Gross tumour volume variations in primary non-small-cell lung cancer during the course of treatment with stereotactic body radiation therapy.

Introduction: We aim to quantify the variations in the gross tumour volume (GTV) during a course of stereotactic body radiotherapy (SBRT) and determine its impact on dosimetric coverage of the GTV.

Methods: The GTVs and dose coverage for 14 patients with 16 primary non-small-cell lung tumours treated with SBRT were investigated. Initial GTVs were calculated from treatment planning CT scans. The prescribed doses ranged from 48 to 60 Gy in three to five fractions. Before each treatment, patients underwent a CBCT scan. For each CBCT scan, the GTV and the dose received by the GTV were determined and followed during the course of therapy.

Results: There was considerable variation in the measured GTVs during the course of therapy. Increases of up to 63.3% of volume measured by initial CBCT were detected during the first few fractions, after which GTV tended to decrease. Dose coverage (V95) for any given fraction deviated no more than 5% from optimised coverage obtained in the initial treatment plan. In the long term, all patients with follow-up scans demonstrated tumour shrinkage with no radiographic evidence of tumour recurrence.

Conclusion: GTV, as evaluated in this study, demonstrates an initial increase in volume followed by a subsequent decrease. This volume change needs to be considered in the design of treatment plans and assignment of treatment margins.