Small-beam calibration by 0.6- and 0.2-cm3 ionization chambers

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• Small radiation beams are crucial in radiotherapy for curative treatments like shrinking field and high-dose techniques.
• Detector response to non-uniform radiation fluence can cause variations in measured field size factors (FSF).
• Accurate FSF measurements are essential for precise dose delivery in small field radiotherapy.

Purpose of the Study:

• To investigate the impact of different detector volumes on field size factor (FSF) measurements for small photon and electron beams.
• To compare FSF values obtained using 0.6-cm3 and 0.2-cm3 Farmer ionization chambers.
• To assess potential dose delivery errors associated with detector choice in small field dosimetry.

Main Methods:

• Measured dose rates of small photon (60Co, 4, 8, 16 MV) and electron (6, 12, 20 MeV) beams in polystyrene using 0.6-cm3 and 0.2-cm3 Farmer ionization chambers.
• Determined FSF as the ratio of dose rates for various field sizes to the dose rate of a 10x10 cm beam.
• Compared FSF values as a function of beam flatness and quality, and assessed potential dose errors.

Main Results:

• Significant discrepancies in FSF were observed between the 0.6-cm3 and 0.2-cm3 chambers, even for fields larger than the chamber volumes.
• The 0.6-cm3 chamber led to potential dose delivery errors ranging from 0% to 2% for photon beams up to 5x5 cm and -1% to 5% for electron beams up to 4 cm diameter.
• Errors exceeding 5% were noted for most beams smaller than 3.5x3.5 cm when using the 0.6-cm3 chamber.

Conclusions:

• Detector volume critically influences FSF measurements in small radiotherapy beams.
• The 0.2-cm3 Farmer chamber provides more accurate FSF determination for small fields compared to the 0.6-cm3 chamber.
• It is recommended to use detectors smaller than the 0.6-cm3 Farmer chamber for FSF measurements of beams less than or equal to 5x5 cm to ensure accurate radiotherapy dose delivery.