Candice Deans
GenesisCare Oncology Queensland
Brachytherapy Services Manager

Holly Stephens
Medical Physics Registrar
GenesisCare Oncology Queensland, Australia/University of Adelaide, South Australia, Australia

David Schlect
Radiation Oncologist
GenesisCare Oncology Queensland, Australia

Tanya Kairn
A/Prof Principal Medical Physicist
Royal Brisbane and Women’s Hospital, Queensland, Australia/Queensland University of Technology, Queensland, Australia

Background

Developing a non-invasive method for treating skin cancer led to the investigation of HDR Brachytherapy. Commercially available skin applicators such as Valencia or Leipzig are not suited to the lower eyelid if the treatment field does not conform to a circle. Designing an applicator to suit the semi-circular area under the lower eyelid led to investigations using a flexible catheter. Limitations of TG43-UI dose calculations within the planning system required a design that provided adequate scatter without being uncomfortable for patients.

Methods

Radiochromic film dosimetry was used to test and identify the minimum thickness of bolus material required to produce the same result as 10mm. Film was placed in blue tissue equivalent blocks at different depths and irradiated.

Both wax and thermoplastic were tested as moulds.

Two types of catheter were tested: Flexible Implant Tubes and Lumencare Azure lumencaths (Elekta AB, Stockholm Sweden) for suitably.

Integrity testing of the lumencath catheters was conducted after immersion in 76˚C thermoplastic and over multiple transfers to ensure multiple fractions of treatment could be completed with a single mould.

Results

The measured dose fall-off in the phantom, when the catheter was surrounded by 3 mm of thermoplastic, was found to be sufficiently similar to the dose fall-off when the catheter was covered by a thick layer of bolus. This indicates that 3 mm of thermoplastic provides adequate backscatter to allow the TG-43-based treatment planning dose calculation to provide an accurate dose distribution, even though the target volume (in the patient's skin) is surrounded by air. Practical testing of thermoplastic applicator construction indicated that a 6 mm thickness of thermoplastic between the flexible catheter and skin produced fewer air gaps and while reducing the dose uncertainty arising from the close proximity of the source in the high dose region.

Conclusion

6mm of thermoplastic between skin and catheter produced equivalent results and satisfactory dosimetry on the phantom used to assess appropriateness for clinical use. The use of a lumencath, despite not designed for such a purpose, was proven to be a superior alternative to standard flexible catheters.

References

1. T. Aland, T. Kairn, J. Kenny, “Evaluation of a Gafchromic EBT2 film dosimetry system for radiotherapy quality assurance”, Australasian Physical and Engineering Sciences in Medicine34(2): 251-260 (2011)

2. S. C. Peet, R. Wilks, T. Kairn, J. V. Trapp, S. B. Crowe, “Calibrating radiochromic film in beams of uncertain quality“, Medical Physics 43(10): 5647-5652 (2016)


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