Potential therapeutic misadministration due to inappropriate electron beam field shaping

Area of Science:

• Medical Physics
• Radiation Oncology

Background:

• Lead or cerrobend blocking strips are utilized for electron treatment field shaping when custom inserts are unavailable.
• On the Varian 2100C accelerator, applicator structure limits strip placement on the insert frame, favoring unimpeded placement at the applicator's top.

Purpose of the Study:

• To investigate the dosimetric consequences of placing field-shaping strips at the top of the 15x15 applicator for electron beams.
• To evaluate the impact on dose delivery accuracy and treatment efficacy.

Main Methods:

• Dosimetric measurements were performed for 6, 9, and 16 MeV electron beams.
• Field shaping strips were placed at the top of the applicator and compared to placement at the insert level.
• Dose decrease and penumbra changes were quantified.

Main Results:

• Placing field-shaping strips at the top of the applicator resulted in dose decreases of up to 30%.
• A penumbral increase of up to 2 cm was observed compared to insert-level placement.
• These dosimetric deviations can lead to significant underdosing of the target volume.

Conclusions:

• The dosimetric errors associated with top-level placement of lead strips are clinically significant and may constitute therapeutic misadministration.
• Routine use of lead strip blocking should be discouraged in favor of custom inserts to ensure accurate electron field shaping.
• Proper placement of shielding devices is critical for safe and effective radiation therapy delivery.