PO10: Case Presentation and Workflow for a 3D Printed Mold for HDR Brachytherapy of the Lower Lip

Abstract

<h3>Introduction</h3> Interstitial head and neck brachytherapy is a versatile tool but requires intense patient and clinical resources as well as a high level of physician skill. For certain lesions, 3D printed brachytherapy molds may provide an "easier" alternative to interstitial brachytherapy. Recently, a patient presented with a progressive recurrent squamous cell carcinoma (SCC) in situ of the lower lip two years after prior Mohs surgery. As surgery for removal of the lesion would be disfiguring, radiotherapy was pursued. Electron radiotherapy would be difficult due to the curved surface. Thus, brachytherapy options were considered, however the use of interstitial brachytherapy would have been technically difficult as well as invasive. Commercial surface brachytherapy applicators would not fit the lesion and location, and therefore, a 3D printed brachytherapy applicator was utilized. <h3>Materials and Methods</h3> To create the 3D printed mold, two CT scans were required prior to mold design - one scan with radiopaque wires to define the target volume on the CT scan, and a second scan without wires, used to accurately define the patient surface for the 3D-printed mold. In this instance, the lower lip had to be rolled outward to expose the lesion and had to be immobilized in this position. Immobilization of the lower lip was achieved with adhesive attached to the lip and chest (Figure 1.a.). A standard bite block was inserted into the patient's mouth to keep the teeth in a reproducible position with the final intent that the 3D mold could be designed to attach to this bite block. CT scans were acquired at 1.25 mm thickness. A 3D mold was designed using our standard process in the Adaptiiv 3D bolus software. A pre-plan was created with the mold design in BrachyVision (Varian, Palo Alto, CA) to determine if the desired dosimetry to the target volume was achievable. Once a final mold design was approved, printing utilized an Axiom20 3D printer with Cheetah TPU. <h3>Results</h3> The final mold is shown in Figure 1.b and 1.c., illustrating that the mold was designed to fit over the pegs of the bite block. Several mold designs were considered, and ultimately an open bottomed mold was selected (Figure 1.d.) to allow for visualization of setup and also in case the lip swelled during treatment. A final CT of the patient with the mold in place was obtained to evaluate final mold fit and perform final CT planning (Figure 1.f.), as well as to acquire setup measurements (Figure 1.e.). Air gaps were minimal and only in two locations over the treatment volume which were all ≤1.5mm. The patient received 40 Gy in 10 fractions every other day. Near the end of treatment, topical lidocaine was applied to the patient's lip to help with applicator tolerance. Follow-up pictures are shown in Figure 1.g-i. Some moist desquamation was visible at the 1 week follow-up but resolved by six weeks posttreatment. At 10 months, the patient shows no signs of recurrence and no significant treatment-related toxicities. <h3>Conclusions</h3> A recurrent SCC in situ of the lower lip was successfully immobilized, planned, and treated with HDR brachytherapy using a custom 3D-printed mold. The benefits of the 3D mold allow for rigid immobilization of the lip for a reproducible and comfortable setup. Further research is needed into the optimal indications and applicator designs for 3D-printed applicators for head and neck brachytherapy.