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

backgroundLinear 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. PURPOSE: To achieve optimization in higher dose rates for smaller fields with FF, one small flattening filter (SF) for fields covering up to 16 × 16 cm

methodsRecently, 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.

resultsAs 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.

conclusionAs 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.