Treatment Parameters Reported for a Post-Market Surveillance Study of Electronic Brachytherapy (EBt) in Non-Melanoma Skin Cancer Patients

Abstract

Nonmelanoma skin cancer (NMSC), consisting of basal cell carcinoma (BCC) and squamous cell carcinoma (SCC), is the most common malignancy in the United States with over 5 million cases being diagnosed every year. Surgery is the standard of care for treating NMSC. Comorbidities, lesion location, cosmetic concerns, and other patient factors can lead to selection of alternative treatments. Low energy X-rays (50–150 kV) and radionuclides have been used for >100 years to effectively treat NMSC. An electronic, high dose rate brachytherapy (EBt) devices have been recently introduced but there is limited data on local control and safety. This report is focused on the dosing schemes of an EBt device used to treat NMSC in patients enrolled in a multicenter Phase IV study. Seven sites have accrued prospective and retrospective subjects for EBt treatment of BCC and SCC, with a 5-year follow up. The choice of prescription dose was based on the successful treatment of NMSC using isotope-based brachytherapy (42 Gy total dose resulted in 98% local control). This study uses fractionation schemes based on a constant biologically equivalent dose (BED) of about 70 Gy(BED) at the prescription depth (1-4 mm). Fractionation schemes with EBt allow for 6 or more fractions, twice per week, at 69-71.5 Gy(BED) at a depth of 1-4 mm. Total dose ranged from 42 – 49 Gy. At each follow up, cosmesis was graded by both patient and physician and adverse events recorded using RISRAS (acute) and CTCAE Version 4 (long-term), with the Skin Cancer Index obtained to assess patient quality of life. To date 139 subjects (151 lesions) have been accrued. Six subjects (4.3%) have exited, none for recurrence. Most subjects were male (65%) with median age of 74 years. All subjects were white, with the most common comorbidities being anticoagulant use (36.7%), smoking (22.3%), and diabetes (9.4%). The most common reason for choosing EBt was patient preference (92.8%), lesion location (65.5%) and age (60.4%). 30.2% of subjects have follow up of 12 months with 6.5% at 24 months. Lesion diameters ranged from 1-20 mm, with a median diameter of 8 mm. Lesion depth ranged from 1-4 mm (median = 2 mm). The median number of fractions (#fx) was 8 with median dose/fraction of 5.6 Gy (range 4 to 7 Gy). The most frequent applicator diameters were 15mm (33.8%) and 20mm (40.4%). Treatment depths median value was 3 mm and ranged from 2.0 mm to 3.5 mm. There was a weak association between #fx and; lesion depth (R2 = 0.24, p=0.021) and none with lesion depth (R2 = 0.11, p=0.24). A trend was seen in higher #fx in highly visible lesion locations (head, neck and arms). This is the first report of a combined prospective/retrospective phase IV study using EBt to treat NMSC. With a short median follow up (<12 mo), there were no recurrences and the adverse events were expected given the type of treatment (low energy x-rays). Subjects will continue to be followed until 5 years post-EBt to document longer-term local control, cosmesis, and adverse event rates.