Is internal target volume accurate for dose evaluation in lung cancer stereotactic body radiotherapy?

PURPOSE: 4DCT delineated internal target volume (ITV) was applied to determine the tumor motion and used as planning target in treatment planning in lung cancer stereotactic body radiotherapy (SBRT). This work is to study the accuracy of using ITV to predict the real target dose in lung cancer SBRT. MATERIALS AND METHODS: Both for phantom and patient cases, the ITV and gross tumor volumes (GTVs) were contoured on the maximum intensity projection (MIP) CT and ten CT phases, respectively. A SBRT plan was designed using ITV as the planning target on average projection (AVG) CT. This plan was copied to each CT phase and the dose distribution was recalculated. The GTV_4D dose was acquired through accumulating the GTV doses over all ten phases and regarded as the real target dose. To analyze the ITV dose error, the ITV dose was compared to the real target dose by endpoints of D(99), D(95), D(1) (doses received by the 99%, 95% and 1% of the target volume), and dose coverage endpoint of V(100)(relative volume receiving at least the prescription dose). RESULTS: The phantom study shows that the ITV underestimates the real target dose by 9.47%∼19.8% in D(99), 4.43%∼15.99% in D(95), and underestimates the dose coverage by 5% in V(100). The patient cases show that the ITV underestimates the real target dose and dose coverage by 3.8%∼10.7% in D(99), 4.7%∼7.2% in D(95), and 3.96%∼6.59% in V(100) in motion target cases. CONCLUSIONS: Cautions should be taken that ITV is not accurate enough to predict the real target dose in lung cancer SBRT with large tumor motions. Restricting the target motion or reducing the target dose heterogeneity could reduce the ITV dose underestimation effect in lung SBRT.