Optimization of electron arc therapy doses by multi-vane collimator control.
Retrospective computer simulations, based on clinical treatment planning data available from over 50 patients treated by electron arc radiotherapy to the chestwall following mastectomy, show that a dramatic improvement in dose uniformity can, in many clinical situations, be achieved by dynamic shaping of the electron arc collimator, under computer control, as a function of gantry angle and distance superior or inferior to the central plane. The greatest improvement in dose uniformity is seen in calculational planes in which the patient contour has the greatest departure from a circular shape. Dosimetric studies demonstrate this improvement. Indicators for use of variable-width multi-vane electron arc collimators include the following: (1) Mechanical constraints of the therapy equipment may limit the placement of isocenter to an inadequate depth which causes large variation in the SSD around the arc; (2) Out of the central plane, the shape of the chest wall may change dramatically across the limits of the arc, creating large variations in the dose distribution; (3) Clinical definition of the treatment surface to include surgical scars or other at-risk volume may create an irregularly shaped treatment surface, thereby changing the fraction of the arc included in the treatment surface from one plane to the next. Electron arc collimator shape determines both the dose rate and the electron arc beam profile. Both the dose rate and the beam profile must be included in the integration of dose to a point within the arc. The dose to a point within the arc can be modified by as much as a factor of 1.5 to 2.0 by increasing the collimator width from 3 cm to 7 cm. A multi-vane collimator allows these changes to be made in each specific plane to compensate for changes in patient contour.
Leavitt
D D
DD
Department of Radiology, University of Utah Medical Center, Salt Lake City 84132.
Stewart
J R
JR
Moeller
J H
JH
Earley
L
L
D001943:Breast Neoplasms / Q000532:radiotherapy; D003198:Computer Simulation; D004583:Electrons; D005260:Female; D006801:Humans; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D011882:Radiotherapy, High-Energy / Q000295:instrumentation*