Improvements in dose calculation accuracy for small off-axis targets in high dose per fraction tomotherapy

Authors

  • Hardcastle Nicholas,

    1. Department of Human Oncology, University of Wisconsin-Madison, WI, 53792; Department of Physical Sciences, Peter MacCallum Cancer Centre, Melbourne, VIC 3002, Australia; and Centre for Medical Radiation Physics, University of Wollongong, Wollongong, NSW 2522, Australia
    Search for more papers by this author
  • Bayliss Adam,

    1. Department of Human Oncology, University of Wisconsin-Madison, WI 53792
    Search for more papers by this author
  • Wong Jeannie Hsiu Ding,

    1. Centre for Medical Radiation Physics, University of Wollongong, Wollongong, NSW 2522, Australia and Department of Biomedical Imaging, Faculty of Medicine, University of Malaya, 50603 Kuala Lumpur, Malaysia
    Search for more papers by this author
  • Rosenfeld Anatoly B.,

    1. Centre for Medical Radiation Physics, University of Wollongong, Wollongong, NSW 2522, Australia
    Search for more papers by this author
  • Tomé Wolfgang A.

    1. Department of Medical Physics, University of Wisconsin-Madison, Madison, Wisconsin 53792; Department of Biomedical Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53792; Einstein Institute of Oncophysics, Albert Einstein College of Medicine of Yeshiva University, Bronx, New York 10461; and Centre for Medical Radiation Physics, University of Wollongong, Wollongong, NSW 2522, Australia
    Search for more papers by this author

Abstract

Purpose:

A recent field safety notice from TomoTherapy detailed the underdosing of small, off-axis targets when receiving high doses per fraction. This is due to angular undersampling in the dose calculation gantry angles. This study evaluates a correction method to reduce the underdosing, to be implemented in the current version (v4.1) of the TomoTherapy treatment planning software.

Methods:

The correction method, termed “Super Sampling” involved the tripling of the number of gantry angles from which the dose is calculated during optimization and dose calculation. Radiochromic film was used to measure the dose to small targets at various off-axis distances receiving a minimum of 21 Gy in one fraction. Measurements were also performed for single small targets at the center of the Lucy phantom, using radiochromic film and the dose magnifying glass (DMG).

Results:

Without super sampling, the peak dose deficit increased from 0% to 18% for a 10 mm target and 0% to 30% for a 5 mm target as off-axis target distances increased from 0 to 16.5 cm. When super sampling was turned on, the dose deficit trend was removed and all peak doses were within 5% of the planned dose. For measurements in the Lucy phantom at 9.7 cm off-axis, the positional and dose magnitude accuracy using super sampling was verified using radiochromic film and the DMG.

Conclusions:

A correction method implemented in the TomoTherapy treatment planning system which triples the angular sampling of the gantry angles used during optimization and dose calculation removes the underdosing for targets as small as 5 mm diameter, up to 16.5 cm off-axis receiving up to 21 Gy.

Ancillary