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Matchline dosimetry in step and shoot IMRT fields: a film study
P Tangboonduangjit1, P Metcalfe, M Butson
1Department of Engineering Physics, University of Wollongong, NSW, Australia.
Physics in Medicine and Biology
|October 9, 2004
Summary
Optimizing multileaf collimator leaf offsets minimizes dose hotspots in intensity-modulated radiation therapy. A 0.07 cm offset effectively reduced matchline effects, ensuring accurate dose delivery for cancer treatment.
Area of Science:
- Medical Physics
- Radiation Oncology
- Radiotherapy Dosimetry
Background:
- The Varian millennium 120 multileaf collimator (MLC) features curved leaf ends, potentially causing asymmetric penumbral dose effects.
- This design can lead to dose 'hot spots' between adjacent beam segments during step-and-shoot intensity-modulated radiation therapy (IMRT).
- Observed matchline discrepancies with film dosimetry necessitate verification of optimal leaf offset to mitigate these effects.
Purpose of the Study:
- To determine the optimal leaf offset for the Varian millennium 120 MLC to minimize dose discrepancies at beam matchlines.
- To evaluate the effectiveness of leaf offsetting in reducing hot spots during step-and-shoot IMRT delivery.
- To compare the performance of radiographic and radiochromic films in measuring dose distributions with varying leaf offsets.
Main Methods:
- An in-house program controlled MLC leaf banks in 2 cm steps with a 2 cm gap, varying the offset from 0.0 to 0.1 cm.
- Radiographic (Kodak EDR, XV) and radiochromic (Gafchromic MD-55-2) films were used to measure optical density maps in a solid water phantom.
- Point doses were measured using an ion chamber (IC4) for normalization at the beam's minimum umbral dose position.
Main Results:
- Without leaf offset, relative mean peak-to-valley dose ratios were approximately 1.30-1.31 for all film types.
- A leaf offset of 0.07 cm significantly reduced the central matchline dose to approximately 1.0 for all dosimeters.
- Mini-peaks adjacent to the matchline were measured at 1.05-1.08, indicating effective reduction of hot spots.
Conclusions:
- An optimal leaf offset of 0.07 cm effectively minimizes dose discrepancies at beam matchlines for the Varian millennium 120 MLC.
- This offset strategy significantly reduces hot spots, improving the accuracy of dose delivery in step-and-shoot IMRT.
- Radiographic and radiochromic films provide comparable and reliable measurements, agreeing within +/-2% with ion chamber readings.