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[The estimation of the absorbed dose with wedge fields]
1Servizio di Fisica Sanitaria, Policlinico S. Orsola-Malpighi, Azienda Ospedaliera di Bologna.
La Radiologia Medica
|February 18, 1998
Summary
Treatment planning systems often approximate wedged and open beam output factors, leading to systematic errors in absorbed dose calculations. Measuring wedge factors reveals field size dependencies crucial for accurate megavoltage X-ray dosimetry.
Area of Science:
- Medical Physics
- Radiation Oncology
- Dosimetry
Context:
- Megavoltage X-ray therapy relies on accurate dose calculations.
- Treatment planning systems (TPS) utilize algorithms for dose determination.
- Wedged beams introduce complexities in radiation field characteristics.
Purpose:
- To investigate the systematic errors in absorbed dose calculations due to approximations in TPS algorithms.
- To evaluate the field size dependence of wedge factors (WF) for 25-MV X-rays.
- To quantify the impact of neglecting WF field size dependence on dose accuracy.
Summary:
- Current TPS algorithms often fail to differentiate between wedged (w) and open (o) output factors (OF) and do not account for field size-dependent wedge factors (WF).
- This approximation neglects changes in back-scattered and phantom-scattered radiation when a wedge is inserted, impacting dose calculations.
- Measurements of WF(s) for 25-MV X-rays showed variations from -0.7% to +2.6% across different field sizes, highlighting a significant source of systematic uncertainty.
Impact:
- Accurate absorbed dose calculation is critical for effective radiation therapy and patient outcomes.
- Understanding and correcting for WF field size dependence can improve the precision of TPS.
- This study emphasizes the need for TPS to incorporate field size-dependent wedge factors for enhanced dosimetric accuracy in clinical practice.