Background and purpose: Lung tissue is a special challenge for a dose calculation algorithm, especially in the case of extracranial stereotactic radiotherapy (ESRT) due to small field sizes in combination with large variations in tissue density. The present study investigates the choice of dose calculation algorithm for 18 patients with a single lung tumor and 8 patients with a single liver tumor. The dose calculation is performed with both the pencil beam convolution algorithm and the collapsed cone convolution algorithm with the same number of monitor units in both cases. In addition, the dose calculation with the collapsed cone convolution algorithm is also performed with modified field sizes in order to match the Planning Target Volume (PTV) peripheral dose of the pencil beam based treatment.
Results: For liver tumors, the mean Clinical Target Volume (CTV) dose calculated by the collapsed cone convolution algorithm and the pencil beam convolution algorithm is almost identical. For lung tumors, the mean CTV dose determined by the collapsed cone convolution algorithm differs up to 20%. Plans obtained by the two algorithms have field sizes which differ up to 8mm for the same number of monitor units and minimum dose to the lung PTV.
Conclusions: The choice of dose calculation algorithm can have a large influence on a treatment plan for ESRT of the lungs.