Related Experiment Videos
Ionization chamber volume averaging effects in dynamic intensity modulated radiation therapy beams
Daniel A Low1, Parag Parikh, James F Dempsey
1Department of Radiation Oncology, Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, Missouri 63110, USA. low@castor.wustl.edu
Medical Physics
|August 9, 2003
Summary
Commercial ionization chambers accurately integrate dose in homogeneous fields but show errors in partial scans. Larger chambers under-respond in small fields, highlighting the need for careful application in dynamic radiotherapy.
Area of Science:
- Medical Physics
- Radiotherapy Dosimetry
Background:
- Accurate dose measurement is crucial for radiotherapy.
- Dynamic treatment fields present unique challenges for ionization chamber dosimetry.
Purpose of the Study:
- To evaluate the ionization integration accuracy of commercial cylindrical ionization chambers under dynamically moving fields.
- To assess the performance of different ionization chambers in narrow and scanned radiation beams.
Main Methods:
- Ionization chambers were scanned beneath fixed linear accelerator portals using a computer-controlled table.
- Measurements were performed with narrow fields (0.6-4.0 cm) at 6 MV and 18 MV in a water-equivalent phantom.
- Charge integration was performed using computer-controlled electrometers, with dose rate variations accounted for by a reference chamber.
Main Results:
- All tested ionization chambers accurately integrated ionization in homogeneous dose regions across scanned portals.
- Larger chambers (Farmer, waterproof scanning) showed under-response at the centers of small fields.
- Partial integration resulted in significant errors (>1% and >2% for 6 MV and 18 MV, respectively), with the Farmer chamber exceeding 10% error for a 4 cm field.
Conclusions:
- Ionization chambers provide accurate integrated charges in homogeneous dose regions.
- Errors arise during partial integration, especially with larger chambers and narrow fields.
- Careful consideration of chamber size and integration limits is necessary for accurate dosimetry in dynamic radiotherapy applications.