Assessment of pretreatment setup variation in cervical cancer patients undergoing conformal radiation therapy
Abstract
Introduction: The principal of radiotherapy in the 3D era is to eradicate tumor cells contained in the gross tumor volume (GTV) with a margin of subclinical disease in the clinical target volume (CTV) while sparing surrounding organs at risks (OARs). It is achieved by conforming the treatment portals to the target volume and using appropriate margins to rationalize for uncertainties in organ shape and motion, beam geometry and patient set-up. The patient’s reproducibility and its accuracy are essential for precise radiotherapy delivery. A significant variation/error in the patient’s positioning during conformal cervical cancer radiation therapy delivery leads to a compromise in the ultimate precision of idealized treatment.
Objectives: To assess the different sources of variation in pre-treatment setup of cervical cancer patients undergoing conformal radiation therapy (CRT), to determine the solutions to eradicate the sources of variation in pre-treatment set-up of cervical cancer patients undergoing CRT.
Methods: This research study was carried out at Uganda cancer institute, selected because it is the only specialized hospital offering radiation therapy in Uganda. A total of 10 cervical cancer patients undergoing CRT treatment courses were included in the study. They were treated in supine position with a knee wedge and ankle rest to a dose of 50 Gy in 25 fractions for five weeks. Anterior-posterior (AP) and lateral onboard images were taken daily, five days a week for five weeks. Setup verification was done using kilo-voltage orthogonal on-board images obtained using an on-board imager (OBI -Varian medical system). Setup variations were estimated using 2D/2D matching (Kv onboard image/DRR) of bony landmarks such as the widest portion of the pelvic brim, anterior border of S1 vertebrae, and the pubic symphysis in the lateral (left to right), vertical (anterior to posterior), and longitudinal (superior to inferior) axes respectively during the course of treatment. The shifts between the origin of the initial treatment and the isocenter of each fraction represents the directional variation setup error (∆) due to baseline deviation between the initial treatment and the fractionated treatment. The setup accuracy at the treatment unit relative to the simulator setup was measured for 10 patients foe at least 5 fractions per patient. Setup corrections were applied, based on an online decision protocol.
Results: The random error (day to day variation, σ) in the lateral, longitudinal and vertical directions were 1.41 mm, 1.67 mm, and 0.74 mm respectively.
Conclusion: Based on this study, the use of daily DRR allows estimating and making up for pre-treatment setup variation/ random errors according to my configuration. Daily DRR can be a suitable method of accurate patient positioning in treating cervical cancer patients with high precision techniques. This resulted in avoiding geographical miss. Close surveillance through onboard imaging (OBI) is thus recommended for high quality radiation therapy delivery for patients undergoing CRT.