Abstract
Objective : AAA (analytical anisotropic algorithm) dose calculation, which shows a better performance for heterogeneity correction, was tested for lung stereotactic radiation therapy (SBRT) in comparison to conventional PBC (pencil beam convolution method) to evaluate its impact on tumor dose parameters.
Materials : Eleven lung SBRT patients who were treated with photon 4MV beams in our department between April 2003 and February 2007 were reviewed.
Methods : Clinical target volume (CTV) was delineated including the spicula region on planning CT images. Planning target volume (PTV) was defined by adding the internal target volume (ITV) and set-up margin (SM) of 5mm from CTV, and then an MLC penumbra margin of another 5mm was also added.
Six-port non-coplanar beams were employed, and a total prescribed dose of 48Gy was defined at the isocenter point with four fractions. The entire treatment for an individual patient was completed within 8days. Under the same prescribed dose, calculated dose distribution, dose volume histogram (DVH), and tumor dose parameters were compared between two dose calculation methods. In addition, the fractionated prescription dose was repeatedly scaled until the monitor units (MUs) calculated by AAA reached a level of MUs nearly identical to those achieved by PBC.
Results : AAA resulted in significantly less D95 (irradiation dose that included 95% volume of PTV) and minimal dose in PTV compared to PBC. After resealing of each MU for each beam in the AAA plan, there was no revision of the isocenter of the prescribed dose required. However, when the PTV volume was less than 20cc, a4% lower prescription resulted in nearly identical MUs between AAA and PBC.
Conclusions : The prescribed dose in AAA should be the same as that in PBC, if the dose is administered at the isocenter point. However, planners should compare DVHs and dose distributions between AAA and PBC for a small lung tumor with a PTV volume less than ap-proximately 20cc.
