To analyze the effect of the new conformal index (nCI) and the conventional conformal index (CI) on the treament planning quality of lung stereotopic radiotherapy (SBRT).

A total of 19 peripheral lung cancer patients, treated with SBRT in Fujian Medical University Union Hospital from 2014 to 2017, were analyzed retrospectively. Each patient was planned twice yielding identical CI and nCI. The prescription to 95% of planning target volume (PTV) was 48 Gy in four fractions, and renormalization was performed when needed for nineteen nCI plans. The Wilcoxon signed-rank test was used to examine the dosimetric index.

The dose conformity plots indicate that nCI does not only reflect the dose to the organ at risk outside tumor, but also represents the dose distribution in the PTV. In addition, nCI was stricter with treatment planning qualities when the dose around PTV was closer to the prescribed dose. The value of target coverage (TC), the ratio of out-of-target volumes receiving 105% prescribed dose to the target volume (R_105%), the ratio of volume covered by 50% isodose line to the target volume (R_50%), and the ipsilateral lung V₂₀ were 98.70%, 0.56, 5.53, 15.59% in the CI plans, vs. 90%, 0, 4.99, 14.42% in the corresponding nCI plans, respectively. All index were significantly lower in the nCI group (Z=-3.823, -3.180, -3.823, -3.783, respectively, P<0.05). The ratio of the maximum dose to the 2 cm external margin from the PTV (D_{2 cm}) to the maximum dose to the PTV were 63.70% and 64.07% respectively in the two groups, and the differences were not statistially significant (P>0.05). The conformity values denoted a clinically favorable value as 1 between D_95% and D_99% of nCI plans, yet were not applicable to CI plans.

It is more clinically relavant to evaluate lung SBRT plans using nCI, TC and other indicators collectively than using CI alone.

Peripheral lung cancer; Stereotactic body radiation therapy; Conventional conformity index; New conformity index