The utility of non-axial treatment beam orientations for lower lobe lung cancers

Abstract

Traditional treatment beams for non-small-cell lung cancer are limited to the axial plane. For many tumor geometries, non-axial orientations appear to reduce the dose to normal tissues (e.g. heart, liver). We hypothesize that non-axial beams provide a significant reduction in incidental irradiation of the heart and liver, while maintaining adequate target coverage. CT scans of twenty-four patients with lower lobe lung cancers were studied. For each patient, an opposed oblique axial beam pair and a competing non-axial opposed oblique pair were generated, both off-cord. The competing plans delivered comparable doses/margins to the GTV. DVHs and integral doses were computed for all structures of interest for the two competing plans. The integral dose was compared for axial and non-axial beams for each contoured organ using a paired t-test. Dose to the heart was significantly lower for the non-axial plans ( p = .0001). For 20/24 patients, the integral heart dose was reduced by using non-axial beams. In those patients with tumors located in the inferior right lower lobe, a lower dose to the liver was achieved when non-axial beams were used. There were no meaningful differences in dose to the GTV, lungs, or skin between axial and non-axial beams. Non-axial beams can reduce the dose to the heart and liver in patients with lower lobe lung cancers. Non-axial beams may be clinically beneficial in these patients and should be considered as an option during planning.

Figure 1

Potentially reducing the field size through rotation of the beam out of the axial plane. This approach may better align the central ray with the long axis of the GTV. This image also demonstrates the exclusion of the heart with the non‐axial beam.

Figure 5

Schematic illustration of the longer path length, or greater separation, for non‐axial vs. axial beams (a); graph (b) demonstrates that the increase in path length (calculated using the cosine of the angle created by the deviation of the selected path from the axial plane) should be minimal when the off‐axis deviation is less than approximately $30°$. Clinically, this path length is commonly referred to as the patient's “separation.”

Figure 2

Sample dose‐volume histograms for the GTV, heart, normal lung, and liver for one of the studied patients.

Figure 3

The relationship between relative normal lung dose (integral lung dose using non‐axial plan/integral lung dose using axial plan) and degree of rotation off of the axial plane is shown. A trend toward increasing dose to the normal lung tissue with increasing rotation off the axial plane is seen (${\mathrm{R}}^2=0.28$, $p=0.008$).

Figure 4

The relationship between relative field size (size of the non‐axial field/size of the axial field) vs. degree of rotation off of the axial plane is shown. There is a slight trend, albeit nonsignificant, toward decreasing size of the non‐axial field as the beam is rotated farther away from the axis (${\mathrm{R}}^2=0.04$, $p=0.327$).