Set-Up Errors, Organ Motion, Tumour Regression and its Implications on Internal Target Volume–Planning Target Volume During Cervical Cancer Radiotherapy: Results From a Prospective Study

Abstract Aims Uterocervical motions and organ filling during cervical cancer conformal radiotherapy is complex. This prospective, observational study investigated set-up margins (clinical target vo, ume [CTV] to planning target volume [PTV]) for pelvic nodal CTV and internal margin (CTV to internal target volume [ITV]) expansions for uterocervical movements during cervical cancer radiotherapy. Materials and methods During cervical cancer radiotherapy, a daily kilovoltage, cone-beam computed tomography (CBCT) scan was acquired. Bony anatomy-based rigid co-registration and matching to vessels/pelvic nodal region was carried out to document shifts, errors (systematic and random) and to calculate CTV to PTV margins. Subsequently, soft-tissue matching was carried out at the mid-cervical region and uterine fundus to record shifts, errors and to calculate CTV to ITV margins. Results In 67 patients, 1380 CBCT scans were analysed. The mean (±standard deviation) couch shifts for CTV pelvic nodal region in all directions were within 4.5–5.3 mm, systematic and random errors 3.0–3.6 mm and set-up margins of within 10 mm (except anterior margin 10.3 mm). For the mid-cervical region, mean shifts were 4.5–5.5 mm, systematic and random errors 2–4 mm amounting to Conclusion Our study suggests anisotropic CTV to ITV and CTV to PTV margins for cervical cancer radiotherapy.