Program Information

Investigate of Physical Penumbra Definition

K Hadad

h alipour , K Hadad*,

Presentations

SU-I-GPD-J-34 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: In IMRT because of small size of the beamlets, determination of dosimetric parameters such as physical penumbra is important and small changes in the penumbra will have a great impact on output. The physical penumbra width is defined as the lateral distance between two specified isodose curves at a specified depth of phantom.In this study the physical penumbra as the important factor was evaluated and physical penumbra measurement in different condition and in two directions (parallel X and Y axis) was performed. Factor assessed is impact of the location of leaves as well as field size and field shape in physical penumbra in two directions (parallel X and Y axis).Surface between two isodose curve (20% and 80%) in specified depth of phantom (z=5 cm) and X-Y plane was calculated.

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: According to the result physical penumbra width for each condition on right and left sides of isodose curve is not equal.The same phenomena is observed for the physical penumbra width in y direction.

Conclusion: When defining a parameter , the constrains of physical conditions under which that parameter is valid should be clearly defined.The results presented in this study demonstrates the physical penumbra definition is not conclusive. consequently in TPS and medical dosimety the physical penumbra should be defined as a two or three dimensional parameter to avoid miscalculation in dose delivery, So surface penumbra and percent surface penumbra as the conclusive quantity was defined.


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