Procedures for high precision setup verification and correction of lung cancer patients using CT-simulation and digitally reconstructed radiographs (DRR).

PURPOSE: In a recent study, large systematic setup errors were detected in patients with lung cancer when a conventional simulation procedure was used to define and mark the treatment isocenter. In the present study, we describe a procedure to omit the session at a conventional simulator to remove simulation errors entirely. Isocenter definition and verification was performed at a computed tomography (CT) simulator, and digitally reconstructed radiographs (DRRs) were used for setup verification and correction at the treatment unit. METHODS AND MATERIALS: A CT simulation protocol was developed, in which radiopaque markers were used to verify the coincidence of the isocenter marked on the patients' skin with the isocenter defined in the planning CT scan. This protocol was evaluated for 20 patients. Subsequently, electronic portal images were acquired at the treatment unit. The three-dimensional setup error was established from a template match of the appropriate anatomy visible in two orthogonal beams with the corresponding anatomy in DRRs. An offline setup correction protocol was applied to reduce systematic setup errors. RESULTS: For all patients, the skin marks defined the planning CT scan isocenter to within +/- 1.5 mm in each of the three main directions. Random setup errors at the treatment unit were 1.8, 2.0, and 1.9 mm (1 SD) for the lateral (x), the superior-inferior (y), and the anterior-posterior (z) directions, respectively. With the use of the correction protocol, the systematic errors for x, y, and z were 1.5, 1.5, and 1.3 mm (1 SD). CONCLUSIONS: Because the distributions of treatment setup errors measured against DRRs obtained in our CT simulation were equal to previously obtained distributions measured against simulator films, conventional simulation can be omitted and DRRs are well-suited for setup verification. By adopting our CT simulation procedure, the large systematic simulation setup errors, which remain hidden if a conventional simulation is performed, can be avoided.

Can errors in reconstructing pre-chemotherapy target volumes contribute to the inferiority of sequential chemoradiation in stage III non-small cell lung cancer (NSCLC)?

Concurrent chemo-radiotherapy (CT-RT) has been shown to be superior to sequential CT-RT for stage III non-small cell lung cancer (NSCLC). Pre-chemotherapy gross tumor volumes (GTV) are commonly contoured for sequential CT-RT and, as significant inter-clinician variability exists in defining GTV's for lung cancer, we postulated that the poorer local control observed with sequential CT-RT may partly be due to the larger errors in defining GTV after chemotherapy-induced tumor regression. Pre-and post-chemotherapy CT scans for RT planning (RTP) were performed in ten patients who received induction chemotherapy for NSCLC. Image registration of pre- and post-chemotherapy RTP scans was performed for all patients. GTV's were first contoured in the conventional manner by two clinicians, i.e. by visual reconstruction from hard copies of the pre-chemotherapy diagnostic CT scans ('GTV-visual'). A 'GTV-match' was then contoured after image-registration, and the 'gold standard' volume was considered to be the overlap of the 'GTV-match' generated by both clinicians. The 'GTV-match' was on average 31-40% larger than 'GTV-visual'. The mean percentage of the 'gold standard', which was not covered by the 'GTV-visual' was similar for both clinicians, i.e. 26.3+/-12.5 and 28.0+/-15.0%. The inter-clinician agreement in contouring improved after image registration. These data suggest that conventional visual contouring of pre-chemotherapy GTV's may fail to treat the actual pre-chemotherapy tumor volume, and thus confound studies evaluating optimal sequencing of chemo-radiotherapy in NSCLC.

What margins are necessary for incorporating mediastinal nodal mobility into involved-field radiotherapy for lung cancer?

PURPOSE: The mobility of mediastinal nodes was studied on multiple CT scans of the thorax from patients with non-small-cell lung cancer. PATIENTS AND METHODS: A total of 10 enlarged mediastinal nodes/masses were identified in 8 patients with non-small-cell lung cancer. Nodal locations were classified using the Naruke/ATS-LCSG system, and between 3 and 6 scans were available for each site. The CT data sets were coregistered, and the contoured nodes were automatically projected onto the initial planning CT scan. An encompassing nodal volume (ENV) of all contours of a particular node was manually contoured on all scans. Individual nodal volumes were expanded in three dimensions to establish additional margins required to encompass the ENV. RESULTS: The mean volume of nodes studied ranged from 0.8 to 23.2 cc. The addition to individual nodes of a margin of 5 mm was found to result in a mean ENV coverage of >or=95% at all sites. For individual nodes at locations N4R, N5, and N6, however, the coverage ranged from 87.8% to 92.6%. CONCLUSION: The addition of a margin of 5 mm to individual mediastinal nodes seems to be adequate to account for variations in both contouring and mobility.