A Beam Angle Selection Method to Improve Plan Robustness Against Position Error in Intensity-Modulated Radiotherapy for Left-Sided Breast Cancer

Abstract

Purpose: We report a dosimetric study in whole breast irradiation (WBI) of plan robustness evaluation against position error with two radiation techniques: tangential intensity-modulated radiotherapy (T-IMRT) and multi-angle IMRT (M-IMRT).

Methods: Ten left-sided patients underwent WBI were selected. The dosimetric characteristics, biological evaluation and plan robustness were evaluated. The plan robustness quantification was performed by calculating the dose differences (Δ) of the original plan and perturbed plans, which were recalculated by introducing a 3-, 5-, and 10-mm shift in 18 directions.

Results: M-IMRT showed better sparing of high-dose volume of organs at risk (OARs), but performed a larger low-dose irradiation volume of normal tissue. The greater shift worsened plan robustness. For a 10-mm perturbation, greater dose differences were observed in T-IMRT plans in nearly all directions, with higher ΔD_98%, ΔD_95%, and ΔD_mean of CTV Boost and CTV. A 10-mm shift in inferior (I) direction induced CTV Boost in T-IMRT plans a 1.1 (ΔD_98%), 1.1 (ΔD_95%), and 1.7 (ΔD_mean) times dose differences greater than dose differences in M-IMRT plans. For CTV Boost, shifts in the right (R) and I directions generated greater dose differences in T-IMRT plans, while shifts in left (L) and superior (S) directions generated larger dose differences in M-IMRT plans. For CTV, T-IMRT plans showed higher sensitivity to a shift in the R direction. M-IMRT plans showed higher sensitivity to shifts in L, S, and I directions. For OARs, negligible dose differences were found in V₂₀ of the lungs and heart. Greater ΔD_max of the left anterior descending artery (LAD) was seen in M-IMRT plans.

Conclusion: We proposed a plan robustness evaluation method to determine the beam angle against position uncertainty accompanied by optimal dose distribution and OAR sparing.

Keywords: beam angle; intensity-modulated radiotherapy; left-sided breast; position-error; robustness.

Figure 1.

Beam arrangements of T-IMRT and M-IMRT plans. (A). T-IMRT; (B) M-IMRT.

Figure 2.

Steps in robustness evaluation of T-IMRT and M-IMRT plans.

Figure 3.

A sample of dose-volume histograms (DVHs) of the nominal and perturbed T-IMRT and M-IMRT plans for different isocenter shifts. (A) PTV of T-IMRT plans with a 3-mm shift; (B) PTV of T-IMRT plans with a 5-mm shift; (C) PTV of T-IMRT plans with a 10-mm shift; (D) PTV of M-IMRT plans with a 3-mm shift; (E) PTV of M-IMRT plans with a 5-mm shift; (F) PTV of M-IMRT plans with a 10-mm shift; (G) PTV Boost of T-IMRT plans with a 3-mm shift; (H) PTV Boost of T-IMRT plans with a 5-mm shift; (I) PTV Boost of T-IMRT plans with a 10-mm shift; (J) PTV Boost of M-IMRT plans with a 3-mm shift; (K) PTV Boost of M-IMRT plans with a 5-mm shift; (L) PTV Boost of M-IMRT plans with a 10-mm shift; PTV, plan target volume; PTV Boost, plan target volume boost; T-IMRT, tangential intensity-modulated radiation; M-IMRT, multiple-angle intensity-modulated radiation. L, left; R, right; A, anterior; P, posterior; S, superior; I, inferior.

Figure 4.

TCP reduction (ΔTCP) of CTV and CTV Boost between the reference and perturbed T-IMRT and M-IMRT plans for different isocenter shifts. (A). ΔTCP of CTV (3 mm); (B). ΔTCP of CTV (5 mm); (C) ΔTCP of CTV (10 mm); (D). ΔTCP of CTV Boost (3 mm); (E). ΔTCP of CTV Boost (5 mm); (F) ΔTCP of CTV Boost (10 mm).

Figure 5.

Dose difference between the reference and perturbed T-IMRT and M-IMRT plans for different isocenter shifts. (A) ΔV₂₀ of Lung L; (B) ΔV₅ of Lung L; (C) ΔD_mean of Lung L; (D) EUD of Lung L; (E) ΔV₂₀ of heart; (F) ΔV₅ of heart; (G) ΔD_mean of heart; (H) EUD of heart; (I) ΔD_mean of Lung R; (J) ΔD_mean of Breast R; (K) ΔD_max of LAD; (L) ΔD_mean of LAD.