Individualized image guided iso-NTCP based liver cancer SBRT

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Abstract
A highly individualized stereotactic body radiotherapy (SBRT) strategy was developed to allow a wide spectrum of patients with liver cancer to be treated. This phase I/II study encompasses individualization of immobilization, radiation planning, PTV margin determination, image guidance strategy and prescription dose. Active breathing control breath hold is used to immobilize the liver when feasible. Image guidance strategies include orthogonal MV images and orthogonal kV fluoroscopy using the diaphragm for a surrogate for the liver, and kV cone beam CT using the liver or tumour for guidance. The prescription dose is individualized to maintain the same estimated risk of radiation-induced liver disease (RILD), based on a normal tissue complication probability (NTCP) model, with a maximum permitted dose of 60 Gy in 6 fractions. Since August 2003, 79 patients with hepatocellular carcinoma (33), intrahepatic cholangiocarcinoma (12) and liver metastases (34) were treated. The median tumour volume was 293 cm3 (2.9-3 088 cm3). The median prescribed dose was 36.6 Gy (24.0 Gy-57.0 Gy) in 6 fractions. The median effective liver volume irradiated was 45% (9-80%). Sixty percent of patients were treated with breath hold to immobilize their liver. Intra-fraction reproducibility (sigma) of the liver with repeat breath holds was excellent (1.5 mm); however inter-fraction reproducibility (sigma) was worse (3.4 mm). Image guidance reduced the residual systematic and random setup errors significantly.