Objective: The outcome of post-surgical recurrences of cervical cancer may be improved through radiation dose escalation, which hinges on accurate identification and treatment of the target. The present study quantifies target motion during course of image-guided radiotherapy (IGRT) for vault cancers.
Methods: All patients underwent planning CT simulation after bladder-filling protocol. A daily pre-treatment megavoltage CT was performed. All translations and rotations were recorded. Post-registration displacement of gross tumour volume (GTV) and centre of mass (COM) of GTV was independently recorded by two observers for fractions one to seven. Day 1 image sets served as reference images against which the displacements of COM were measured. We calculated the displacements of common volume (CV) and encompassing volume (EV) of GTV for both the observers.
Results: A total of 90 image data sets of 15 patients were available for evaluation. Individual patient GTV and average GTV by both the observers were comparable. The average shifts for EV were 2.4 mm [standard deviation (SD) ±1.2] in the mediolateral, 4.2 mm (SD ±2.8) in the anteroposterior and 4.0 mm (SD ±2.1) in superoinferior directions. Similarly, the average shifts for CV were 1.9 mm (SD ±0.6) in the mediolateral, 3.7 mm (SD ±2.7) in the anteroposterior and 4.4 mm (SD ±2.7) in superoinferior directions. Using Stroom's/van Herk's formula, the minimum recommended margins would be 4.5/5.2, 8.2/9.4 and 7.3/8.3 mm, respectively, for lateral, anteroposterior and superoinferior directions.
Conclusion: Differential directional internal margin is recommended in patients undergoing IGRT for post-surgical recurrence of cervical cancers.
Advances in knowledge: Internal organ motion of vault cancers can be accounted for by a directional margin to the gross tumour.