Small-field electron dosimetry for the Philips SL25 linear accelerator

Area of Science:

• Medical Physics
• Radiation Oncology
• Accelerator Physics

Background:

• Accurate characterization of electron beams is essential for effective radiation therapy.
• Philips SL25 linear accelerators are widely used in clinical settings.
• Understanding beam output variations with field size and shaping devices is critical for dose delivery.

Purpose of the Study:

• To systematically investigate the electron-beam characteristics of a Philips SL25 linear accelerator.
• To determine the energy and field-size dependence of output factors for standard cones and cerrobend cutouts.
• To evaluate methods for predicting output factors in small and rectangular fields.

Main Methods:

• Measured central-axis percentage depth doses, cross-beam profiles, and beam output factors for 6, 10, and 20 MeV beams.
• Analyzed output factors across a range of field sizes (down to 2x2 cm) using standard cones and cerrobend cutouts.
• Applied least squares fitting with a second-order polynomial for small field output factors.
• Compared output factor prediction methods (one-dimensional, square-root, equivalent-square) for rectangular fields.

Main Results:

• Output factors exhibit complex energy dependence, varying with cone size.
• Output factors decrease with increasing cone size for 10 and 20 MeV beams, but increase for 6 MeV.
• Output factors fall below unity for fields <6 cm at 6 and 10 MeV, remaining stable at 20 MeV.
• The one-dimensional formula accurately predicts output factors for elongated fields (+/- 1%).
• Variations in scattered electron contamination from cones affect dose response.

Conclusions:

• The study provides comprehensive data on Philips SL25 electron beam characteristics.
• Output factor behavior is energy and field geometry dependent, requiring careful consideration in treatment planning.
• The one-dimensional formula is recommended for accurate output factor prediction in small, elongated fields.