Dose-to-medium vs. dose-to-water: Dosimetric evaluation of head and neck VMAT cases using Monaco treatment planning system

Abstract

Purpose: In this paper, we evaluate the dosimetric differences between absorbed dose to water and absorbed dose to medium in Monte Carlo (MC)-based calculations used for radiation therapy treatment plans. Methods: Thirty-four treated Head and Neck simultaneously integrated boost cases were analyzed retrospectively. All of them were planned by Monaco treatment planning system (TPS), calculated and reviewed on absorbed dose to medium (D m ) calculations and treated in Elekta Versa HD LINAC. Absorbed dose to medium D m was converted to absorbed dose to water D w in Monaco treatment planning system using the procedure based on stopping power ratios and the Bragg-Gray cavity theory. Dosimetric parameters were then compared and analyzed with respect to absorbed dose to medium (D m ) calculations for multiple planning target volumes (PTVs) and critical organs such as brainstem, spinal cord, left and right lens, left and right parotids, larynx, left and right middle ear and lips. Results: It was found that mean and minimum D w (i.e. D w mean and D w min ) of organs at risk did not differ much (hardly differing by 0.8-2%) with respect to those of the absorbed dose to medium. However maximum D w (i.e. D w max ) in case of lips, left and right middle ear were found to differ more than 4% with respect to D m max . For serial organs brainstem and spinal cord, maximum dose D w max were found to vary around 1% and 2%, respectively, with respect to absorbed dose to medium dose calculation. In case of PTVs, the mean percentages variation of D w min and D w mean were found to be less than 1 %, although the variation of maximum D w was found to be high around 5-7% with respect to that of D m . Conclusion: The comparative analysis of dosimetric parameters in the present study shows that the selection of either D m or D w in Monaco planning system is less likely to produce any significant clinical effect in tumor control and to the damage of organs at risk.