Dose Response of a 2D Diode Array for Non-Primary Photon Radiation From a Dynamic MLC
A Chu1*, W Feng2, Z Chen3, R Nath4, (1) Yale New Haven Hospital, New Haven, CT, (2) New York Presbyterian Hospital, TENAFLY, NJ, (3) Yale New Haven Hospital, New Haven, CT, (4) Yale Univ School of Medicine, New Haven, CTSU-E-T-356 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall
Purpose: To quantify the dose responses of a 2D-diode array for 'non-primary' radiations (scatter and transmission from MLC and gantry-head) under dynamic MLC dose deliveries. The 'non-primary doses' have been overlooked by the 2D-diode dose-calibration because the primary radiation dominates over a large calibration field (10x10cm²). If non-primary dose responses of diode are significant, it would cause systematic errors when non-primary radiations become important (in small field).
Methods: The dose response is the diode-dose normalized by chamber-dose. Three setups under a 6MeV photon beam (Trilogy) were measured by detector (diode and chamber) at central-axis (CAX): (1) the CAX diode was accurately located at places: either within MLC-leaf collimation (2.5 mm) or between leaves. This measurement field is entirely blocked by MLC. (2) Dynamic MLC exposures are tested by 6 different MLC-gaps (1, 5, 10, 20, 50 and 100 mm) sweeping over the CAX detector. (3) To estimate the scatter dose but without MLC involved, the CAX detector is blocked from primary beam by jaws only but received scatter doses from narrow-slit apertures with distances from CAX.
Results: (1) Results showed the diodes tends to underestimate dose of lower energy radiation, e.g. scatters. Experiment (3) confirmed this by showing its decreased dose response by scatter source away from CAX; i.e. more lower-energy scatters in larger lateral scattered angles. As the slit is narrowed, less scatters and higher diode response were observed. (2) 3% difference in diode response to MLC-leakage vs. non-leakage was detected. However the impact of MLC leakage is not substantial in dynamic MLC delivery unless MLC-gap is narrow enough (e.g. 1 mm). (3) Overall non-primary radiations still significantly present as gap<10mm by 3% criterion.
Conclusions: The diode dose responses to non-primary radiations can cause more than 3% systematic errors for small-aperture dynamic MLC QA unless appropriate corrections are made.