Eclipse 4

The immobilization systems used were the wing board with raised arms or the thermoplastic head-neck mask. A planning non-contrast CT scan was taken with 25 mm slices. By using PET-CT and/or CT with i.v. contrast, the residual mediastinal mass after chemotherapy was identified and, through precise fusion procedures, contoured as gross tumor volume (GTV) by two experienced radiation oncologists (VDS and MFO). The planning target volume (PTV) was created by an isotropic expansion of 5 mm from the GTV, taking into consideration surrounding healthy structures. RSRT was administered by conventional fractionation (fractions per day: 2.0 Gy) five days per week by a 6-MV linear accelerator for a total dose of 30 Gy.
The techniques chosen for RT delivery were intensity-modulated radiotherapy (IMRT) in 28 patients and three-dimensional conformal radiotherapy (3D-CRT) in 3 patients only. All along the RT course, the portal vision was planned every 2 days for patients treated with 3DCRT. Daily on-board imaging with cone-CT was planned for patients treated with the IMRT technique. No deep inspiration breath-hold technique was adopted in our series of patients. The Eclipse 4.5.5 (Varian) treatment planning system was used for all patients’ radiotherapy plans. Details of target and organ at-risk delineation and an example plan are shown in Figure 1.

Patients underwent CT simulation in a supine position with a slide of 2.5 mm. All patients were immobilized using a vacuum-assisted body mold to recreate exact positioning during daily sessions. Target lesion was not readily identified on the CT simulation, and the planning data set was registered to a diagnostic contrast CT or PET-CT, using a mutual information algorithm from our in-house treatment system, to facilitate gross tumor volume (GTV) delineation. A 3–8 mm isotropic expansion was generated from GTV to obtain planning target volume (PTV). Organs at risk (OARs) were delineated depending on the target lesion location without margins. The treatment planning system was Eclipse 4.5.5 (Varian), and VMAT/IMRT technique on a 6-MV linear accelerator Varian were used for treatment. The dose of SBRT was converted to the biologically effective dose (BED) to compare different dose-fractionation schedules. The BED was calculated using the linear-quadratic model with α/β = 10 Gy for the tumor and α/β = 3 to the organs at risk. Dose schedules were chosen with the aim of delivering ablative treatments respecting dose constraints for OARs.