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The characteristics of photon beams from FF linac with jaw collimators and multileaf collimators (MLCs) have comprehensively been studied in the last five decades and all the aspects related to its clinical applications have been standardized. The presence of the FF in the linac substantially reduces the photon beam dose rate and is also thought to be the major source of head scattered photons which causes the variation of in-air output with field size and the exchange effect of secondary collimators. The FF is made up of high Z materials and is usually conical in shape to flatten the forward peaked bremsstrahlung spectrum of megavoltage photons. The flat dose profiles with a homogenous dose variation across the beam provide the ease in patient dose calculation during treatment planning. The FF in a standard linac is located between the primary collimator and the monitor chamber and its main role is to make the photon beam dose distribution uniform at reference depth within the allowed variations. A standard linac uses a flattening filter (FF) in photon mode operation while helical and robotic linacs do not have flattening filters.

A large number of standard linacs are used worldwide and their number is increasing rapidly in radiation beam management of increasing cancer cases. For high energy photon beam therapy, three different types of medical linacs, namely standard linac (C-arm linacs of Elekta, Siemens, and Varian), helical linac (Hi-Art Tomotherapy), and robotic linac (Accuray Cyber knife) are used in clinics. Medical electron linear accelerators (linacs) are the most commonly used beam delivery devices for radiation therapy of varieties of cancer cases.