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is a typical IMRT plan verification. The analyzed film in this example
is a central axis section of a phantom, irradiated to a dose of 100cGy
to isocenter. This film is compared to its associated dose matrix slice
on the same phantom plan, planned for the same dose. The dose matrix
was imported in DICOM RT-Dose format from the treatment planning system. Total multi-slice comparison
analysis can be performed – in parallel to –multi-slice 3D planning
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The original case is
a plan for the irradiation of the Cavernous Sinus. The dose distribution
shape represented on the actual anatomy is very similar to that of the
phantom’s Adequately sized phantoms are
recommended to avoid over-distorting the actual dose distribution
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MLC strips of 1 cm width are irradiated in sequence from
left to right, with MLC banks traveling left to right.
An exposure is made every other 1 cm starting from –9.5cm
to +11.5cm. Every row of “little blue squares” should represent
exposures of a leaf pair with an interleaf width of 1cm at various locations
starting from –9.5cm to +11.5cm. The small analysis line segment on the film – towards the
mid-upper left – produces the profile to the right, which has precisely 10mm
width at the 50% FWHM |
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The same analysis of a different interleaf pair separation
– the yellow analysis line towards the upper right of the film – shows an
inter leaf width of 7.42mm at FWHM. Indicating the necessity to undergo MLC
re-calibration. The dosimetric error between a correct width (top) and an
incorrect one (bottom) is excessive in this case and can drastically affect
dosimetric results of IMRT plans which employ small interleaf spacing. The same analysis can be repeated for every leaf pair at
positions spanning the area of interest. Calibration of leaf pair positioning
can be made based on the 50% FWHM and peak to axis measurements. |
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