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.