Setup errors during curative radiation therapy in intracranial neoplasms - is a 5 MM PTV margin an overestimation?

The aim of the study was to determine an ideal planning target volume (PTV) margin using the Van Herks formula by auditing and reporting the setup errors in 6 dimensions of patients undergoing curative radiation therapy to the brain in a newly set up Radiation Oncology Department in Southern India.

All patients with intracranial neoplasms who underwent curative intracranial radiotherapy with an on board kV cone beam computed tomography (CBCT) in our department between December 2022 and December 2024 were selected for this retrospective analysis. Displacements in each direction - 3 translational and 3 rotational - were retrieved and recorded using the offline review feature of the radiation planning and delivery software. Clinical target volume (CTV) to PTV margin was calculated using the Van Herk formula (2.5 Σ + 0.7 σ) in the translational axes.

We evaluated 474 on-board CBCT images of 21 patients who had undergone curative intracranial fractionated external beam radiotherapy (EBRT). Matching the CBCT and simulation CT was done and verified by 2 expert radiation oncologists. The mean setup errors in the lateral, longitudinal and vertical axes were 0.03, -0.01, and -0.06 cm, respectively, and the mean rotational errors for roll, pitch and yaw were 0.1, 0.2, and -0.28 degrees, respectively. Using the Van Herk formula, the calculated CTV to PTV margins in the lateral, longitudinal and vertical axes were 0.28, 0.37, and 0.34 cm, respectively. This was well below the institutional PTV margin of 5 mm.

The results showed that a reduced PTV Margin of 4mm is sufficient in intracranial malignancies. Daily Online image matching with a 6D couch further enhances the accuracy of the treatment.