Evaluation of set-up uncertainties with daily kilovoltage image guidance in external beam radiation therapy for gynaecological cancers.

AIMS: To assess the set-up uncertainty for gynaecological cancer patients treated with external beam radiation therapy using daily kilovoltage image guidance and to estimate set-up margins for treatment and factors that would predict higher set-up uncertainty.
MATERIALS AND METHODS: Alignment data from daily two-dimensional kilovoltage planar images and three-dimensional kilovoltage cone beam images for 52 patients treated on a Varian 2300iX linear accelerator with On Board Imaging (OBI; version 1.4) capability were analysed. The mean displacements of translational shifts, population systematic errors and random errors were calculated. Using van Herk's formula, the clinical target volume (CTV) to planning target volume (PTV) margins for set-up uncertainties were calculated. The differences in set-up error were calculated with respect to the type of cancer, imaging type and body mass index (BMI).
RESULTS: Population systematic and random errors were 1.1 mm, 2.3 mm, 2.3 mm and 3.9 mm, 5.0 mm, 3.5 mm in the anterior-posterior (AP), medial-lateral (ML) and superior-inferior (SI) directions, respectively, for the entire patient population. Using van Herk's formula, the CTV to PTV margins for set-up uncertainties were found to be 5.5, 9.1 and 8.3 mm in the AP, ML and SI directions respectively. The mean displacements in the AP, ML and SI directions for BMI ≥ 30 (28 patients) versus <30 (24 patients) were -0.1 mm, 0.9 mm and 1.0 mm versus -0.1 mm, 0.1 mm and 0.4 mm, respectively, (P = 0.02).
CONCLUSIONS: Daily imaging helps to assess set-up uncertainty. The set-up margin for CTV to PTV was larger for patients with BMI ≥ 30 without image guidance and these patients would benefit more from daily image guidance.