Integral small field output factor measurements using a transmission ionisation chamber.

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Dosimetry

Background:

• Accurate measurement of dose-area product (DAP) and output factors (OFs) is crucial for small megavoltage photon fields in radiotherapy.
• Traditional methods using large area parallel-plate chambers may require additional equipment purchase.
• Transmission ionization chambers are commonly available in radiotherapy departments for water tank scanning.

Purpose of the Study:

• To investigate the feasibility of using a large transmission ionization chamber (IBA Stealth chamber, SC) for output factor measurements in stereotactic fields.
• To evaluate the SC's capability for measuring DAP in small fields as an alternative to dedicated chambers.
• To compare SC-derived output factors with those measured by a stereotactic diode.

Main Methods:

• Measurements were performed using a 6 MV photon beam on an Elekta Infinity linear accelerator.
• The IBA Stealth chamber (SC) was used to measure DAP over the 2D dose distribution of small fields (5-50 mm).
• Conversion of DAP to central axis point dose was achieved using 2D dose maps from Gafchromic EBT3 films and filmless water tank profiles.

Main Results:

• Output factors derived using the SC and film showed good agreement with a stereotactic diode (within 2% for fields ≥ 6 mm, 3% difference at 5 mm).
• The SC demonstrated feasibility as an alternative to traditional parallel-plate chambers for DAP measurement and small field OF derivation.
• A filmless approach using water tank profiles for DAP to point dose conversion is feasible but requires extensive scanning time.

Conclusions:

• The IBA Stealth chamber is a viable and cost-effective alternative for measuring DAP and deriving output factors in small stereotactic fields.
• This method reduces the need for additional equipment purchases in radiotherapy departments.
• While filmless dosimetry is possible, the time investment for accurate 2D dose reconstruction from profiles is significant.