SU‐FF‐T‐178: Dosimetric Evaluation of Penumbra for Conformal and Intensity Modulated Radiotherapy

• Published in: Medical physics (Lancaster) 2007-06, Vol.34 (6), p.2442-2442
• Main Authors: Subramani, V, Rath, GK, Chander, S, Ganesh, T, Joshi, RC, Gopishankar, N
• Format: Article
• Language: English
• Subjects: Collimation; Dosimetry; Electromagnetic radiation detectors; Field size; Intensity modulated radiation therapy; Ion beam detectors; Linear accelerators; Materials analysis; Multileaf collimators; Particle beam detectors
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1118/1.2760837

Purpose: To evaluate the penumbral characteristics as function of collimation systems, conventional detector, depth, energy and MLC travel orientations using diode detectors. Methods and Materials: The experiment was carried out in a Clinac 2300C/D linear accelerator for 6 and 15 MV. For penumbral width determination, cross‐beam profiles were measured using Blue phantom radiation field analyzer, 0.13cm3 ionchamber and diode detectors, CU‐500E dual‐channel electrometer and OmniPro beam‐data acquisition system. The penumbra was measured for both Jaw and MLC‐defined field sizes of 4×4 to 30×30cm2 depths at dmax, 5, 10, 15 and 20cm and also for cross‐plane and in‐plane MLC travel orientations at 100cm SSD. The penumbra width was calculated as the distance between 80%–20% intensity in the profiles. Results: The MLC penumbra was in the range of 3.6mm to 4mm for 6MV and 5.4mm to 7.1mm for 15MV at dmax depth for field sizes of 4×4 to 30×30cm2 as measured by diode detector. The mean differences between Jaw and MLC‐defined field's penumbra were 0.8mm ± 0.2, and 1mm ± 0.2 for 6MV and 1.2mm ±0.2 and 1.8mm ±0.5 for 15MV at dmax and 10cm depths. The maximum difference of 1.9mm ±0.3 for 6MV and 2.2mm ±0.2 for 15MV between diode and 0.13cm3 detectors were found. Use of diode detectors, maximum differences of 3.4mm ±3 at 20cm depths between energies and the differences of 3.3mm ± 2.2 for 6 MV and 1.2mm ± 0.3 for 15MV between cross‐plane and in‐plane MLC penumbra were found. Conclusions: The rounded‐end MLC penumbra was larger as compared to Jaw penumbra. The penumbra measured by diode was increasing with energy upto a certain depths and field sizes and then it increases more with low‐energy at larger depths and field sizes as compared to high‐energy. The adequate accounting of penumbra is important in intensity modulated beam dose calculation.