DIVERT (Diagnostic Imaging for VERTebral metastases) from Conventional Treatment Planning CT Scan - Results of a Prospective Patient Study

Purpose/Aim

To assess the feasibility of using diagnostic CT (dCT) images to create and deliver treatment plans for palliative radiotherapy for vertebral metastases using volumetric modulated arc therapy (VMAT) radiotherapy by comparing differences in the pattern and magnitude of dose deposition between plans created on dCT images and those on treatment planning CT (TPCT) images.

Methods/Process

This is an REB-approved single-center non-randomized prospective cohort study with sample size n=30. A workflow was created to ensure treatment planning parameters were met and the existing dCT covered the extent of vertebral pathology. The dCT position was replicated by CT simulation staff at the time of the TPCT using a curved Styrofoam couch-top. Patients were given setup tattoos. A dedicated pre-treatment template provided setup instructions and CT staff completed a study questionnaire. The clinical target volume (CTV) and 1cm planning target volume (PTV) was contoured on the TPCT, and a VMAT treatment plan was created. Patients were positioned on the treatment couch on a curved Styrofoam couch-top and positioned according to the setup instructions, without viewing tattoos for setup. A CBCT and alignment correction with a 6-DoF treatment couch was performed and treatment delivery was initiated. Treatment delivery staff completed a study questionnaire. The CTV/PTV were independently contoured on both dCT and TPCT, and a separate VMAT plan created on the dCT. The image sets were rigidly registered at the level of the CTV, and contours exported to 3D Slicer for evaluation of common segmentation metrics. The plan created on the dCT was recalculated on the TPCT, and TPCT target coverage was evaluated.

Results or Benefits/Challenges

Results for n=16 (n=5 lumbosacral, n=6 lumbar or thoracolumbar, n=5 thoracic) cases to date, receiving doses of 2000cGy/5 (5) and 800cgy/1 (11). The mean ± standard deviation target coverage as calculated on the TPCT was 98.4 ± 5.2% and 94.8 ± 9.7% for the volume receiving 95% of the prescription dose (V95% Rx), 100.0 ± 3.0% and 95.2 ± 9.8% for the dose (as a percentage of the prescription dose) to 95% of the target volume (D95%), and 106.8 ± 3.0% and 107.4 ± 2.9% for the dose (as a percentage of the prescription dose) to 2% of the volume (D2%), for the CTV and PTV, respectively. The mean ± standard deviation Dice Similarity Coefficient and Hausdorff Distance between dCT and TPCT contours was 0.87 ± 0.05 and 0.92 ± 0.03, and 2.0 ± 0.5 mm and 2.0 ± 0.7 mm for the CTV and PTV, respectively. Qualitative questionnaires highlight ‘good’ or ‘acceptable’ agreement between dCT/TPCT reproducibility; localization tattoos were not used for patient setup; and a 6-DoF treatment couch appropriately corrected patient alignment. Booking time was equal to conventional setup and delivery.

Conclusions/Impact

Given the number of cases assessed to date, VMAT planning on previously acquired dCT scans provides acceptable target coverage to vertebral targets without unacceptable hot spots to organs at risk. Eliminating the need for a dedicated TPCT in such palliative cases has the potential to expedite treatment, reducing the burden on both the patient and radiotherapy resources without significant compromise.