Dosimetric changes induced by positional uncertainty of cutout in electron radiotherapy

Abstract

Dosimetric changes caused by the positional uncertainty of centring a small electron cutout to the machine central beam axis (CAX) of the linear accelerator (linac) were investigated. First, six circular cutouts with 4 cm diameter were made with their centres shifted off from the machine CAX for 0, 2, 4, 6, 8 and 10 mm using the 6 · 6 cm 2 applicator. Then, the percentage depth doses (PDDs) at the machine CAX and cutout centre were measured using the 4, 9 and 16 MeV clinical electron beams produced by a Varian 21 EX linac. The cross-and in-line axis beam profiles were measured at depth of maximum dose (d m ) and source-to-surface distance equal to 100 cm using a scanning water tank system and diode detector. When the cutout centre was shifted away from machine CAX for the electron beam with low energy of 4 MeV, the d m , depths of the 80 (R 80 ) and 90% (R 90 ) depth dose at the machine CAX had no significant change (<0.1 mm). For higher energies of 9 and 16 MeV beams, the d m were reduced with 0.45 and 1.63 mm per mm off-axis shift between the cutout centre and the machine CAX, respectively. The R 80 and R 90 were reduced with 0.7 mm per mm off-axis shift for both energies. When there was a 4 mm off-axis shift, the relative output factors for the 4, 9 and 16 MeV beams were reduced with 0.8, 1.6 and 0.5%, respectively. The isodose coverage of the in-line axis beam profile was reduced when the cutout centre was shifted away from machine CAX. It is important for radiation oncologists, dosimetrists, therapists and physicists to note such dosimetric changes in the electron radiotherapy to the patient, because such positional uncertainty is unavoidable in fabricating an electron cutout in the mould room.

Study of the backscattering of electron beams with energies typical of radiotherapy

“…Electron beam scattering is an important issue in electron beam therapy and should be considered during treatment planning according to treatment guidelines [11]. Monte Carlo simulations provided systematic and clinically useful information on electron backscatter factors in the use of lead shielding in electron beam therapy and explored the effect of electron beam orientation and treatment volume thickness on electron backscatter [12]. An electron beam simulation was also carried out to describe the variation of backscatter probability with the scatterer's thickness and its atomic number [13].…”