6 mv linear accelerator

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.

This prospective study was conducted in the Radiotherapy Sector of the
Universidade Federal de São Paulo (Unifesp), from June 2010 to August
2012. It included 30 patients with HNC (oral cavity, pharynx, larynx, or occult
primary tumor) submitted to conventional 3D radiotherapy, with irradiation
fields necessarily encompassing all major salivary glands. The total dose ranged
from 66 to 70 Gy, given in fractions of 2 Gy/day, in weekly combination with
cisplatin (40 mg/m²), accompanied or not by surgery. The radiotherapy
was performed with a 6 MV linear accelerator (Varian) or with a ⁶⁰Co
teletherapy unit (Alcyon II; CGR MeV), in cervicofacial areas and
supraclavicular fossa. All patients were over 18 years of age and had a
Karnofsky index ≥ 70.
Patients with diabetes mellitus, autoimmune diseases, infectious diseases, or
collagen diseases were excluded, as were those with incipient tumors (stage T1
or T2) limited to the larynx, as well as those with trismus (reduced mouth
opening capacity) due to surgical sequelae.
This study was approved by the Unifesp Research Ethics Committee (Ruling no.
0844/10). All research subjects gave written informed consent.

A custom-made acrylic phantom was fabricated to insert cuvettes for delivering a uniform dose to a total of three cuvettes at a time. The dimensions of the phantom were 10 × 10 × 14 cm³. Three cuvettes could be located at the center of the phantom, i.e., the center of each cuvette was located at a depth of 7 cm into the phantom, as shown in Fig 1(A).
The mold phantom with three inserted cuvettes was processed to acquire CT images using a Brilliance CT Big Bore^™ (Philips, Cleveland, OH, USA) with an imaging slice thickness of 1 mm. Based on these CT images, IMRT plans using two opposed bilateral beams were calculated to deliver uniform doses of ±1.0% to all cuvettes at once. The IMRT plans were generated with 6 MV photon beams in the Eclipse^™ system (Varian Medical Systems, Palo Alto, CA USA). Irradiation of the PRESAGE dosimeters was also carried out with a 6 MV linear accelerator (Varian Medical Systems, Palo Alto, CA, USA) according to the IMRT plan. Various radiation doses (0, 10, 20, 50, 80, 100, 150, 200, and 300 cGy) were delivered at a dose rate of 600 cGy/min for all fabricated dosimeters, as shown in Fig 2.