Evaluation of irregular breathing effects on internal target volume definition for lung cancer radiotherapy

Purpose Irregular breathing may compromise the treated volume for free‐breathing lung cancer patients during radiotherapy. We try to find a measure based on a breathing amplitude surrogate that can be used to select the patients who need further investigation of tumor motion to ensure that the internal target volume (ITV) provides reliant coverage of the tumor. Material and methods Fourteen patients were scanned with four‐dimensional computed tomography (4DCT) during free‐breathing. The breathing motion was detected by a pneumatic bellows device used as a breathing amplitude surrogate. In addition to the 4DCT, a breath‐hold (BH) scan and three cine CT imaging sessions were acquired. The cine images were taken at randomized intervals at a rate of 12 per minute for 8 minutes to allow tumor motion determination during a typical hypo‐fractionated treatment scenario. A clinical target volume (CTV) was segmented in the BH CT and propagated over all cine images and 4DCT bins. The center‐of‐volume of the translated CTV (CTVCOV) in the ten 4DCT bins were interconnected to define the 4DCT determined tumor trajectory (4DCT‐TT). The volume of CTV inside ITV for all cine CTs was calculated and reported at the 10th percentile (VCTV10%). The deviations between CTVCOV in the cine CTs and the 4DCT‐TT were calculated and reported at its 90th percentile (d90%). The standard deviation of the bellows amplitude peaks (SDP) and the ratio between large and normal inspirations, κrel, were tested as surrogates for VCTV10% and d90%. Results The values of d90% ranged from 0.6 to 5.2 mm with a mean of 2.2 mm. The values of VCTV10% ranged from 59–93% with a mean of 78 %. The SDP had a moderate correlation (r = 0.87) to d90%. Less correlation was seen between SDP and VCTV10% (r = 0.77), κrel and d90% (r = 0.75) and finally κrel and VCTV10% (r = 0.75). Conclusions The ITV coverage had a large variation for some patients. SDP seems to be a feasible surrogate measure to select patients that needs further tumor motion determination.