Accurate
Heterogeneous Dose Calculation for Lung Cancer Patients without
Jonathan G. Li,
Department of Radiation Oncology,
Purpose
To investigate the accuracy of
megavoltage photon-beam dose calculations using bulk densities applied to 4
distinct regions identified as air, lung, soft tissue, and bone when compared
to dose calculations with a full density CT. We are investigating the accuracy
of this technique for use in an on-board MRI image-guided radiation therapy
device under development at our institution.
Material and methods
Full CT resolution and bulk density treatment
plans were generated for 17 lung patients using a commercial planning system
with a convolution dose calculation algorithm. Bulk densities were applied to
regions identified by an isodensity segmentation tool for each case. Individual
and population-averaged densities were compared to the full resolution plan for
each case. DVH and dose difference distributions were examined for all cases.
Results
The average densities as determined by CT numbers of the segmented air,
lung, soft tissue and bone for the entire set of patients were 0.15, 0.32, 0.98
and 1.11 g/cc, respectively. In all cases, the normal tissue DVH agreed to
better than 1%. In 15 out of 17 cases, the target DVH agreed to better than 1%,
while 2 cases with bullous emphysema showed inconsistent lung density and
agreed to within 5%.
Conclusions
Dose calculation applying bulk tissue density to four regions provides an
accurate method of heterogeneous dose calculation, which can be employed with
MRI planning data. Dose calculation accuracy in cases with ehphysemic
lung can be improved by assigning air to regions with bullous changes in the
lung.