Position of effective electron source for shielded electron beams from a therapeutic linear accelerator

Abstract

Position of effective electron source for shielded electron beams from a therapeutic linear accelerator The effective electron source positions for the standard electron cones and for the shielded field sizes with cerrobend inserts were measured based on Inverse Square Law (ISL) and the Inverse Slope (IS) method for various electron energies. The charge measurements were carried out using a 0.6 cc ion chamber (PTW, Type 30001) connected to a PTW Unidos E digital electrometer in a polystyrene phantom for electron beam energies of 6-18 MeV. The resultant charge for 100 MU setting was measured at nominal source to surface distances (SSDs) of 100-120 cm for cone and cerrobend defined field sizes. The effective SSD (SSDeff) was found to be different for the same field size defined by electron applicator and the cerrobend shield placed in 25×25 cm standard cone. Strong dependency of SSDeff with field size and electron beam energy was noticed. The results from the ISL and IS method are consistent, hence either of the two methods can be used to determine the effective source position. Whenever treatment is to be given with shielded electron portal, the SSDeff for that field needs to be determined. Same SSDeff as that of the standard cone can be used for minimum shielded electron portals.

Accuracy evaluation of distance inverse square law in determining virtual electron source location in Siemens Primus linac

“…Various studies showed that the location of virtual electron source depends on the energy of electron beam and the field size. 7,9,14,15 It was reported that the location of S Vir depends on machine type and the experimental method which was used to determine it. 7 By comparing the results obtained by Monte Carlo simulations and ISL method in the present study, it is shown that ISL method can be applied well in higher electron energies (14 MeV).…”

Monte Carlo study on effective source to surface distance for electron beams from a mobile dedicated IORT accelerator

“…, 11 This originating point of electron beam is called effective electron source. As defined by International Commission on Radiation Units and Measurements, effective electron source is a source that when placed in vacuum at some distance from the phantom surface, produces exactly the same electron fluence as the real beam is employed 10 . The distance between the effective electron source and distal end of the applicator is known as effective source to surface distance (SSD eff ).…”

“…[4][5][6][7][8] Output factor correction for electron beam is highly dependent to the accurate location of electron source. 9,10 Unlike the X-ray beam that is produced from a point on the X-ray target, the electron beams is originated from a focal spot that appears in space and does not coincide with the scattering foil or exit window inside the accelerator head. 10,11 This originating point of electron beam is called effective electron source.…”

“…9,10 Unlike the X-ray beam that is produced from a point on the X-ray target, the electron beams is originated from a focal spot that appears in space and does not coincide with the scattering foil or exit window inside the accelerator head. 10,11 This originating point of electron beam is called effective electron source. As defined by International Commission on Radiation Units and Measurements, effective electron source is a source that when placed in vacuum at some distance from the phantom surface, produces exactly the same electron fluence as the real beam is employed.…”

“…As defined by International Commission on Radiation Units and Measurements, effective electron source is a source that when placed in vacuum at some distance from the phantom surface, produces exactly the same electron fluence as the real beam is employed. 10 The distance between the effective electron source and distal end of the applicator is known as effective source to surface distance (SSD eff ). This parameter is a useful concept in derivation of several characteristic parameters of the electron beam, such as accelerator output correction factor and is dependent on the electron energy, field size and collimation method.…”