First dosimetric evaluation of deformable dose accumulation in human individuals

The aim of this study is to dosimetrically validate deformable dose accumulation in online adaptive radiotherapy using realistic physiological tissue deformation and comparing point dose measurements to the treatment planning system-calculated and accumulated point dose.

For the data analysis, four real human body donors were used after fixation to preserve physiological tissue elasticity. Online adaptive radiotherapy was performed using the Varian Ethos system for variable bladder and bowel filling as created using catheters. The dose to the bladder, urethra, and rectum was measured using thermoluminescent dosimeters. The measured dose at each point was compared with the sum of the single-fraction adaptive doses and with the accumulated dose.

On average, the summation of point doses from the individual fractions (adapted plan as calculated by the Ethos software) is consistent with the accumulated dose, demonstrating internal consistency of the Ethos system regarding deformable dose summation. However, from the 46 point doses analyzed, 22 show deviations larger than ± 10% and 16 more than ± 20%. Comparing the measurements with the treatment planning system summation dose, the Ethos appears to somewhat underestimate the dose.

To our knowledge, these measurements are the first presentation of point dose measurements in soft-fixed human bodies under physiological organ deformation. The experimental set-up is challenging, but feasible, providing an end-to-end validation of deformable dose accumulation. While there is general consistency within the Ethos system between the single-fraction adaptive dose distributions and the resulting accumulated dose distribution, specific locations appear to be matched incorrectly and can result in considerable internal inconsistencies.