A hybrid phantom Monte Carlo-based method for historical reconstruction of organ doses in patients treated with cobalt-60 for Hodgkin's lymphoma

Historical radiotherapy treatment plans lack 3D images sets required for estimating mean organ doses to patients. Alternatively, Monte Carlo-based models of radiotherapy devices coupled with whole-body computational phantoms can permit estimates of historical in-field and out-of-field organ doses as needed for studies associating radiation exposure and late tissue toxicities. In recreating historical patient treatments with Co-60 based systems, the major components to be modeled include the source capsule, surrounding shielding layers, collimators (both fixed and adjustable), and trimmers as needed to vary field size. In this study, a computational model and experimental validation of the Theratron T-1000 are presented. Model validation is based upon in-field commissioning data collected at the University of Florida, published out-offield data from the British Journal of Radiology (BJR) Supplement 25, and out-of-field measurements performed at the University of Wisconsin's Accredited Dosimetry Calibration Laboratory (UWADCL). The computational model of the Theratron T-1000 agrees with central axis percentage depth dose data to within 2% for 6 x 6 to 30 x 30 cm(2) fields. Out-of-field doses were found to vary between 0.6% to 2.4% of central axis dose at 10 cm from field edge and 0.42% to 0.97% of central axis dose at 20 cm from the field edge, all at 5 cm depth. Absolute and relative differences between computed and measured out-of-field doses varied between +/- 2.5% and +/- 100%, respectively, at distances up to 60 cm from the central axis. The source-term model was subsequently combined with patient-morphometry matched computational hybrid phantoms as a method for estimating in-field and out-of-field organ doses for patients treated for Hodgkin's Lymphoma. By changing field size and position, and adding patient-specific field shaping blocks, more complex historical treatment set-ups can be to recreated, particularly those for which 2D or 3D image sets are unavailable.