  Treatment planning system calculations in inhomogeneous regions may present
significant inaccuracies due to loss of electronic equilibrium. In this study,
three different dose calculation algorithms, pencil beam, collapsed cone, and
Monte-Carlo, provided by our planning system were compared to assess their
impact on the three-dimensional planning of lung and breast cases. A total of
five breast and five lung cases were calculated using the PB, CC, and MC
algorithms. Planning treatment volume and organs at risk delineation was
performed according to our institutions protocols on the Oncentra MasterPlan
image registration module, on 0.3 to 0.5 cm computed tomography slices taken
under normal respiration conditions. Four intensity-modulated radiation therapy
plans were calculated according to each algorithm for each patient. The plans
were conducted on the Oncentra MasterPlan and CMS Monaco treatment planning
systems, for 6 MV. The plans were compared in terms of the dose distribution in
target, OAR volumes, and monitor units. Furthermore, absolute dosimetry was
measured using a three-dimensional diode array detector to evaluate the dose
differences in a homogeneous phantom. Comparing the PB, CC, and MC algorithms
planned dose distribution, the PB algorithm provided adequate coverage of the
PTV. The MUs calculated using the PB algorithm was less than those of the other
algorithms. The MC algorithm showed the highest accuracy in terms of the
absolute dosimetry. Differences were found when comparing the calculation
algorithms. The PB algorithm estimated higher doses for the target than the CC
and MC algorithms. The PB algorithm actually overestimated the dose compared
with those calculated by the CC and MC algorithms. The MC algorithm showed
better accuracy than the other algorithms.
