Authors (including presenting author) :
Kwok PW, Lau CT, Tai KK, Lee WY, Tin WY
Affiliation :
Department of Clinical Oncology, Tuen Mun Hospital
Introduction :
Boluses are essential in radiotherapy to mitigate skin-sparing effects from high-energy photon. Since 2022, our department has used 3D-printed boluses for breast cancer radiation therapy.
Traditionally, a secondary CT scan (re-CT) is done after bolus placement. However, this approach increases workload, strains CT resources, prolongs treatment times, and exposes patients to extra radiation. We present an innovative workflow that utilizes the projected light field from the linear accelerator (LINAC) collimator to guide bolus placement, eliminating the need for re-CT.
Objectives :
To evaluate the accuracy and dosimetric outcomes of bolus placement using projected light field guidance and assess the feasibility of omitting re-CT.
Methodology :
Breast cancer patients requiring adjuvant radiotherapy underwent planning CT, followed by the creation of a virtual bolus structure. The bolus was 3D-printed and applied to patient. Prior to the first treatment fraction, the outline of the virtual bolus was projected onto the patient's body from the LINAC collimator. Radiographers positioned the 3D-printed bolus guided by this projection. A re-CT was then performed for verification, and the positional accuracy and dosimetric outcomes of the light field projection-guided placement were analyzed.
Result & Outcome :
From May 2023 to February 2024, 61 breast cancer patients were treated with boluses positioned using light field projection, followed by re-CT verification. Fifty-six patients (91.8%) had their boluses accurately placed with light field projection without need for adjustments. Five patients had positional deviations on re-CT, with displacements ranging from 5mm to 13mm.
For these five patients, dosimetric analysis was performed assuming the bolus placement was guided solely by light field projection. Coverage of the clinical target volume (CTV) remained adequate, with V100% values ranging from 97.7% to 100%.
This is the first report showing re-CT can be safely omitted. Notably, light field projection may distort on angled surfaces (keystone effect). Our phantom analysis revealed light field projection is most accurate when the bolus is positioned within 60°. Therefore, re-CT is still necessary for very lateralised tumors.
We have incorporated this new workflow into our practice, omitting re-CT for most breast cancer patients undergoing adjuvant radiotherapy, leading to improved efficiency.
