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At our institution, different types of stereotactic radiotherapy treatments and techniques are available for early stage lung cancer patients or for oligo lung metastases. The aim was to evaluate the accuracy of different respiratory motion management techniques and calculation algorithms.

The CIRS dynamic thorax phantom was used in this study. Three targets with different sizes (diameter of 1 cm, 2 cm and 3 cm) can be interchanged in the phantom insert, which can be used for measurements with an ionization chamber. The amplitude of the target motion can be set independently in the three directions. The cos4 function of the PTW CIRS Motion Control was used during the measurements and a PTW 31016 3D Pinpoint ionization chamber was placed in the centre of the targets. The FSPB (finite-size pencil beam) and Monte Carlo calculation algorithms with Synchrony Respiratory Tracking system were evaluated on an Accuray CyberKnife M6 machine.  The AAA, AcurosXB dose to water and AcurosXB dose to medium calculation algorithms with gated and ITV method treatments were evaluated on a Varian TrueBeam machine.

In the case of CyberKnife treatments, the differences between the calculated and measured doses for 1 cm, 2 cm and 3 cm targets were -12.1%. -5.6% and -0.9% with FSPB and -0.6%, -1.6%, -1.5% with Monte Carlo, respectively. In the case of gated TrueBeam treatments the differences for 1 cm, 2 cm and 3 cm targets were -8%, -2%, -1.2% with AAA, -8.6%, -3.4%, -3.8% with AXB dose to water, and -7.5%, -2.1%, -2.6% with AXB dose to medium, respectively. In the case of ITV method treatments on the TrueBeam, the differences for 1 cm, 2 cm and 3 cm targets were -10.1%, -8.3%, -4.7% with AAA, -11.4%, -8.4%, -6.2% with AXB dose to water, and -11.7%, -8.6%, -5.6% with AXB dose to medium, respectively. For gated treatments better results could be achieved if the ionization chamber was present in the insert on the gated planning CT.  With ITV method too many artefacts were generated due to the presence of high density parts of the ionization chamber. To increase the accuracy of calculations for small targets in the future, we plan to use a homogeneous insert to replace the ionization chamber during planning CT scans.

Very good results were obtained with Monte Carlo algorithm on CyberKnife regardless of the target size, but the FSPB algorithm is not recommended for the stereotactic treatments of lung. For TrueBeam the best results were achieved with the AAA algorithm. Gated treatments are recommended for tumours with at least 2 cm in diameter. With the ITV method similar results were obtained with the three different algorithms, but generally the calculated dose underestimates the measured dose.