SU-E-T-721: Systematic Dose Discrepancy of Diode Based In-Vivo Dosimetry Due to Short Source-To-Surface Distance and Oblique Beam Angle Incidence




The diode detectors for photon beam In-Vivo dose (IVD) measurement are usually manufactured with intrinsic buildup, which may cause the IVD respond differently from the predicted dose when the source-to-surface distance (SSD) and/or the beam incident angle changes, such as that in breast or chest wall irradiations. This work is to evaluate this systematic discrepancy.


In-Vivo dose was measured on a solid water phantom as a function of varying SSD and beam oblique incidence. The cylindrical IVD detectors were calibrated to 100cGy at nominal gantry angle, 100MU, 10×10cm field, 100 SAD with isocenter at dmax. To study SSD dependence, beams were delivered from nominal gantry angle at SSDs from 105 to 75cm. Doses calculated at the dmax were used as the predicted values. To study the oblique incidence responses, measurements were performed at fixed 100 SSD with gantry angles ranging from the nominal angle (0 degree) to horizontal (90 degree). Two calculation points were used for comparison, one at dmax from nominal angle, a second one at dmax along the oblique beam ray. For both setup, IVD was positioned at the beam CAX on the phantom surface.


Dose discrepancy increases with decreasing SSD and is more pronounced with high photon energy. This discrepancy is as high as 4.6% for an 18MV beam at 75SSD. For oblique measurements, both calculated values decrease as the oblique angle increases; however, the measured IVD doses exhibited little changes, which introduce significant systematic errors (∼20% at 80 degree) for incident angles larger than 60 degrees.


Significant systematic dose discrepancies are observed between IVD measurements and the predicted values for short SSD and oblique incident angles. Cautions must be taken in evaluating IVD results at short SSD and at oblique incidence angles larger than 60 degrees.