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Small Field Stereotactic Radiosurgery Fluence Measurement for Cone and MLC-Based Plans Using SRS MapCHECK

C Geraghty1*, T Li2, J Carroll3, B Hasson4, (1) Anne Arundel Medical Center, Annapolis, MD, (2) ,Annapolis, MD, (3) Versant Medical Physics & Radiation Safety, Tewksbury, MA, (4) Anne Arundel Medical Center, Annapolis, MD

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: To evaluate fluence of stereotactic cones and MLCs for PTVs of 6mm to 10mm diameter using the SRSMapCHECK and StereoPHAN phantom.
Methods: Patients with SRS PTVs of 10mm or smaller were planned using cones with diameters 6mm, 7.5mm, or 10mm using BrainLAB iPlan RT Dose 4.5. Plans used four noncoplanar arcs each with a gantry sweep of approximately 120 degrees. The planning process was repeated using the same treatment couch and gantry angles but with MLC dynamic conformal arcs. All plans were calculated with pencil beam algorithm for a Novalis TX with 120HD MLC and a calculation grid of <1mm. The plans were mapped onto the StereoPHAN with SRS MapCHECK and recalculated dividing the planned MUs by two or three. The phantom was then positioned on the machine and aligned to the room lasers. The plans were delivered on the phantom at the planned couch and gantry angles. The phantom was not repositioned at each table angle with image guidance as is typical of patient treatment delivery. The plans were evaluated in the SRSMapCHECK software using gamma, absolute dose criteria of 1%, 1mm with a threshold of 10.
Results: Cone plan passing rates ranged from 97.1% to 100%. MLC plan passing rates ranged from 91.5% to 96.9% without CAX offset. CAX offset of Y = -0.7mm improved all MLC plan passing rates to 100%. Changing the analysis criteria to 2%, 1mm increased the MLC plan passing rates without CAX offsets to 93.6% and above. The 6mm cone plan and the corresponding MLC plan had the lowest passing rates.
Conclusion: Both cone and MLC based plans had satisfactory passing rates down to 6mm target size under fluence analysis with the SRSMapCHECK. However, there was a systematic difference between the cone and MLC plans indicated by CAX offsets in the analysis.

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Keywords

Stereotactic Radiosurgery, Small Fields

Taxonomy

TH- External Beam- Photons: Small field experimental dosimetry

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