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Description
Small and narrow cutout fields of electron beam are commonly used in keloid radiotherapy. However, they pose several dosimetric challenges, including lateral scatter disequilibrium, source occlusion. This study investigated physical dose distributions for small and narrow fields used in keloid and breast scar boost radiotherapy. We conducted measurements in an Elekta Versa HD linear accelerator with a 6 MeV electron beam. The PTW MP3-M acrylic water tank and two PTW PinPoint ionization chambers were used to perform this experiment. The small fields included 5 × 5 to 2 × 2 cm2 placed in 10×10 cm2 cone. The field widths of narrow fields involve 3, 2.5 and 2 cm with 8 to 16 cm length, respectively. The percent depth dose curves and beam profiles were assessed. For PDD curves, we reported data on the several physical parameters that vary as a function of the cutout size: dmax (depth of maximum dose), R90, R80, R50, (the depth in water at which the absorbed dose falls to 90%, 80%, 50% of its maximum value, respectively), Rp (practical range), reference depth (dref) and entrance dose. For square fields, an increase in entrance dose is observed as the field size decreases, particularly for cutouts as small as 3 × 3 cm². Additionally, the experimental results reveal a noticeable shift of the depth of maximum dose toward the surface with decreasing field size. In term of narrow fields, the depth of maximum dose of shifted within 1 mm for field width > 2 cm, but around 2 mm for the 2 cm field width. This study reveals substantial changes in physical properties when the field size falls below 3 × 3 cm² for square cutouts and reaches 2 cm in field width for narrow rectangular fields. These changes underscore the need for meticulous dosimetric validation.