Independent Calculation of Dose from a Helical TomoTherapy Unit

Abstract

Conventional monitor unit (MU) verification calculations are not applicable for TomoTherapy treatments where intensity-modulated doses are delivered in an arc-therapy mode based on elapsed time.  A new calculation algorithm has been developed for independently verifying doses calculated by the TomoTherapy treatment planning system.  The algorithm is designed to confirm the dose to a point in a high dose, low dose-gradient region.  Patient data used by the algorithm include the radiological depth to the point for each projection angle and the treatment sinogram.  The algorithm uses common dosimetric functions [tissue phantom ratio (TPR) and output factor (S_cp)] combined with lateral and longitudinal beam profile data to quantify the off-axis dose dependence.  Machine data for the dosimetric functions were measured on the TomoTherapy machine and simulated using the TomoTherapy treatment planning system.  Point dose calculations were made for several test phantoms and for 97 patient treatment plans.  Comparisons with Tomotherapy-predicted point doses for the phantom treatment plans demonstrated agreement within 2% for both on-axis and off-axis planning target volumes (PTVs).  Similar comparisons with TomoTherapy-predicted point doses for the patient treatment plans also showed good agreement.  For calculations at sites other than lung and superficial PTVs, agreement between the calculations was within 2% for 94% of the patient calculations (64 of 68).  Calculations within lung and superficial PTVs overestimated the dose by an average of 3.1% (s =2.4%) and 3.2% (s=2.2%), respectively.  Systematic errors within lung are probably due to the weakness of the algorithm in correcting for missing tissue and/or tissue density heterogeneities, whereas errors encountered within superficial PTVs probably result from the algorithm overestimating the scatter dose within the patient.  Our results demonstrate that for the majority of cases, the algorithm could be used without further refinement to independently verify patient treatment plans.