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 (SSD_eff) 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 SSD_eff 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 SSD_eff for that field needs to be determined. Same SSD_eff as that of the standard cone can be used for minimum shielded electron portals.