Dual flatness filter design for single energy photon beam of medical linear accelerator - Rationale and efficacy

Linear accelerators used for cancer treatments, in general, have one flattening filter (FF) for each x-ray energy. In a newly developed 6 MV low energy linac model, beam optimization at higher dose rates is achieved by two FFs.

To achieve optimization in higher dose rates for smaller fields with FF, one small flattening filter (SF) for fields covering up to 16 × 16 cm² and another large flattening filter (LF), to provide flattened fields till 30 × 30 cm² are made available. A smaller thickness filter selectively flattens central part of the beam.

Recently, a new generation low energy Siddharth II model 6 MV linac is manufactured by M/s Panacea Medical Technologies Pvt Ltd in India, with two FFs. Experimentally measured dose rates with SF and FF vis-à-vis un-flattened open beam were compared with theoretical estimates. Furthermore, a comparison is made with beam characteristics of a True Beam Varian linear accelerator for 6 MV beam.

As beam hardening is expected with FF, it was taken appropriate to consider 1.20 MeV mean energy for 6 MV bremsstrahlung continuous spectrum, before entry to FF. Measured transmissions of 0.8140 and 0.4470 for SF and LF, respectively, compare well with theoretically estimated 0.7904 and 0.4130. A dose rate enhancement factor 1.821 is achieved along with better flatness for smaller fields. The measured factor 0.4470 for large filter for Siddharth II, is similar to a transmission of 0.5100 for True Beam 6 MV photons.

As the first of its kind, the thin FF covers smaller solid angle of the total beam, giving higher MU/min in small fields, which might help for radical treatments. Also, there will be less power input requirements because of gain in dose rate.