brain metastases. The current literature remains limited in its understanding of the predictive factors for the development of LMC after stereotactic radiosurgery (SRS) for brain metastases. This case-control study explored multiple risk factors that may predispose patients to LMC after SRS treatment. Materials/Methods: Under an IRB approved protocol, a case-control study of patients with brain metastases who underwent single-fraction SRS between March 2011 and June 2016 at one institution was conducted. Demographic, clinical, and brain lesion information were collected retrospectively from patients’ electronic medical records for 19 LMC cases and 30 controls out of 413 patients, including baseline recursive partitioning analysis (RPA) classification, location of brain metastases, tumor size, tumor volume, history of surgical resection of brain metastases, and WBRT. Controls were matched for age at treatment (3 years), gender, primary cancer, histology, and race. Risk factors of interest were evaluated by univariate and multivariate logistic regression analyses and overall survival were evaluated by Kaplan-Meier survival analysis. Results: About 5% of our patients with brain metastases who received SRS developed LMC. Overall in cases and controls, the 1-year survival rate after SRS was 33.3% while the 2-year survival rate after SRS was 6.7%. Patients with LMC (median 154 days, 95% CI: 33-203 days) demonstrated a poorer overall survival than matched controls (median 417 days, 95% CI: 121-512 days, pZ0.002). The most common primary tumor histologies for LMC were non-small cell lung cancer (36.8%), breast cancer (26.3%), and melanoma (21.1%). No significant association was found between the risk of LMC and the location of the brain lesion or total brain volume of brain metastases with risk for LMC. Yet, a history of prior surgical brain lesion resection before SRS was associated with a 6.5 times higher odds (95% CI: 1.45-29.35, pZ0.01) of developing LMC post-radiosurgery. Conclusion: A history of prior surgical resection of brain metastases before SRS was associated with 6.5 times higher odds of LMC, than those who had no prior resections of brain metastases. Adjuvant WBRT may help to reduce the risk of LMC and can be considered in decision-making for patients who may have had brain metastasectomy. Author Disclosure: R. Ma: None. M. Levy: None. B. Gui: None. S. Lu: None. V. Narra: None. A. Cohler: None. S. Danish: None. S. Goyal: Employee; Princeton Medical Group. Independent Contractor; Hines Associates, Isoray Medical. Consultant; Hines Associates. Senior Editor; Advances in Radiation Oncology. S. Hanft: None. A.J. Khan: Research Grant; Cianna Medical, Elekta. Consultant; Elekta. S.B. Motwani: None. S.K. Jabbour: Research Grant; Merck.

3654 Dose Comparison between Proton Pencil Beam and Monte Carlo Dose Calculation Algorithm in Lung Cancer Patients D. Maes,1 S.R. Bowen,2 A. Fung,3 J. Saini,1 C. Bloch,4 A. Egan,1 J. Zeng,5 R. Rengan,5 and T.P. Wong1; 1Seattle Cancer Care Alliance Proton Therapy Center, Seattle, WA, 2University of Washington Radiation Oncology and Radiology, Seattle, WA, 3SCCA Proton Therapy Center, Seattle, WA, 4University of Washington, Department of Radiation Oncology, Seattle, WA, 5University of Washington, Seattle, WA Purpose/Objective(s): Proton pencil beam dose calculation algorithms (PBA) have limitations in modeling proton scatter dose and its impact on depth dose properties through heterogeneous tissues present when treating lung cancer patients. Monte Carlo dose calculations (MC) may provide more

Abstract 3654 Pencil Beam Median CTV V95 CTV HI Lung V20 Lung VRX Max Dose

100% 0.98 25% 7% 106%

Range 99% - 100% 0.95 - 1.00 15% - 35% 2% - 11% 101% - 113%

Monte Carlo Median 90% 0.91 25% 1% 109%

Range 77% - 94% 0.87-0.94 16% - 34% 0% - 6% 104% - 122%

Sign Rank p 0.002 0.002 1.000 0.002 0.002

Author Disclosure: D. Maes: None. S.R. Bowen: None. A. Fung: None. J. Saini: None. C. Bloch: Oversee the organization of online continuing educational material and related duties; American Association of Medical Physicist. A. Egan: None. J. Zeng: None. R. Rengan: Honoraria; Apollo Oncology Group. Consultant; Apollo Oncology Group. Advisory Board; Apollo Oncology Group. Travel Expenses; Apollo Oncology Group. T.P. Wong: None.

