Repeat 4D-CTs during fractionated radiotherapy of lung cancer: a clinical protocol providing a basis of target definition for gated radiotherapy

Purpose: Planning target volume definition is one of the crucial points for gated radiotherapy. To gain reliable information on variability of tumour position and tumour motion during the course of fractionated radiotherapy a longitudinal study was implemented.

Patients and Methods: Patients with locally advanced, inoperable non small cell lung cancer are enrolled in a clinical protocol for curative treatment with 66 Gy @ 2 Gy. For treatment planning a 4D-FDG-PET/4D-CT with phase correlated attenuation correction of the PET in treatment position was performed (Biograph 16, Siemens). Target volumes are derived from the Internal Target Volume (ITV) + 7 mm (Clinical Target Volume) + 8 mm (Planning Target Volume) for the first week. The latter margin is reduced to 5 mm after 6 fractions if deviations in tumour location are small. For treatments patients are positioned by X-ray verification (ExacTrac-Xray, BrainLab). Repeat 4D-CT scans are acquired with an in-room CT (Primatom, Siemens) directly before treatment for the first 5 fractions and subsequently twice per week (i.e. 16 4D-CTs per patient). Immediate evaluation of the 4D-CTs includes verification of patient positioning and adequateness of treatment fields (Pinnacle, Philips). Further evaluation of data includes intra- and interfractional variability of tumour volume and centre of gravity. Margins will be evaluated simulating gated radiotherapy.

Results: The protocol started in January 2008. Up to now 5 patients are enrolled, 2 of them have completed their treatment. For the first 5 patients a PTV reduction to 79,5 % (SD 2,1 %) after the first treatment week could be reached, which led to a decrease of Mean Lung Dose of 0.7 Gy (SD 0.3 Gy). Preliminary evaluation of the 4D data shows minor intrafractional variability of the tumour volume compared to greater interfractional variations. The preliminary data rather suggest variability in contouring than true variability in tumour volume. The trajectory of centre of gravity associates with the tumour motion.

Conclusion: The preliminary data suggest only small setup variability during treatment. Extension of the data base and further analysis will address if dose escalation by introducing gated radiotherapy is possible.

Supported by BMBF (03ZIK/OncoRay) and Siemens Medical Solutions.