ESTRO 2023 - Abstract Book

S2037

Digital Posters

ESTRO 2023

Existing guidelines and selection of patients for re-CT has demonstrated effectiveness in maintaining setup reproducibility with acceptable alignment of P-PTV and LN-PTV observed. Patients who underwent re-CT exhibited a greater percentage of overall weight loss compared to those without re-CT.

PO-2265 Patient position verification for multi isocenter total body irradiation with VMAT

A. Zoetelief 1 , T. Harderwijk 1 , B. Hoeben 1 , M. Willemsen-Bosman 1 , L. Krikke 1 , E. Seravalli 1

1 University Medical Center Utrecht, Radiation Oncology, Utrecht, The Netherlands

Purpose or Objective In August 2021, we implemented multi isocenter Volumetric Modulated Arc Therapy (VMAT) Total Body Irradiation (TBI) in combination with a rotatable tabletop at our department. The dose optimization and treatment are performed consecutively in Head First Supine (HFS) and in Feet First Supine (FFS) orientations. The patient Position Verification (PV) procedure is performed for the HFS and FFS treatment plans separately on different parts of the patient's anatomy, using cone beam computed tomography (CBCT). PV online corrections could potentially lead to over- or underdosage in the overlap zone (Fig.1). The aim of this retrospective study was to evaluate the necessity of longitudinal online correction of the FFS plan.

Materials and Methods The tabletop (IT-V medizintechnik, Austria) allows rotation from HFS to FFS orientation, while the patient remains immobilized in an individualized whole body vacuum mattress and open face mask. The treatment plan is optimized on the HFS and FFS Computer Tomography (CT)-scans, co-registered on the overlapping anatomical region (Fig.1). Depending on patient-length, 4 to 7 isocenters with overlapping arcs are applied. Prescription dose is 12 Gy in 6 (twice-daily) fractions with mean dose constraint for lungs <8 Gy, kidneys <10 Gy and lenses <6 Gy, conform ESTRO recommendation. The distance between the isocenters is automatically applied to the treatment table from the control room using Theraview (Cablon Medical, Netherlands) software. The intended longitudinal distance between the HFS and FFS isocenters was per treatment compared to the calculated distance between the isocenters with and without online FFS patient position correction. The longitudinal distance between HFS and FFS isocenters was calculated by using the absolute treatment table values. Results The average absolute difference (± stdev) in longitudinal position between planned and per treatment calculated HFS and FFS isocenters was 4 ± 3 mm and 5 ± 4 mm with and without FFS online correction, respectively. On average, the FFS and HFS dose distributions moved 3 mm apart, both with and without FFS online correction (Fig. 2). Setup data of 11 patients (height range: 111-200 cm), 62 fractions, was analyzed.