Monte Carlo vs. pencil beam based optimization of stereotactic lung IMRT

Abstract

Background: The purpose of the present study is to compare finite size pencil beam (fsPB) and Monte Carlo (MC) based optimization of lung intensity-modulated stereotactic radiotherapy (lung IMSRT).

Materials and methods: A fsPB and a MC algorithm as implemented in a biological IMRT planning system were validated by film measurements in a static lung phantom. Then, they were applied for static lung IMSRT planning based on three different geometrical patient models (one phase static CT, density overwrite one phase static CT, average CT) of the same patient. Both 6 and 15 MV beam energies were used. The resulting treatment plans were compared by how well they fulfilled the prescribed optimization constraints both for the dose distributions calculated on the static patient models and for the accumulated dose, recalculated with MC on each of 8 CTs of a 4DCT set.

Results: In the phantom measurements, the MC dose engine showed discrepancies < 2%, while the fsPB dose engine showed discrepancies of up to 8% in the presence of lateral electron disequilibrium in the target. In the patient plan optimization, this translates into violations of organ at risk constraints and unpredictable target doses for the fsPB optimized plans. For the 4D MC recalculated dose distribution, MC optimized plans always underestimate the target doses, but the organ at risk doses were comparable. The results depend on the static patient model, and the smallest discrepancy was found for the MC optimized plan on the density overwrite one phase static CT model.

Conclusions: It is feasible to employ the MC dose engine for optimization of lung IMSRT and the plans are superior to fsPB. Use of static patient models introduces a bias in the MC dose distribution compared to the 4D MC recalculated dose, but this bias is predictable and therefore MC based optimization on static patient models is considered safe.

Figure 1

Lung phantom. Cross sections through the CT scan of the lung phantom which consisted of a low density cork cube surrounding a homogeneous plastic sphere of variable diameter (here: 4.2 cm). The cork cube densities ranged from ρ_min = 0.001 to ρ_max = 1.09 with an average density of ρ_mean = 0.12 , while the plastic sphere had a density of ρ = 1.1 . A film could be positioned through the phantom as marked with a dashed line in A and B. The cork cube, plastic sphere and film were fixed relatively to the high density positioning markers (indicated by arrows on A) on the surface of the plastic container by a plastic plate and four plastic screws as shown in B.

Figure 2

Verification of dose engines - γ-plots. γ-plots of the dose distributions measured with film and calculated with MC and fsPB for all tumour sizes for 6 MV (upper set) and 15 MV (lower set). The acceptance criteria for the γ comparisons was set to 3%/3 mm. The tumour outlines are marked by black circles.

Figure 3

Verification of dose engines - dose profiles. 6 MV (upper plots) and 15 MV (lower plots) in-plane (Y) and cross-plane (X) dose profiles as measured with film (solid) and calculated with both MC (dotted) and fsPB (dashed) algorithms for tumour I-III.

Figure 4

Verification of dose engines - depth dose curves. 6 MV (upper row) and 15 MV (lower row) depth dose profiles (Z) calculated with MC (solid) and fsPB (dashed) algorithms for tumour I-III.

Figure 5

Dose distribution isodoses. Example of the resulting dose distributions from optimization with fsPB and MC with 6 MV beam energy and using the average CT patient model (a). The same plans recalculated with 4DMC (b).

Figure 6

Target dose distribution DVHs. Target (PTV) DVHs of the PB and MC static plans (solid) and their 4DMC calculation (CTV accumulated dose DVHs) (dashed). Calculated for 6 MV (left column) and 15 MV (right column) for the one phase static CT (a, b); the minimum density overwrite one phase static CT (c, d) and the average CT (e, f) patient models.

Figure 7

Ipsilateral lung dose distribution DVHs. Ipsilateral lung DVHs of the PB and MC static plans (solid) and their 4DMC calculation (accumulated dose DVHs) (dashed). Calculated for 6 MV (left column) and 15 MV (right column) for the one phase static CT (a, b); the minimum density overwrite one phase static CT (c, d) and the average CT (e, f) patient models.