Description: Publication date: Available online 22 December 2016 Source: Radiotherapy and Oncology Author(s): Fredrik Qvarnström, Martin Simonsson, Jan Nyman, Ingegerd Hermansson, Majlis Book, Karl-Axel Johansson, Ingela Turesson Background and purpose Previously we reported that hyper-radiosensitivity (HRS) was evidenced by quantifying DNA double strand break (DSB) foci in epidermis biopsies collected after delivering radiotherapeutic one and five dose fractions. The aim of this study was to determine whether HRS was preserved throughout a 7-week radiotherapy treatment, and also to examine the rate of foci decline and foci persistence between dose fractions. Materials and methods 42 patients with prostate cancer received 7-week fractionated radiotherapy treatment (RT) with daily dose fractions of 0.05–1.10 Gy to the skin. Before RT, and at several times throughout treatment, skin biopsies ( n = 452) were collected at 30 min, and 2, 3, 24, and 72 h after dose fractions. DSB-foci markers, γH2AX and 53BP1, were labelled in epidermal keratinocytes with immunofluorescence and immunohistochemical staining. Foci were counted both with digital image analysis and manually. Results HRS in keratinocytes was evidenced by the dose–response relationships of DSB foci, observed throughout the treatment course, independent of sampling time and quantification method. Foci observed at 24 h after dose fractions indicated considerable DSB persistence. Accordingly, foci significantly accumulated after 5 consecutive dose fractions. For doses below 0.3 Gy, persistent foci could be observed even at 72 h after damage induction. A comparison of γH2AX and 53BP1 quantifications in double-stained biopsies showed similar HRS dose–response relationships. Conclusions These results represented the first evidence of preserved HRS, assessed by γH2AX- and 53BP1-labelled DSB foci, throughout a 7-week treatment course with daily repeated subtherapeutic dose fractions.