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<p>Introduction: Stereotactic body radiation therapy (SBRT) is a common treatment
techniquethat can be used to treat tumors for multiple cancer sites. Density heterogeneity
in the
target volume and beam path combined with small treatment fields has made dose calculation
in lung SBRT difficult. Dose calculation algorithms used historically have difficulty
modelling the extreme density heterogeneity present in lung SBRT and have been shown
to overestimate the dose delivered to tumors situated in the lung parenchyma. Recently,
more advanced algorithms that directly model heterogeneity have been implemented for
clinical treatment planning. The limited accuracy of historically utilized dose calculation
algorithms has raised questions about their effects on local control due to the possibility
of tumor underdosing. The first part of this work establishes a proper dose normalization
technique when implementing these advanced algorithms for treatment planning in order
to keep consistent radiation beam settings and to quantify the dosimetric effect of
various
dose normalizations. The second aim is to quantify the effects dosimetric accuracy
has on
local control in lung SBRT.</p><p>Materials/Methods: 87 lung SBRT plans with doses
originally calculated with the AnisotropicAnalytical Algorithm (AAA) had their doses
recalculated with the new Acuros XB (AXB)
algorithm, which is able to directly model the heterogeneity of the lungs and treatment
volume. After recalculation, the plan was normalized to the planning target volume
(PTV)
D95%, internal target volume (ITV) D99%, and to match the original PTV coverage. The
percentage change in total monitor units (MU) between the AXB renormalized plans and
the original AAA plans were calculated to quantify how the delivered radiation would
change when implementing the AXB algorithm for treatment planning. Percentage changes
in relevant PTV and ITV dose metrics as well as absolute changes in relevant organ
at risk.
(OAR) dose metrics were quantified to compare plan dosimetry. OAR doses were also
compared to the current institutional planning objectives to investigate the feasibility
of
meeting the current objectives with the new algorithm.
162 patients previously treated with SBRT were selected from a retrospective protocol
comparing the efficacy of SBRT and surgery for treatment of early-stage non-small
cell
lung cancer. Plans had their doses originally computed with the Pencil Beam Convolution
(PBC, n = 8) algorithm or AAA (n = 156). Each plan was recalculated with AXB
with identical beam settings. A subset was also recalculated with Monte Carlo to validate
the accuracy of the AXB calculations. Percentage changes in relevant PTV and ITV biologically
effective doses (BED) were calculated between the original and AXB plans to
quantify the magnitude of the dosimetric differences between the old and new algorithm.
A multivariable linear regression was performed to investigate which patient and treatment
parameters influenced the magnitude of these dosimetric changes. A competing risk
analysis
was performed to quantify the association between the magnitude of the dosimetric
changes and local failure.</p><p>Results: Normalizing the AXB plan to the PTV D95%
and keeping the original PTVcoverage typically resulted in a total MU increase (average
increase of 7.0% and 7.9%,
respectively) while normalizing to the ITV D99% resulted in similar total MU (average
increase
of 0.31%). When normalizing to the PTV D95%, the AXB plans had increased PTV
and ITV D1%[Gy] (median increases of 3.4% and 3.2%, respectively) while normalizing
to the ITV D99% showed a median 1.9% decrease. Normalizing the AXB plans to the
PTV D95% typically resulted in increased OAR dose for all OARs and an inferior ability
to meet the OAR planning constraints. Reoptimization of the renormalized plans showed
the current OAR objectives to be manageable when using the AXB algorithm.
The AXB dose calculations were much more consistent with Monte Carlo than were the
original dose calculations. A large range of dosimetric decreases upon recalculation
with
AXB were observed for both patients who failed locally and those who were controlled.
Higher beam energy was found to increase the magnitude of the dosimetric decreases
(expected
decrease in PTV mean BED of 3.6%, 5.9%, and 9.1% when using 6X, 10X, or
15X, respectively). Total lung volume was also associated with an increased magnitude
of
dosimetric decrease (expected decerease of 0.8% per 500 cc for the PTV mean BED).
The
median follow-up time of the cohort was 26 months. 15 patients experienced local failures.
Upon univariate analysis, the dosimetric decreases in the PTV and ITV D1% BED
were found to be associated with local failure (hazard ratio (HR) of 0.89 (p=0.04)
and 0.87
(p=0.02), respectively). Upon multivariate analysis, the dosimetric decrease in the
ITV
D1% BED remained significant when controlling for PTV volume (HR=0.89 (p=0.04)).</p><p>Conclusions:
More accurate dose calculation algorithms are beginning to be implementedfor clinical
treatment planning. When implementing these new algorithms, issues arise
with dose normalization due to the potential for vast differences between the dose
distributions
calculated with the different algorithms. Normalizing the dose to the PTV D95%
in the AXB plan will result in a delivered dose increase relative to a AAA plan while
normalizing to the ITV D99% will keep similar delivered doses between the plans. Dose
metrics typically increase when normalizing to the PTV D95% (for targets and OARs)
while normalizing to the ITV D99% typically decreased the reported dose metrics. The
OAR planning objectives are manageable using the AXB algorithm.
Many factors are related to the magnitude of the dosimetric decreases observed when
recalculating plans with AXB, including but not limited to beam energy and lung volume.
Most of the investigated dose metrics were not associated with local failure, but
the change
in the PTV and ITV D1% BEDs were found to be associated with local failure in the
univariate analysis.</p>
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