(Sunday, 7/29/2018) 3:00 PM - 6:00 PM
Room: Exhibit Hall
Purpose: Because of small size of the beamlets in IMRT, determination of physical penumbra is important and small changes in the penumbra will have a great impact on output. Physical penumbra definition is a one dimensional quantity indicative of the lateral distance between two specified isodose curves at a specified depth of the phantom. In this study the physical penumbra was evaluated and physical penumbra measurement in two directions ( X and Y axes) was performed. Factor assessed is impact of the location of leaves and field shape in two directions. Surface between two isodose curve (20% and 80%) in z=5 cm and X-Y plane was calculated. Two new dosimetric parameters proposed and calculated, those are intended to be suitable for MLC fields
Methods: Calculations were performed by using IAEA phase space data for Varian IX just above MLC and BEAMnrc/DOSXYZnrc for source to surface distance (SSD)=100 cm and in a homogenous water phantom.
Results: Physical penumbra width for each condition on right and left sides of isodose curve is not equal . The same phenomena are observed for the physical penumbra width in y direction .Physical penumbra width is depend on location of measurement.
Conclusion: The intended purpose from physical penumbra in its conventional definition is not satisfied for non-uniform MLC setups. For such cases, the newly defined surface penumbra and uniformity index are demonstrated to be more suitable and indicative of the field non-uniformity. CTV to PTV margin recipe is based on random and systematic errors, as well as the beam penumbra while defining the physical penumbra is incomplete and one dimensional parameter; the surface penumbra is proposed as a substitute for the physical penumbra to determine the PTV and CTV. It is also shown that for an idle treatment planning system, the uniformity index approaches unity.
Keywords
Penumbra, Intensity Modulation
Taxonomy
TH- External beam- photons: General (most aspects)
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