Stereotactic body radiotherapy with a single isocentre for multiple pulmonary metastases

A 45-year-old male developed a second set of pulmonary metastases 5 years after surgery for extraskeletal mucinous chondrosarcoma of the left shoulder. He already underwent a lobectomy and two segmentectomies for a first set of pulmonary metastases 2 years ago. The closely grouped three nodules within the left lower lung formed a planning target volume (PTV) for stereotactic body radiotherapy (SBRT) with a single isocentre, which was focused on the centre of the largest nodule (the simultaneous plan). Dose-volume histogram analysis confirmed that the plan was superior to an alternative plan, in which SBRT plans would have been produced for each individual tumour (the individual plan). The mean, maximum and minimum PTV doses were 54.0, 57.5 and 47.3 Gy, respectively, in the simultaneous plan, and 65.6, 87.2 and 52.3 Gy, respectively, in the individual plan. The homogeneity index, conformity index, and the maximum dose delivered to the surrounding healthy lung were 1.21, 0.71, and 37.7 Gy, respectively, in the simultaneous plan and 1.66, 4.44, and 46.2 Gy, respectively, in the individual plan. The patient developed Grade two pneumonitis, but remained healthy until 4 years after the SBRT. When multiple closely grouped metastases are treated using SBRT, the use of a single isocentre should be considered.