MLC dosimetric characteristics for small field and IMRT applications

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Technology

Background:

• Accurate beam shaping is crucial for intensity modulated radiation therapy (IMRT).
• Multileaf collimators (MLCs) are key devices for IMRT beam modulation.
• Performance characteristics of MLCs, such as penumbra and leakage, require thorough evaluation for clinical application.

Purpose of the Study:

• To measure the performance characteristics of a double-focus multileaf collimator (MLC).
• To specifically evaluate variations in penumbra and leakage for narrow fields across a defined IMRT treatment space.
• To assess the suitability of the MLC for segmented IMRT applications.

Main Methods:

• Measurements were conducted using 6 MV x-rays and EDR2 film at a depth of 10 cm in solid water.
• Interleaf, intraleaf, and leaf-end transmission were measured at various positions.
• Penumbra was assessed in-plane and cross-plane for narrow (1 cm) fields across a 20 cm range.
• Films were digitized with 0.17 mm resolution and converted to dose data.

Main Results:

• Interleaf leakage ranged from 1.0%-1.5%, intraleaf transmission from 0.6%-0.8%, and leaf-end transmission from 0.8%-2.7%.
• These transmission values were consistent across leaf pairs at given abutment positions.
• The measured penumbra for 1-cm-wide fields was consistently 0.36 cm ± 0.03 cm for 99% of measurements.

Conclusions:

• The double-focus MLC exhibits remarkable consistency in penumbra and leakage across studied leaf positions.
• These dosimetric characteristics are well-suited for IMRT segments, particularly where opposing leaf pairs are closely spaced.
• The findings support the clinical utility of this MLC for precise radiation delivery in IMRT.