Margriet Kwint

Chapter 4 72 radiotherapy (>44 Gy), histology or cytology proven lung cancer and image guided radiotherapy (IGRT) with the use of 3 or 4D-CBCT. All CBCT-scans were available for all patients. Several radiotherapy regimens, all planned with Intensity Modulated Radiotherapy (IMRT) and concurrent or sequential chemotherapy or radiotherapy alone were included. Radiotherapy preparation A 3D-midventilation-CT (MidV-CT) was selected for all patients from a respiration correlated 4DCT, in which the moving tumor was closest to its time-averaged mean position [20]. The gross tumor volume (GTV) and pathological lymph nodes were delineated on the MidV-CT. A recent flu-deoxyglucose-positron-emission- tomography-(FDG-PET)-scan was registered to the MidV-CT[21]. The GTV was expanded to a planning target volume (PTV) using margins of 12 mm +¼ of the 4DCT peak-to-peak tumor amplitude in orthogonal directions. A uniform PTV margin of 12 mm was used for the lymph nodes [22] according to our institutional protocol. The planning-constraints used for the OAR were; esophagus V35<65% (physical dose), Mean Lung-Dose ≤20Gy (EQD 2 α/β=3Gy), spinal cord ≤50Gy (EQD 2 α/β=2Gy), total heart ≤40Gy, ⅔ of the heart ≤50Gy and ⅓ of the heart ≤66Gy (physical dose). Equally spaced, 7-field IMRT-plans were designed using 6/10 MV photons and direct machine parameter optimization (Pinnacle version 9.0, Philips, Best, the Netherlands) on the homo-lateral lung [23]. The prescription-dose was specified at a representative point in the PTV. The dose distribution within 99% of the PTV was >90% and <115% of the prescribed dose. Setup correction protocol An off-line shrinking action level setup correction protocol was used for all patients [24] with Nmax=3 and α=9mm. For this off-line shrinking action level setup correction protocol, CBCT’s were acquired the first three fractions using Elekta Synergy 4.2 (Elekta Oncology Systems Ltd., Crawley, UK). If no correction was necessary (average setup error over 3 fractions <5.2mm in each direction) then weekly follow-up scans were acquired. If a correction was required, the protocol restarted with three fractions with CBCT verification. This resulted in a minimum of 7 CBCT’s per patient. 4D-CBCT were acquired if the motion of the tumor, measured on the 4DCT, was ≥8 mm. The CBCT’s were registered, by two RTTs, to the MidV-CT based on the bony anatomy of the vertebrae [25].