SU-E-T-170: A Viable Approach to Patient Specific QA for Spine VMAT SRS Using EPID-Based Dosimetry

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Technology

Background:

• Volumetric Modulated Arc Therapy (VMAT) involves complex multi-parameter modulations, posing significant challenges for patient-specific quality assurance (QA).
• Stereotactic Radiosurgery (SRS) for spine tumors demands precise dose delivery and rigorous QA protocols.

Purpose of the Study:

• To present a viable approach for patient-specific QA of spine VMAT SRS cases utilizing Electronic Portal Imaging Device (EPID)-based dosimetry.
• To evaluate the accuracy and feasibility of EPID dosimetry for VMAT QA in clinical practice.

Main Methods:

• Spine VMAT SRS plans were optimized using 6 MV photons on a Varian Trilogy LINAC.
• QA plans were created by maintaining MLC apertures and MU weights while resetting gantry angles.
• Absolute dose was measured in a solid water phantom using EPIDose software, converting EPID images to dose distributions.

Main Results:

• The EPID-based dosimetry method achieved a pass rate typically exceeding 95.0% for the 2%/2 mm criteria.
• Discrepancies between measured and planned doses at the isocenter were generally around ±1.0%.
• The technique accurately detected sharp dose gradients at the planning target volume (PTV)-cord interface.

Conclusions:

• EPID-based dosimetry offers a real-time, financially viable solution for VMAT QA.
• This method achieves sub-millimeter dosimetry accuracy without complex inter-diode interpolations.
• EPID-based VMAT dosimetry presents a competitive alternative to existing dosimetry technologies.