Session Item

3D printing allows for brachytherapy (BT) skin applicators adapted to challenging body contours like nose, ears, etc. Such patient specific applicators require proper verification. Therefore, a workflow for 2D dosimetric verification of these applicators was investigated. Here, we use EBT3 Gafchromic film measurements in an anthropomorphic head and neck (H&N) phantom (CIRS, USA) for an end-to-end verification of a high-dose-rate BT nose-skin treatment.

To generate the patient specific applicator, a CT scan (Somatom definition edge, Siemens Healthcare GmbH, Germany) was acquired of the H&N phantom in supine position. Delineation of a CTV in the region of the left nose wing and cheek was performed by a physician. Next, a 3D mesh of the body and CTV were exported from Eclipse (Varian, USA) using scripting. Subsequent design of the applicator was done in TriMatic (Materialise, Belgium). Within a 10 mm thick bolus, nine channels (2.2 mm diameter) were designed 5 mm from the patient surface and spaced 1 cm apart from each other. The direction, location and number of channels were manually optimized for target coverage and minimal local curvature to assure proper passage of the BT-source. The applicator was printed on a Raise3D N2+ 3D printer (Raise3D, Netherlands) using PLA (ICE filaments, Belgium) with 100% infill and 2 shells.

For treatment planning, a second CT was acquired of the applicator positioned on the phantom, with 5f BT catheters and radio-opaque markers in place. Next, the dosimetric planning was performed in Oncentra Brachy 4.5.2 (Elekta, Netherlands) with a 7 Gy prescription at 5 mm under the body surface.

The treatment was delivered with an Ir-192 Flexitron afterloader (Elekta, Netherlands) with laser cut EBT3 films positioned in the three sagittal planes of the phantom. The films were scanned and converted to dose using a calibration procedure for page-sized films and triple channel correction methodology¹.

Dose comparison of the three films to the Oncentra dose calculation showed excellent agreement with an overall agreement score (AS) greater than 98% for gamma analysis² with a 5% dose difference, 2mm distance-to-agreement and 10% dose threshold criteria. For each analysis, a ROI was manually selected corresponding to the outline of the film. The figure below shows the good agreement between the measured (blue) and calculated (red) dose profiles trough the high dose region.

The presented E2E verification proves to be a useful verification methodology for customized 3D printed BT applicators. Here, it was applied to a relative complex surface geometry. The presented results showed excellent agreement between dose calculation and measurements using the 3D applicator. Future work is the investigation of more challenging situations including cavities (e.g. nostrils) and interstitial BT. Although EBT3 film shows its usefulness, reusable OSL films can be explored as an instant alternative allowing for pre-treatment BT dosimetry.