The incidence of Non-Melanoma Skin Cancer (NMSC) in the Australian population is estimated to be around 2% and it continues to be an ongoing disease due to the nature of the climate and lifestyle (Perera et al., 2015). Whilst the use of Mohs Micrographic Surgery is considered the gold standard for treatment of skin cancers, the use of superficial x-rays for the treatment of skin cancers has proven to be the preferred treatment for many patients (McGregor et al., 2015). The major recipients of superficial x-ray treatment (SXRT) will be patients >70 years who may present with other co-morbidities. Further, many of the sites that may be difficult to treat with surgical means, are ideal candidates for radiotherapy. Dosimetry of kilovoltage beams is well established, but with new treatment machine design, new issues can present. This work is related to a newly released machine and the issue of electron surface dose enhancement.
In 2019 a new WOmed T105 SXRT machine (Wolf Medical, Germany) was installed in a standalone treatment clinic. As a standalone centre, they treat only with superficial energies with a high patient load. The newly installed machine was accepted, and commissioning was undertaken. As part of that process, the recommended testing, as per the ACPSEM recommendations were undertaken (Hill et al., 2018). The factory measurements for output at the end of the reference cone was compared with those undertaken during commissioning. While the work here was performed with a recommended reference chamber for output (Farmer-type), it was noted that electron enhancement at the surface maybe present due to the design of the cones.
The circular cones provided with the treatment unit were 1cm, 2cm, 3cm and 4cm open-ended stainless-steel cones at 15cm SSD. In addition, there were two circular close-ended cones (Perspex); 5cm at 15cm SSD and 10cm at 30cm SSD. All the reference measurements were performed using the 5cm close-ended cone.
Electron dose enhancement is known consequence of the metal cones used for treatment. Close-ended cones, with a thin window of Perspex or other medium, provide sufficient shielding to remove this enhanced dose. Without the addition of an absorbing material the dose measured at the end of the cone and delivered to the patient can be enhanced significantly due to electrons depositing in the first few nm of skin.
Reference documents recommend that chambers used sufficiently absorb these electrons, and as such the Farmer chamber measurements are a true measure of dose due only to kilovoltage photons. Unfortunately, in some open cones the dose being received by the surface skin layers will have this enhanced dose present and could potentially have higher than expected dose to treatment volumes (Klevenhagen et al., 1991).
The Australasian recommendations to remove the electron dose is to coat the inner 10cm of the cone with nail varnish (Hill et al., 2018). The acrylic of the nail varnish sufficiently absorbs the electrons produced. This does provide an inexpensive method to alleviate the issue but is not clear in how many coats are sufficient. Further, the nail varnish will degrade over time, requiring it to be reapplied and potentially being a hygiene hazard when the cones are positioned directly in skin contact.
Nail varnish is most commonly a nitrocellulose dissolved in a solvent like ethyl acetate. As the solvent evaporates, the resin in the nitrocellulose hardens forming a coating on the surface. Application of the nail varnish to the inner surface of the cones can be both time consuming and may give a non-uniform thickness throughout the cone. An alternative proposed here is to use a 3D printed sleeve inserted into the cone.
3D printing is available within many clinical departments with the application including bolus for megavoltage treatment and to provide customised shielding in SXRT applications (Crowe et al., 2021). Further, many home users have been able to print small items using available resin. Online CAD software means that the design of and printing is now within reach of any user. Small commercial companies providing reliable printing services can offer a range of materials once the design has been sent directly to them.
Polylactic Acid (PLA) is the most used material in desktop extrusion printers as it requires a low heat to print, does not require a heated bed, and is relatively inexpensive. It has a physical density of 1.210–1.430 g/cm3, and when printed in a thin layer is still relatively robust (Polyactic Acid, 2015). It was proposed that a sleeve of PLA could be inserted into the treatment cone as an alternative to the nail varnish method.
PLA has been assessed in its use for external beam radiotherapy (Van der Walt et al., 2019) and found to be a safe and effective material to use for electron and photon modalities for bolus. Other investigators have examined the use of PLA in 3D printing and the uncertainty and reproducibility in manufacture (Craft et al., 2018). In applications where the radiation beam will be passing through the material, the issues related the density and production are integral, but in this application, any minor variation in production will have no consequence. The work here examines the reduction in electron enhancement dose when using the PLA sleeve.
Anecdotally, the use of plastic wrap has been proposed to reduce the enhanced dose at the surface. Plastic wrap is most used in the food industry and home applications. It is generally a form of PolyVinyl Chloride but depending upon manufacturer could also be a low-density polyethylene. It is generally found in thicknesses in the range of 7.6-10 µm (Polyvinyl Chloride PVC: Properties, Benefits & Applications, 2019). It was investigated here as another method of dose reduction that may be achievable in the clinic.