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

Previous studies describe the use of a large area parallel-plate chamber, the PTW Bragg Peak chamber, for measuring dose-area product (DAP) and output factors in small megavoltage photon fields. However, in radiotherapy departments without protons, this detector would have to be purchased separately for this purpose. This work investigated the feasibility of alternatively using a large transmission ionisation chamber, the IBA round Stealth chamber (SC), for output factor measurements of stereotactic fields. This type of detector is more commonly found in radiotherapy departments as a reference chamber for water tank scanning of small fields, and hence DAP could be performed without an additional purchase. The SC's large sensitive area (diameter of 94 mm) measures the integral dose, also known as DAP, over the whole two-dimensional (2D) dose distribution of the small field. The measurements were performed using a 6 MV beam from an Elekta Infinity linear accelerator. Conversion of DAP to central axis point dose was performed using 2D dose maps from Gafchromic EBT3 films. The field sizes measured ranged from side length of 5 mm to 50 mm (all square). The resultant output factors were compared against measurements with a stereotactic diode. The small field output factors measured using SC + film were in good agreement with the stereotactic diode (within 2% for field sizes as small as 6 mm; 3% difference at 5 mm). The new proposed method showed that a transmission chamber like SC is a good alternative large-area parallel plate chamber to measure DAP and derive small field OFs. Furthermore, the feasibility of using 2D reconstructed dose maps from water tank profiles and hence filmless approach was investigated. Results showed that filmless conversion of DAP to central axis point dose is feasible using profiles. However, a large number of profiles are required (i.e. 15° increments (star pattern) are required for accurate 2D dose reconstruction), and hence the water tank scanning for this approach may be prohibitively time-consuming.