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Investigation of Off-Isocenter Field Misalignment in Single-Isocenter Multiple-Target VMAT SRS and Its Implication On CTV Margin

Y Song1*, C Velton2 , X Tang3 , Z Saleh4 , M Zhang5 , B Mueller6 , B Mychalczak7 , (1) Memorial Sloan Kettering Cancer Center, Montvale, New Jersey, (2) Columbia University, New York, NY, (3) Memorial Sloan Kettering Cancer Center, West Harrison, NY, (4) Memorial Sloan Kettering West Harrison, West Harrison, NY, (5) Memorial Sloan Kettering Cancer Center, New York, NY, (6) Memorial Sloan Kettering Cancer Center, Montvale, NJ, (7) Memorial Sloan Kettering Cancer Center, West Harrison, NY

Presentations

(Tuesday, 7/31/2018) 11:00 AM - 12:15 PM

Room: Karl Dean Ballroom A1

Purpose: One potential risk for single-isocenter multiple-target VMAT SRS is the increased geometric misalignment between the actual treatment field and planned field for off-isocenter targets. This phenomenon of field shift is mainly caused by mechanical uncertainty and asymmetric penumbra. As the dose gradient is about 13~18%/mm for SRS plans, a 1-mm geometric misalignment will result in a significant dose deviation. Here, we present a systematic methodology to quantify the field misalignment and assess its impact on CTV margin definition. A literature search indicates that this is the first such systematic evaluation on a TrueBeam machine.

Methods: Fourteen test plans were computed with EclipseV13. Each test plan contained MLC-defined static fields (3x3cm) with different combinations of gantry, collimator, couch angles, and off-isocenter distances (0, 2, 3, 4, 5, and 7 cm). A total number of 448 fields were created. All fields were measured with MV imaging on a TrueBeam machine. A Python program was written to batch process the images.

Results: We found that field center misalignment changed with off-isocenter distance. It was primarily the maximum misalignment that increased with off-isocenter distance, while the mean and minimum misalignments remained fairly constant. Few fields had large field center shifts, while some even had decreasing center shifts as off-isocenter distance increased. We also found that mechanical rotations did not induce any systematic field misalignment when off-isocenter distance ≤ 3 cm. The field misalignment became apparent when off-isocenter distance was ≥ 5 cm. Finally, we determined that the ratio of measured/planned field size was primarily affected by MLC leakage and not by off-isocenter distance.

Conclusion: Based on the results of this study, we recommend an additional 0.4 mm margin to CTV when off-isocenter distance is ≤ 3 cm and 0.6 mm margin to CTV when off-isocenter distance is ≥ 4 cm for TrueBeam machines.

Keywords

Brain, Stereotactic Radiosurgery

Taxonomy

TH- External beam- photons: intracranial stereotactic/SBRT

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