3655 Breast Cavity Boost Imaging: Comparison of kV Imaging versus kV Cone Beam Computed Tomography Techniques in Deep Seated Tumor Cavity W.L. Magalhaes,1 S. Herchko,2 D. Miller,1 T. Kaleem,1 K.S. Tzou,1 and L.A. Vallow1; 1Mayo Clinic, Jacksonville, FL, 2Mayo clinic, Jacksonville, FL Purpose/Objective(s): The most common location of an ipsilateral breast recurrence is the surgical cavity. Therefore an additional dose is provided to a tight surgical cavity volume with utilization of a breast boost. To evaluate the accuracy of localization during radiation treatment, Kv imaging was compared to conebeam CT to determine the difference in alignment and adequate margins of treatment. Materials/Methods: Twenty breast cancer patients with deep seated tumors were selected in whom the breast surgeon had placed surgical clips to delineate the tumor cavity for adjuvant radiation therapy. Whole breast radiation therapy was delivered, followed by a boost to the breast cavity.

Volume 99  Number 2S  Supplement 2017

Poster Viewing E695

Breast cavity boost treatments using photon conformal arc technique were performed after initial treatment with whole breast tangents. For each boost cavity the surgical clips and tumor bed were contoured prior to treatment, and Kv imaging and CBCT were then utilized to evaluate the tumor bed location. The patients were initially set up by using skin marks placed at the time of simulation. Kv imaging was then performed. The surgical clips were initially aligned with Kv imaging, which was then followed by CBCT. The differences in the position of couch coordinates when matched to the surgical clips were then calculated to compare the localization accuracy of the initial Kv images to CBCT for radiation treatment. Results: The average couch coordinate differences are seen in the table below. A largest average difference of 2.4 mm was seen on the vertical positioning. It is reasonable to conclude that Kv imaging alone is appropriate if some margin is included in the treatment plan to account for setup variations. We found no statistical significance for the couch coordinates between Kv imaging and CBCT.

Abstract 3656; Table 1

TE (Echo Time)/TR (Repetition Time) (ms) Echo train length (ETL) Voxel size (mm) BW (Bandwidth) (Hz/pixel) Parallel imaging factor (iPAT PE) Parallel imaging factor (iPAT 3D) Number of signals acquired (NEX) Partial Fourier (slice direction) Acquisition time (min)

MR-A

MR-B

MR-C

MR-D

147/1500

147/1500

147/1500

147/1500

217 200 217 200 1.3x1.3x1.3 1.3x1.3x1.3 1.3x1.3x1.3 1.3x1.3x1.3 814 814 814 814 3

4

3

4

3

2

2

1

1

1

1

1

Off

Off

Off

6/8

1:15

1:30

1:56

2:41

Conclusion: The results of this study found that the reproducibility of the intended treatment target can be achieved solely using Kv imaging with an accuracy of +-2.5mm. If a margin of 3 mm is used for the tumor cavity including all surgical clips a certainty of tumor cavity location can be achieved using Kv imaging. This reduces radiation doses from the CBCT, minimizes patient motion and improves treatment accuracy. Author Disclosure: W.L. Magalhaes: None. S. Herchko: None. D. Miller: None. T. Kaleem: None. K.S. Tzou: Employee; Mayo Clinic Florida. L.A. Vallow: None.

Results: Quantitatively, MR-A, with the shortest acquisition time, had a significantly larger shift error (pZ0.007) in the Z translation direction with an average difference of 0.165mm compared with MR-RefA. MRD, with the longest scan time, had a significantly larger yaw rotation (pZ0.014) with a mean difference of 0.13o compared with MR-RefD. Qualitatively, the observers favored MR-D and MR-B sequences. MR-D and MR-B were not significantly different in image quality scores compared with their references. MR-B sequence had a high score, comparable to MR-RefB with bladder outline and visualization (pZ0.212) and bladder motion artifact (pZ0.124). MR-B is a promising imaging sequence. Not only was image quality not impeded to affect accurate registration but good visuals of the OARs and overall little artifact and noise were also reported. Conclusion: In this study, four MR sequences optimized for pelvic positional verification have been tested and the recommended image protocol, MR-B, has been identified based on image quality, registration accuracy and acquisition time. Author Disclosure: S. Man: None. W. Fung: None. G. Chiu: None. O. Wong: None. W. Luk: None. L. Fung: None.

