title: Effectiveness of compensating filters in the presence of tissue inhomogeneities.
abstract: CT based 3D treatment planning systems (3DTPS) can be used to design compensating filters that, in addition to missing tissue compensation, can account for tissue inhomogeneities. The use of computer-driven systems provides a practical, convenient, and accurate method of fabricating compensating filters. In this work, we have evaluated a commercially available PAR Scientific DIGIMILL milling machine linked with FOCUS 3DTPS. Compensating filters were fabricated using refined gypsum material with no additives. Thus, filters were of manageable dimensions and were not sensitive to common machining errors. Compensating filters were evaluated using a homogeneous step phantom and step phantoms containing various internal inhomogeneities (air, cork, and bone). The accuracy of two planning algorithms used to design filters was experimentally evaluated. The superposition algorithm was found to produce better agreement with measurements than the Clarkson algorithm. Phantom measurements have demonstrated that compensating filters were able to produce a uniform dose distribution along the compensation plane in the presence of tissue inhomogeneity. However, the dose variation was greatly amplified in planes located beyond the inhomogeneity when a single compensated beam was used. The use of parallel-opposed compensated beams eliminated this problem. Both lateral and depth-dose uniformity was achieved throughout the target volume.
authors.affiliation: Department of Radiation Oncology, Loyola University Medical Center, Maywood, Illinois 60153, USA. asethi@lumc.edu
authors.forename: A
authors.initials: A
authors.lastname: Sethi
authors.forename: L
authors.initials: L
authors.lastname: Leybovich
authors.forename: N
authors.initials: N
authors.lastname: Dogan
authors.forename: G
authors.initials: G
authors.lastname: Glasgow
mesh_terms: D001703:Biophysics / Q000295:instrumentation / Q000379:methods; D001842:Bone and Bones / Q000528:radiation effects; D002133:Calcium Sulfate / Q000737:chemistry / Q000528:radiation effects / Q000627:therapeutic use; D019047:Phantoms, Imaging; D011137:Polystyrenes / Q000528:radiation effects; D011829:Radiation Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation* / Q000379:methods*; D011882:Radiotherapy, High-Energy / Q000295:instrumentation / Q000379:methods; D014934:Wood
chemical_list: D011137:Polystyrenes; C024075:styrofoam; D002133:Calcium Sulfate