SU-E-T-176: Improved Collimator Scattering Factor (Sc) Measurements for Small Fields Using Build-Up Caps in Robotic...

Area of Science:

• Medical Physics
• Radiotherapy
• Radiation Dosimetry

Background:

• CyberKnife robotic radiosurgery utilizes Monte Carlo calculations for treatment planning.
• In-air measurements are essential for the Monte Carlo algorithm's accuracy.
• The impact of build-up caps on these measurements, particularly for small fields, requires investigation.

Purpose of the Study:

• To compare the effect of different build-up cap thicknesses on in-air measurements using a diode detector.
• To evaluate the impact of build-up caps on Sc factors for small fields in CyberKnife radiosurgery.

Main Methods:

• Two custom-made acrylic build-up caps (1.5 cm and 5 cm) were used with an SFD diode detector.
• Measurements were performed in air at 80 cm SAD using a Blue Phantom.
• Comparisons were made across 12 fixed cones ranging from 5 mm to 60 mm.

Main Results:

• Significant differences (up to 20%) in Sc factors were observed for small fields (5-7.5 mm) with build-up caps compared to free-in-air measurements.
• Agreement within 2% was not achieved until cone sizes exceeded 40 mm.
• Observed differences were attributed to electron contamination, with both caps yielding similar Sc factors.

Conclusions:

• Build-up caps significantly affect Sc factors in small fields, a factor not explicitly addressed in CyberKnife planning manuals.
• The choice of build-up caps is critical for accurate commissioning data, especially for fields smaller than 10 mm.
• Further research into build-up cap selection for small field dosimetry is warranted.