Towards the development of a new technique for mastectomy‐scar electron beam treatment using a variable collimator and computer‐automated treatment table control

Abstract

Breast cancer in women is the most prevalent cancer in both developed and less developed countries, with an incidence of about 2.3 million new cases diagnosed in 2020, which is about 25% of all cancers in women. Several treatment options and techniques are available for the treatment of breast cancer, including surgery, chemotherapy, hormone therapy and radiotherapy, with treatment sometimes involving a combination of these techniques. Nonrandomized studies comparing breast conservation therapy with either electron beam therapy or interstitial implants, showed that there was no statistical difference between the treatment techniques in terms of disease-free survival, cosmesis, local tumour control or morbidity. The use of orthovoltage x-ray beam treatment has also been considered. It has some advantages, such as less shielding requirements and lower equipment costs. However, it has some drawbacks, such as the limitation on the beam penetration, leading to the maximum dose being deposited on the patient's skin, and also requiring longer treatment times because of the lower dose rates. From a South African context, orthovoltage therapy has become obsolete, as the handful of machines are not in operation with a decline worldwide in favour of using linacs. With linacs becoming more widely used and accessible, electron beam therapy remains a mainstay in treating breast cancer compared to other available treatment modalities.

Patients showing locally advanced breast cancer have a risk of both distant and local-regional recurrence. Postmastectomy radiation therapy after surgery reduces the risk of recurrence and improves disease-free and overall survival. A patient undergoing postmastectomy radiotherapy may receive a mastectomy scar electron boost to reduce the risk of local recurrence. Compared with other treatment modalities such as orthovoltage, brachytherapy or photon therapy, electron beam treatment reduces the dose to underlining tissue beyond the target volume and offers a more uniform dose distribution (Khan, 2003). The energy of the electron beam to be used for treatment is determined by the thickness of the breast tissue from the chest wall to the skin (Griem and Hardin, 1998). The physical properties of electron beams make them suitable for chest wall treatment in breast treatment; the crucial motivation for the use of electron beam irradiation is the shape of the depth dose curve. The electron beam, however, needs to be collimated right down to the patient's surface using applicators and cutouts, which are positioned on the applicator as close as possible to the patient to provide a more customized field shape. Due to the disadvantages of traditional electron beam shaping using cutouts, this study proposes an alternative technique which retains the applicator but replaces the end of the applicator with a variable field shaping device allowing the collimator to deliver the dose over the entire region that would have commonly been treated using a cutout. To achieve the required dose distribution, the new technique makes use of the sliding window technique and incorporates the automated movement of the treatment table. This study aimed to develop a new treatment technique for treating the mastectomy scar using a collimated electron beam and a computer-controlled automated treatment table. The research set out to achieve the following objectives: develop a functional variable field shaping device that will allow treating the mastectomy scar; conduct measurements for the new technique; automate the couch movement for automated patient treatment of breast scar and have a working treatment technique. The proof of concept was carried out in developing a technique for treating mastectomy scars that used a variable collimator and automated treatment table. The treatment technique is an alternative to conventional treatment techniques that use lead and Wood’s-alloy electron cut-outs.