Abstract
The volume of irradiated liver is strongly related to target volume size in extracranial stereotactic radiation therapy (ESRT) of liver tumours. Our ESRT dose escalation study treats liver cancer with prescription doses that are individualized to maintain a constant risk of radiation induced liver disease (RILD) for all patients. The impact of liver volume on normal tissue complication probability (NTCP) calculation was assessed using the whole liver, liver minus gross tumour volume (GTV), liver minus clinical target volume (CTV), and liver minus planning target volume (PTV). Assuming that liver minus GTV is the most appropriate volume to base NTCP on (since it includes all potentially functional liver), each volume was used to calculate the dose for 5% NTCP. NTCP was then recalculated using the liver minus GTV, but with doses determined from the other liver volumes. The relationship between target volume size and dose is also investigated. NTCP calculated with liver minus CTV or liver minus PTV results in extremely high risks of RILD, while using whole liver underestimates the risk and is safer. For all target volumes, the prescription dose can be increased as the target size decreases. Predicted dose for individualized dose escalation for liver cancer is strongly dependent on the liver volume analyzed. We suggest that liver minus CTV and liver minus PTV volumes cannot safely be used to individualize prescription doses for dose escalation for liver cancer. Based on these substantial changes in NTCP, uniform reporting of volumes and NTCP is desirable.
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