Commissioning a miniature multileaf collimator for small field radiotherapy

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• Accurate dosimetry is crucial for small radiation fields used in modern radiotherapy.
• Miniature multileaf collimators (MLCs) like the Siemens ModuLeaf present unique challenges for precise dose measurement.
• Evaluating various measurement techniques and detectors is essential for reliable data acquisition.

Purpose of the Study:

• To determine key dosimetric parameters for the Siemens ModuLeaf miniature MLC.
• To assess the accuracy and limitations of different detectors and measurement techniques for small field dosimetry.
• To provide guidance on obtaining reliable dosimetric data without specialized equipment.

Main Methods:

• Output factors, head scatter factors, beam profiles, and percentage depth doses (PDDs)/tissue maximum ratios (TMRs) were measured.
• Various detectors were employed, including diamond detectors, radiographic film, and ionization chambers (cylindrical and advanced Markus).
• Measurements were performed using brass and Perspex miniphantoms at different setups and distances, with and without a low scatter support stand.

Main Results:

• Detectors showed under-response as field size decreased; diamond detectors and film agreed best for fields < 2x2 cm.
• Head scatter factor measurements varied with phantom material and setup, with agreement below 3x3 cm fields only in specific configurations.
• Beam profile artifacts were observed due to phantom markings; PDDs and TMRs showed agreement for fields down to 3x3 cm.

Conclusions:

• Standard dosimetry protocols may require adaptation for miniature MLCs at small field sizes.
• Diamond detectors and radiographic film are suitable for measuring output factors in very small fields.
• Careful phantom selection and setup are critical for accurate head scatter factor measurements with miniature MLCs.