Feasibility study of stereotactic body radiotherapy for peripheral lung tumors with a maximum dose of 100 Gy in five fractions and a heterogeneous dose distribution in the planning target volume

Abstract

We evaluated toxicity and outcomes for patients with peripheral lung tumors treated with stereotactic body radiation therapy (SBRT) in a dose-escalation and dose-convergence study. A total of 15 patients were enrolled. SBRT was performed with 60 Gy in 5 fractions (fr.) prescribed to the 60% isodose line of maximum dose, which was 100 Gy in 5 fr., covering the planning target volume (PTV) surface (60 Gy/5 fr. - (60%-isodose)) using dynamic conformal multiple arc therapy (DCMAT). The primary endpoint was radiation pneumonitis (RP) ≥ Grade 2 within 6 months. Toxicities were graded according to the Common Terminology Criteria for Adverse Events, version 4.0. Using dose-volumetric analysis, the trial regimen of 60 Gy/5 fr. - (60%-isodose) was compared with our institutional conventional regimen of 50 Gy/5 fr. - (80%-isodose). The enrolled consecutive patients had either a solitary peripheral tumor or two ipsilateral tumors. The median follow-up duration was 22.0 (12.0-27.0) months. After 6 months post-SBRT, the respective number of RP Grade 0, 1 and 2 cases was 5, 9 and 1. In the Grade 2 RP patient, the image showed an organizing pneumonia pattern at 6.0 months post-SBRT. No other toxicity was found. At last follow-up, there was no evidence of recurrence of the treated tumors. The target volumes of 60 Gy/ 5 fr. - (60%-isodose) were irradiated with a significantly higher dose than those of 50 Gy/5 fr. - (80%-isodose), while the former dosimetric parameters of normal lung were almost equivalent to the latter. SBRT with 60 Gy/5 fr. - (60%-isodose) using DCMAT allowed the delivery of very high and convergent doses to peripheral lung tumors with feasibility in the acute and subacute phases. Further follow-up is required to assess for late toxicity.

Keywords: Phase I study; dose-escalation study; homogeneity index; lung cancer; prescription dose; stereotactic body ridiotherapy.

Fig. 1.

Axial and coronal plane computed tomography images with superimposed dose distribution curves of stereotactic body radiation therapy (SBRT). (A, B) Our institutional conventional regimen of 50 Gy in 5 fractions prescribed to the 80% isodose line of maximum dose covering the planning target volume (PTV) surface. The maximum dose in the PTV was 62.5 Gy in 5 fractions. (C, D) The trial regimen of 60 Gy in 5 fractions prescribed to the 60% isodose line of maximum dose covering the PTV surface. The maximum dose in PTV was 100 Gy in 5 fractions. Isodose lines from outer to inner represent 20 Gy, 40 Gy, 50 Gy, 60 Gy and 80 Gy of the maximal dose, respectively.

Fig. 2.

Dose–volume histograms for internal target volume (ITV) (A), planning target volume (PTV) (B), normal lung ( = lung minus internal target volume) (C), and chest wall (D), for the same patient as shown in Fig. 1. Mean PTV doses of the 60 Gy/5 fractions − (60% isodose) regimen and 50 Gy/5 fractions − (80% isodose) regimen were 93.7 Gy and 60.4 Gy, respectively. For the two regimens, the mean ITV was 80.5 Gy and 57.1 Gy, respectively, the volume irradiated > 20 Gy (V₂₀) in the normal lung was 6.8% and 6.5%, respectively, and the V₃₀ in the chest wall was 46.4 ml and 41.0 ml, respectively.