INTRODUCTION
Radiotherapy for non-melanoma skin cancer (NMSC) offers significant advantages over surgical treatment as a tissue-preserving modality, with good local control rates. Therefore, to increase the local control rate with radiotherapy without surgery, it is important to accurately apply high doses of radiation [1]. Facial NMSCs located on the nose, including the nose dorsum or tip, require a concentrated dose of radiation to the skin by supplementing for anatomical non-uniformity. Even with bolus coverage, the irregular shape of the human body creates a gap between the bolus and the skin, leading to errors and uncertainty in patient positioning [2]. Previous studies have demonstrated the effectiveness of 3D bolus therapy [3,4], but this cannot be applied to the patient immediately when computed tomography (CT) simulation is performed, and it takes time to manufacture. In this study, we aimed to develop a patient-specific immobilization device for radiotherapy that reflects the nasal morphology of the patient and controls the irregular skin contours to minimize the gap.
We initially designed the patient-specific immobilization device for patients with NMSC of the nose and attempted to verify its application to radiotherapy.
MATERIALS AND METHODS
A patient-specific immobilization device was developed during CT simulation and radiotherapy. During the development of the immobilization device, non-metallic materials were used to prevent radiation attenuation, and a marking structure was considered to ensure reproducibility during expansion within the nose. The overall appearance of the device is shown in Figure 1. A rail-type structure was used to facilitate the bilateral movement of the extension tip, and a graduated structure was attached to the rail to ensure reproducibility of the tip position. A rounded nail tip model was used for the nasal extension portion to expand irregular areas on the side of the nose. A rubber facial model was created for the application of the developed device. The model was made of rubber silicone using a facial frame, and space was secured to replicate the human nasal cavity. A 2-mm-thick CT scan was performed using the patient-specific immobilization device to confirm the bolus application. However, to verify the effectiveness of the developed device, no strong force was applied when using tape to adhere the bolus to the skin surface, and no device was used to increase the adhesive strength.
The acquired images were fused using the ridge body registration method in the Eclipse RTP System (Varian Medical System, Palo Alto, CA, USA). The same slices of both images were then visualized using the slice synchronization function.
To determine geometric differences, the length of the nasal cavity was measured before and after device insertion. Additionally, the air gap between the bolus and skin surface on the nose was measured based on the point at which the air gap was reduced by the immobilization device in each slice.
As this study involved an immobilization device and simulation phantom, approval from an institutional review board or informed consent were not required.
RESULTS
When the patient-specific immobilization device was applied, the geometric distance between the skin and bolus was narrowed. Among the 25 slices with the bolus attached, the air gap in seven slices affected by immobilization was analyzed three times in each slice. The reduced value at each slice indicated a value in the rage of 1.2 to 2.4 mm, and the average distance decreased was 1.8 mm. As shown in Figure 2, when the device was used at a specific location, the distance between the skin and the bolus was confirmed to be reduced from 4.4 to 3.2 mm, a narrowing of 1.2 mm. When comparing the point with the 2.4 mm gap of the skin and bolus before device application, after the device was applied, it was confirmed that the air gap disappeared at the same position. The vertical distance from the tip of the nose to the point where the bolus was in contact with the skin was 8.3 mm when the device was not used and 15.9 mm when the device was used, indicating that it could be improved to approximately 7.6 mm, as shown in Figure 3.
DISCUSSION
By measuring the gap between the skin and bolus at the same location in the same cross-section of the CT image, we confirmed that the gap could be reduced to an average of 1.8 mm (range, 1.2-2.4). Previous studies have shown that unnecessary air gaps of 4-10 mm can affect the surface dose by approximately 4-10% [3,5,6]. Reducing the size of the air gap is clinically important for maintaining the planned surface dose in radiotherapy involving the skin. Therefore, the patient-specific immobilization device designed in this study addresses the well-known limitations of standard flat-bolus materials, which often leave residual air pockets over irregular facial contours such as the nose.
The design emphasizes non-metallic materials to prevent beam attenuation and incorporates a rail and scale structure that enables reproducible positioning of a rounded nail-tip expander that stretches the nasal skin locally. This concept is similar to patient-specific or 3D-printed bolus fixation systems that improve conformity and reduce air gaps, but provide an inexpensive and readily applicable option at the time of CT simulation without a delay in the 3D printing workflow [2,7-9]. A silicone rubber facial phantom with a realistic nasal cavity was fabricated to verify the mechanical feasibility of the device and to demonstrate that image registration and gap measurements in the treatment planning system can capture geometric effects prior to clinical application.
However, this study has several limitations. First, despite the use of plastic, high-density artifacts resembling metals were observed in some components of the CT images, suggesting the need for further material optimization to minimize artifacts while maintaining sufficient mechanical strength for expansion. The nail tip itself may not have sufficient mechanical strength to securely anchor to the skin and may need to be replaced or reinforced with a lower-density polymer or an improved belt adhesive system that does not create additional high-density structures. Second, because this study relied solely on rubber phantoms without clinical patients, we were unable to quantify the dosimetric effects on the extent of the actual tumor and skin toxicity. We also did not know the reproducibility between fractions in vivo.
The patient-specific immobilization device developed in this study can reduce the air gap between the skin and bolus, enabling personalized applications. Therefore, future studies should conduct a patient-based feasibility study to integrate this immobilization device into routine CT simulation and treatment and analyze the correlation between geometric air-gap reduction, reproducibility, detailed dosimetric analysis, and clinical outcome measures such as local control and acute skin reactions. Comparative studies with existing standard treatments, such as commercial planar boluses, handmade customized boluses, and 3D-printed boluses, will clarify whether this device is suitable for the treatment of nasal NMSC in terms of cost, preparation time, and dosimetric performance. Furthermore, a modular design, which can be applied to various nasal morphologies and adjacent facial areas, could broaden the scope of application, and the systematic evaluation of artifact-free materials could improve image quality and treatment planning accuracy.
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