AbstractID: 6875 Title: Improvement of Missing Tissue Compensation in IMRT
Variations in patient external contour result in an inhomogenous target dose distribution.
In 3D treatment planning, a variety of methods have been used to reduce the dose
inhomogeneity: wedge modifiers, missing tissue compensators, etc. In the case of
intensity modulated radiation therapy (IMRT), the effect of missing tissue is compensated
by appropriate modulation of beam intensity. In this work we have studied tomographic
IMRT dose distributions produced by CORVUS treatment planning system (TPS) when a
steep gradient in patient contour is present. In studied cases, the missing tissue thickness
ranged up to 6 cm (as is encountered in head and neck radiation). It was found that
treatment plans produced by CORVUS demonstrate necessary compensation for missing
tissue. However, the measured dose distributions differed from planned doses by up to
17%. A good agreement (within 2%) between measured and planned dose distributions
was obtained when the junction between beamlet banks corresponded to the beginning of
the external slope. In all other cases, the measured dose significantly differed from the
planned dose. This may have important consequences in tumor control and/or critical
structure sparing. We present a method whereby the dose inhomogeneity can be reduced
by appropriately modified treatment arc positions. This can be achieved by the
introduction of a pseudo-target located outside the patient. Prescription dose to the
pseudo target is a small fraction of the target dose. The magnitude of shift in arc
positions is determined by the dimension of the pseudo target in the direction of couch
indexing.