Beam Focal Spot Offset Determination for Linear Accelerators: A Phantom less Method

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Quality Assurance

Background:

• Radiotherapy effectiveness depends on precise beam focal spot positioning.
• Periodic verification of the beam focal spot is crucial for patient safety and treatment efficacy.
• Existing methods for beam focal spot offset measurement can be complex.

Purpose of the Study:

• To develop and validate an electronic portal imaging device (EPID)-based technique for measuring beam focal spot offset.
• To assess the accuracy and reproducibility of the proposed EPID-based method.
• To provide a reliable tool for routine quality assurance in radiotherapy.

Main Methods:

• Utilized a combination of jaws and multileaf collimator (MLC) to create symmetric fields.
• Employed collimator rotations (180° or 360°) to determine radiation isocenters.
• Correlated the difference between isocenters with beam focal spot offset for linear accelerators (Varian ClinaciX, Elekta Versa HD).

Main Results:

• The EPID-based method demonstrated agreement with the co-rotational penumbra modulation (CPM) technique within 0.00923±0.009360 mm (SD).
• The method exhibited excellent reproducibility, with variations within 0.0365 mm (SD).
• A modified approach was successfully implemented for linear accelerators lacking dual jaw sets (e.g., Elekta Agility® head).

Conclusions:

• The developed EPID-based technique is accurate and reproducible for measuring beam focal spot offset.
• This method is suitable for routine quality assurance (QA) protocols.
• Ensuring beam focal spot offset is within tolerance enhances radiotherapy treatment precision.