3656

3657

The Optimization of Isotropic 3D TSE (SPACE) MR Sequence for Daily Verification Imaging in Radiation Therapy of the Pelvis S.Y. Man,1 W.W.K. Fung,1 G. Chiu,2 O. Wong,1 W.P. Luk,1 and L.H. Fung1; 1Hong Kong Sanatorium and Hospital, Happy Valley, Hong Kong, 2Department of Radiotherapy, Hong Kong Sanatorium and Hospital, Happy Valley, Hong Kong

Evaluating the Consequences of MLC Leaf Positioning Errors in Dynamic IMRT Treatments K. J. Maria Das,1 A. Agarwal,2 N. Rastogi,2 S. Yoganathan,1 D. Udayakumar,1 and S. Kumar1; 1Sanjay Gandhi Postgraduate Institute of Medical Sciences, Lucknow, India, 2IFTM University, Moradabad, India

Abstract 3655 Differences in couch coordinates after kV and CBCT imaging when matching to surgical clips

# of Fractions Average Difference (cm) Maximum Difference (cm) Minimum Difference (cm) P Value (average difference)

Vertical

Lateral

Longitudinal

Rotation*

94 0.24 1.2 0 0.6969

94 0.15 0.7 0 0.9997

94 0.14 1.3 0 0.9984

74 0.03 0.9 0 0.6228

* Rotation differences reported in degrees

Purpose/Objective(s): The integration of MR-Simulator in RT workflow has sparked interest in the development of MRI-guided RT treatment protocols. The purpose of this study is to optimize an isotropic 3D TSE (SPACE) MR sequence for daily positional verification using MR-MR based imaging guidance in pelvic RT. Materials/Methods: T2-weighted (T2W) pelvic images of 10 male and 10 female healthy volunteers (age 23-45) were acquired in a 1.5T MRISimulator with a dedicated 18-channel phased-array body coil. Images were scanned in RT treatment position immobilized in a customized vacuum pillow system. Rapid imaging was achieved with partial Fourier techniques and with adjustments to ETL, NEX and iPAT, to generate a total of four axial 3D verification image sequences (MR-X) with a short acquisition time. Signal-to-noise ratio (SNR), TR, TE, BW, NEX and voxel size remained constant (Table 1). To avoid positional deviation, a high-resolution axial T2W 3D SPACE sequence (MR-Refx) was scanned as a reference (TR/TE: 1500/149ms, 1.17mm isotropic voxel) prior to the acquisition of each MR-X. Using four radiation therapist observers (105 years of experience), the impact of image quality of the four verification image sequences was investigated by employing both a qualitative analysis using a 7-point scale evaluation, based on delineation of organ outline and motion artifact, and a quantitative approach using a 6-DoF offline manual registration. The results were analysed using paired t-tests.

Purpose/Objective(s): MLC positioning accuracy in dynamic IMRT is very important for precise treatment delivery and any mismatch between planned and actual treatment delivery can lead to serious dose discrepancies. The purpose of this simulation study was to evaluate the dosimetric impact of leaf positional errors in dynamic IMRT treatments. Materials/Methods: Dynamic IMRT plans of five head-neck (HN) (66Gy/30# for CTV1 and 54Gy/30# for CTV2) and five brain (54Gy/ 30#) patients were retrospectively included in this study. MLC files of these plans were exported to in-house developed software that introduced known leaf positional errors. Two types of errors were considered; random errors (uniform distribution between -2.0 to +2.0mm) and systematic errors (0.5, 0.75, 1.0 & 2.0 mm. The error introduced MLC files were imported into the TPS and dose distributions were re-calculated. The error plans were compared against the respective original plans. The DVH files were exported to in-house software for TCP and NTCP calculation. Results: The impact of random errors was observed to be negligible (<0.5%) while that of systematic errors was substantial. On average, the changes in TCP and NTCP for all systematic errors were less than 5% for brain. On the other hand, the effect was observed to be more prominent in HN. Compared to reference plans the change in TCP of CTV1 was 4% for 0.5mm and 7.0-26.5% for 0.75mm to 2.0mm; whereas, the same for CTV2 was <1.0% up to 1.0mm and 5.1% for 2.0mm error. Similarly, the changes in NTCP of left and right parotid were 10.8% (2.5Gy difference in mean