Clinical quality improvement of high-dose-rate brachytherapy surface treatment: 3D-printed applicators and model-based planning

Mira Moran
Waikato Regional Cancer Centre, Hamilton, New Zealand
mira.moran@waikatodhb.health.nz

Benjamin Norton
Industrial Designer, 3Done Australia Pty Ltd⁺, Brisbane, Australia
ben.norton@3d1.com.au

Cosmo Huxley
Senior Industrial Designer & Production Process Supervisor, ⁺
cosmo@3d1.com.au

Mike Chapman
Business Development Manager, ⁺
mike.c@3d1.com.au

Christian Langton
Chief Technology Officer, ⁺
christian.langton@3d1.com.au

Background and Purpose
HDR Brachytherapy Surface Applicators (BSAs) offer an alternative radiotherapy modality to external-beam electrons and kV/MV x-rays for skin lesion treatment. Issues with proprietary sheet-style BSAs arise in cases with lesions of small dimension, which may benefit from custom channel-spacing, or where the treatment geometry is curved. In-house or vendor BSAs also often lack backscatter material. TG-43 remains the international standard for calculating dose, which assumes the entire treatment area water-equivalent in density. Until TG-186 calculations are accepted clinically, dose will not be modelled accurately for BSAs where air abounds (1). Sheet bolus above the BSA is often the clinical solution, although any air gaps introduced lead to further unknown and uncontrollable uncertainties. Our in-house manufactured BSAs have also experienced source obstruction at sharp angles and the lengthy manufacturing process has rendered HDR surface treatment unfavourable. A novel 3D-printed BSA was investigated for quality improvement of the service.

Methods
A 3D-printed BSA was designed from a lower-limb CT–scan and created in soft thermoplastic-urethane (TPU). The water-equivalence of the printed BSA was investigated using film dosimetry. The effect of sheet bolus of varying thickness on the dose calculation accuracy of the treatment planning system was investigated via film and thermoluminescent dosimeters(calibrated following the methods of Haworth et al (2)).
Results
The in-house applicator with TG-43 dose calculation was found to underestimate dose to the skin surface by 10-20%. The 3D-printed BSA with inbuilt backscatter material improved dose calculation accuracy to within 5%. TG-186 further improved this accuracy to within clinically acceptable tolerance.
Conclusions
In a world where continual improvement is paramount, model-based planning should be embraced for brachytherapy to stay current and in favour. The proposed 3D-printed BSA provides a more accurate treatment option until TG-186 prescriptions are accepted internationally. Further, 3D-printed BSAs have the potential to significantly improve both conformity and workflow of brachytherapy surface treatments.

References
1) Kirisits, Christian et al, Review of clinical brachytherapy uncertainties: Analysis guidelines of GEC-ESTRO and the AAPM, Radiotherapy and Oncology 110 (2014) 199–212
2) Haworth, Annette et al, Comparison of TLD calibration methods for 192Ir dosimetry, J app clin med phys 14:1 (2013)

Mira Moran

Waikato District Health Board
Brachytherapy Physicist

Mira is in her 4th year working at the Waikato Regional Cancer Centre as the lead brachytherapy physicist. At Waikato, the brachytherapy service is always under scrutiny and development. Prior to this, she worked at Royal Berkshire Hospital in Reading, England where she carried out the NHS Scientist Training Programme. Her research experience includes R&D roles within radiobiology, ultrasound-based cell therapy and designing clinical trials for new medical devices. In her spare time, she enjoys feeding herself and others by growing her own food as well as spearfishing and ocean foraging. Other hobbies include oil painting, kayaking, skiing and travelling.