title: Surface and build-up region dosimetry for obliquely incident intensity modulated radiotherapy 6 MV x rays.
abstract: This study investigates the surface dose and build-up region dosimetry for oblique IMRT beams. The dependence of surface and build-up region doses of 0 degrees (perpendicular incidence) and 75 degrees (oblique incidence) IMRT fields on field size was measured and compared with open field dosimetry. Measurements were performed using a parallel-plate chamber and KODAK EDR2 films in a polystyrene phantom for a 6 cm x 6 cm and a 12 cm x 12 cm, 6 MV photon beam at depths of 0 mm (surface) through dmax. Data were normalized to the dmax value of each field. Four intensity modulated delivery patterns were created and delivered using step-and-shoot IMRT: (1) six static 1 cm x 6 cm strips (IMRTstrip), (2) 12 static 1 cm x 12 cm strips (IMRTstrip), (3) intensity modulated beam patterns created by using the inverse planning optimization software (IMRTopt) for 6 cm x 6 cm, and (4) IMRTopt for 12 cm x 12 cm field sizes. The percent depth doses (PDDs) of 0 degrees, 6 cm x 6 cm IMRTstrip beam at the surface and 5 mm were lower by 8.8% and 1.6%, respectively, compared to the open field. The PDDs of 75 degrees, 6 cm x 6 cm IMRTstrip beam at the surface and 5 mm were lower by 6.7% and 2.4%, respectively, compared to the open field. This study showed that IMRT itself is not contributing to greater skin doses.
authors.affiliation: Loyola University Chicago Medical Center, Department of Radiation Oncology, Maywood, Illinois 60153, USA. ndogan@lumc.edu
authors.forename: Nesrin
authors.initials: N
authors.lastname: Dogan
authors.forename: Glenn P
authors.initials: GP
authors.lastname: Glasgow
mesh_terms: D003198:Computer Simulation; D005373:Film Dosimetry / Q000379:methods*; D006801:Humans; D008954:Models, Biological; D011835:Radiation Protection / Q000379:methods*; D011874:Radiometry / Q000295:instrumentation* / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation / Q000379:methods*; D020266:Radiotherapy, Conformal / Q000295:instrumentation*; D015203:Reproducibility of Results; D018570:Risk Assessment / Q000379:methods*; D012680:Sensitivity and Specificity; D012879:Skin Physiological Phenomena