A 71-year-old woman was found to have a solitary pulmonary nodule in the right lower lobe on computed tomography (CT) scans during work-up for an episode of pleuritic chest pain. This 1.4-cm, 18F-fluorodeoxyglucose-avid (maximum standardized uptake value: 5.5) lesion was biopsied, and results revealed a high-grade neuroendocrine tumor. Multiple extrapulmonary lesions on positron emission tomography, including a colonic lesion and adenopathy within the mediastinum and abdomen, were biopsied and found to be consistent with the patient's history of sarcoidosis.
Postbiopsy, the patient developed a pneumothorax requiring a chest tube and 2-day hospital admission. Given the patient's poor pulmonary function (forced expiratory volume in 1 second: 1.1 L; 43% of predicted), the patient was deemed medically inoperable. She was referred to radiation and medical oncology where options including radiation therapy alone, sequential chemoradiotherapy, and concurrent chemoradiotherapy were discussed. The patient refused any treatment involving chemotherapy, and given the size and location of the lesion, the patient consented to move forward with SABR of 6000 cGy delivered over 8 fractions on alternate days.
A 4-dimensional CT simulation was performed on a 16-slice Philips Big Bore CT Scanner (Philips Healthcare, Cleveland, OH). During respiration, the tumor moved 11 mm in the superoinferior direction, greater than our institutional standard of 10 mm for requiring respiratory gating. The internal target volume (ITV) was delineated on the axial slices of a subset average CT scan, composed of phases between 40% and 60% of the respiratory cycle. The planning target volume (PTV) was created using a 5 mm uniform margin around the ITV (Fig 1). The ITV and PTV were 4.2 cm3 and 17.0 cm3, respectively. A dose of 6000 cGy was prescribed to the PTV using a single isocenter, with a maximum point dose of 8170 cGy within the ITV. Treatment was delivered on alternate days, with daily cone beam CT, gated 2-dimensional kV/kV image guidance, and respiratory gating.
After four fractions, cone beam CT scans revealed a change in location, shape, and size of the lung mass. The treating radiation oncologist decided to replan the patient using the same technique described previously. At the time of re-simulation, the GTV was determined to have shifted approximately 8 mm inferiorly, 9 mm posteriorly, and 1 mm laterally (3-dimensional distance: 12 mm; Fig 2). The new ITV and PTV were 1.4 cm3 and 9.0 cm3, respectively. The patient completed the remaining four fractions without incident, and her treatment was restarted 4 days after re-simulation was initiated.