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Tools and Methods for On-Line Adaptive Radiation Therapy


J Chang

J Chang1*, R Heaton1,2, Y Cho1,2, D Jaffray1,2, M Islam1,2, (1) Princess Margaret Hospital, Toronto, ON, (2) University of Toronto, Toronto, ON

TH-C-BRB-6 Thursday 10:30:00 AM - 12:30:00 PM Room: Ballroom B

Purpose:
Essential tools required for daily field modifications for compensating small target variations have been investigated. A deformable registration method was used to generate deformation vector field (DVF), which is subsequently used to modify treatment fields. An on-line beam monitor was evaluated for validating the adapted fields.

Methods:
An adaptive treatment process consists of the following steps: (1) Determine the DVF using the irradiated volumes (IV) of the reference and daily image sets. The IV is constructed by back-projections of respective complete irradiation area outlines of all beams. (2) The deformed contours (CD) were calculated using DVF and the reference contours. (3) Determine MLC leaf adjustments using CD and DVF to form the adapted fields. This approach was investigated using two CT image sets, one acquired during the treatment course, and corresponding IMRT plan data for patients undergoing prostate treatment. A planning study was performed to compare the dose distributions between optimized IMRT and the adapted plans. The adapted field delivery was validated by an online beam monitoring system, termed IQM; which consists of a spatially sensitive area ion chamber mounted below the MLC to provide a fluence-area-product signal and a calculation module to predict the signal.

Results:
Approximately 10 minutes were required to generate the DVF between the reference IV and a daily IV using readily available computer hardware. The dose distribution using adapted fields compare well with the optimized plans in terms of DVH and conformity indices. The average agreement between the IQM predicted signals of the adapted fields with those of measured were within 2%.

Conclusions:
The method of generating the DVF was promising, in terms of speed and effectiveness for small variations of target. The IQM system was able to validate the delivery of appropriate adapted fields, fulfilling the requirement for pre-treatment quality control.

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