An Inter-observer Study to Determine Radiotherapy Planning Target Volumes for Recurrent Gynaecological Cancer...

Area of Science:

• Radiotherapy physics and oncology
• Medical imaging in cancer treatment
• Gynaecological oncology

Background:

• Target delineation uncertainty is a major geometric uncertainty in radiotherapy, impacting treatment accuracy.
• Safety margins are applied to gross tumor volumes (GTVs) to create planning target volumes (PTVs), accounting for delineation uncertainties.
• No prior studies have quantified delineation uncertainty or calculated PTV margins specifically for recurrent gynaecological cancers (RGC).

Purpose of the Study:

• To quantify delineation uncertainty for RGC GTVs and calculate associated PTV margins.
• To compare GTV delineation uncertainty and PTV margins between computed tomography-magnetic resonance imaging (CT-MRI) and MRI-only workflows for RGC.

Main Methods:

• Seven clinicians delineated RGC GTVs on co-registered CT-MRI and MRI-only datasets for 20 patients.
• Delineation error was measured as the standard deviation of distances to a reference outline.
• Planning target volume (PTV) margins were calculated based on measured delineation errors.

Main Results:

• Overall delineation error decreased from 3.1 mm (CT-MRI) to 2.5 mm (MRI-only).
• Resulting PTV margins reduced from 8.5 mm (CT-MRI) to 7.1 mm (MRI-only).
• MRI-only workflow yielded 15% smaller average tumor volumes and showed significant variation in delineation errors among patients.

Conclusions:

• This is the first study to report delineation error and PTV margins for RGC, with determined margins exceeding literature values.
• The significant inter-patient variation in delineation error questions the suitability of single, population-based margins.
• An MRI-only treatment workflow may offer benefits for RGC patients through reduced tumor volumes and safety margins.