(a) Photon energy spectra at 80, 100, and at a total filtration of Al as generated from spectrum processor of IPEM Report 78. (b) Percent depth dose in water with (absorbed dose) and without (kerma) secondary electron transport for the spectrum. Kerma depth dose (solid line) was generated using histories, whereas histories were required for the absorbed dose (dashed line) values.
Graphical representations of 8 possible radiotherapy beam entry directions for treatment of ARMD in (A) frontal and (B) perspective views. Beam 0° is directed caudally (downward) within the medial plane of the treated eye, whereas beam 180° is directed cranially (upward). A cylindrical cuboidal optic nerve model is situated right behind the puckered-shaped macula target. Its vertical tilt angle is allowed to vary from to , from the horizontal visual axis.
Experimental setup of air kerma measurements along with a sagittal cross section of the MCNPX model of the ion chamber (Type 34013, PTW, Freiburg, Germany. A total of four materials were assigned to the ion chamber model: polyethylene entry foil, polyethylene wall, effective air volume, and surrounding air.
Mean absorbed dose (Gy) to the macula, lens, and optic nerve as function of treatment beam azimuthal entry direction (0°–360°) and assumed vertical tilt angle of the patient’s optic nerve (from to ) in the cranial (+) to caudal (−) direction.
Dose volume histograms for the macula target, the lens, and optic nerve when (A) optic nerve vertical tilt angle was and (B) optic nerve vertical tilt angle was . In both cases, the azimuthal beam entry direction was set at 225° as per Fig. 3.
Dose volume histograms (dose per unit air kerma) for the macula target, the lens, and optic nerve for three different x-ray energy spectra: 80, 100, and . The maximum dose per unit air kerma is provided in the legend of each plot for all targets. Additional assumptions are an optic nerve vertical tilt angle of 0° and a beam entrance azimuthal angle of 180°.
Cross-sectional profile of the absorbed dose to the macula target for a x-ray beam as a function of focal spot size. Vertical lines are placed at and , thus, outlining the geometric dimensions of the diameter macula target. To simply the MCNP geometrical setup, a nonclinical normally incident beam angle is assumed (polar angle set at 0°).
(A) Dimensions and locations of tissue structures within the human eye (sagittal cross section) as provided in NCRP Report No. 130 (all dimensions are in mm). (B) Final MCNPX geometrical eye model (patient’s right eye in transverse cross section). The macula target is defined as a puckered disk within the retina at a diameter of .
Material and density assignments to substructures within the 3D MCNPX eye model with corresponding literature sources.
Comparison of percent depth dose from surface (%) between ion chamber measurements (mean and experimental error) and Monte Carlo simulations (mean and statistical error) for verification the MCNPX-based simulation model. A total photon histories were considered in the simulation.
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