Patient-specific compensation for Co-60 TBI treatments based on Monte Carlo design: A feasibility study

To develop an AP-PA treatment technique for the delivery of total body irradiation (TBI) at extended SSD using a modified Co-60 unit equipped with flattening filter and patient-specific compensators supported by Monte Carlo (MC) simulations and measurements.

An existing Eldorado-78 Co-60 teletherapy unit was stripped of its original collimator and equipped with two beam-defining cerrobend blocks. An acrylic flattening filter was numerically designed based on detailed mapping of the dose distribution of the large open field at a 10 cm depth in water using a primary radiation attenuation calculation. An EGSnrc/BEAMnrc MC model of the resulting unit was developed and experimentally validated and was used to calculate MC dose distributions in whole-body supine and prone CT images of a patient. AP-PA patient-specific compensators were designed based on the supine and prone mid-plane dose distributions.

The designed flattening filter flattens the beam to within ±2% over a 200 cm × 70 cm area at 10 cm depth in water. Experimental validation of the calculated dose profiles in the open and flattened beams shows agreement of better than 2% and 1%, respectively. Patient MC dose calculations in the flattened, uncompensated beam showed dose deviations from prescription dose most notably in lung, neck and extremities ranging from -5% to +25%. The use of patient-specific compensators reduced inhomogeneities to within -5% to +10%.

This work demonstrates that a Co-60 TBI setup upgraded with patient-specific compensators, numerically designed using MC patient dose calculations, is feasible and considerably improves the dose homogeneity.