Objectives Feline injection-site sarcomas (FISSs) are soft tissue tumours typically characterised by an interscapular location and highly infiltrative behaviour. CT is considered the modality of choice for FISS staging and double positioning (dynamic approach) was reported to successfully detect the exact extent of infiltration into the muscles. The aim of the present study was to investigate the utility of the dynamic approach in feline patients referred for preoperative staging of interscapular FISS. Methods Tumour volume estimates were compared between the ellipsoid and the semi-automated segmentation methods. Two radiologists blinded to the patient coding used images from each position to assess the extent of muscular infiltration. The distance between the neoplasm and the adjacent skeletal structures (scapulae, spinous processes) was recorded in both positions by a single radiologist. Results Fifty-nine of 84 neoplasms invaded the adjacent muscular structures, with up to 15 muscles infiltrated. Between the extended and flexed position the average estimated numbers of muscles infiltrated were 1.9 (extended) and 1.84 (flexed) for observer A and 1.89 (extended) and 1.85 (flexed) for observer B. Good agreement between observers was established, with higher tumour volumes detected via the ellipsoid method. Moreover, tumours with smaller volumes showed slightly decreased muscular infiltration. Marked difference in the recorded distance between the skeletal structures and the neoplasm in the two different positions was established (mean ± SD difference spinous processes: 9.74 ± 9.57 mm; mean ± SD difference scapulae: 15.15 ± 11.76 mm). Conclusions and relevance A dynamic approach should be used for a complete evaluation of the invasiveness of FISS along with appropriate methodology for tumour volume measurement, which could potentially alter the tomographic estimation of the real dimension of the neoplasms.