Radiobiology of glioblastoma Pulsed low-dose irradiation of orthotopic glioblastoma multiforme (GBM) in a pre-clinical model: Effects on vascularization and tumor control Joshua T. Dilworth, Sarah A. Krueger, Mohamad Dabjan, Inga S. Grills, John Torma, George D. Wilson, Brian Marples ⇑ Department of Radiation Oncology, Beaumont Health System, Royal Oak, USA article info Article history: Received 22 February 2013 Received in revised form 23 April 2013 Accepted 26 May 2013 Available online 19 June 2013 Keywords: PLRT Pulsed radiation Orthotopic Glioblastoma multiforme Animal model abstract Background and purpose: To compare dose-escalated pulsed low-dose radiation therapy (PLRT) and stan- dard radiation therapy (SRT). Methods and materials: Intracranial U87MG GBM tumors were established in nude mice. Animals received whole brain irradiation with daily 2-Gy fractions given continuously (SRT) or in ten 0.2-Gy pulses sepa- rated by 3-min intervals (PLRT). Tumor response was evaluated using weekly CT and [ 18 F]-FDG-PET scans. Brain tissue was subjected to immunohistochemistry and cytokine bead array to assess tumor and nor- mal tissue effects. Results: Median survival for untreated animals was 18 (SE ± 0.5) days. A significant difference in median survival was seen between SRT (29 ± 1.8 days) and PLRT (34.2 ± 1.9 days). Compared to SRT, PLRT resulted in a 31% (p < 0.01), 38% (p < 0.01), and 53% (p = 0.01) reduction in normalized tumor volume and a 48% (p < 0.01), 51% (p < 0.01), and 70% (p < 0.01) reduction in tumor growth rate following the administration of 10 Gy, 20 Gy, and 30 Gy, respectively. Compared to untreated tumors, PLRT resulted in similar tumor vascular density, while SRT produced a 40% reduction in tumor vascular density (p = 0.05). Compared to SRT, PLRT was associated with a 28% reduction in degenerating neurons in the surrounding brain parenchyma (p = 0.05). Conclusions: Compared to SRT, PLRT resulted in greater inhibition of tumor growth and improved sur- vival, which may be attributable to preservation of vascular density. Ó 2013 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 108 (2013) 149–154 Glioblastoma multiforme (GBM) accounts for the majority of primary malignant central nervous system tumors in adults. The diagnosis of GBM portends a poor prognosis, with a median sur- vival of approximately fifteen months for a patient treated with the current standard of care [1]. When feasible, surgery constitutes the initial management of GBM, and the extent of resection has been shown to correlate with clinical outcomes [2]. The infiltrative nature of these tumors, however, makes eliminating clinically oc- cult disease virtually impossible and highlights the need for post- operative therapy. A landmark study defined the current paradigm for adjuvant treatment, reporting a 1-year overall survival of 61% for patients treated with surgery, radiotherapy, and the systemic agent temozolomide [1]. Patients with GBM almost uniformly relapse, and the vast majority of tumor recurrences occur within the irradiated field. While one randomized clinical trial showed that increasing the prescription dose from 45 Gy to 60 Gy improved progression-free and overall survival [3], subsequent randomized studies have re- ported no additional survival benefit to doses beyond 60 Gy [4,5]. Furthermore, doses as high 90 Gy did not alter failure patterns: al- most 80% of the time, greater than 95% of the recurrent tumor fell within the irradiated field [4]. These data may reflect a limit to the effectiveness of irradiation with respect to tumor kill when admin- istered in standard (i.e., continuous 2-Gy) daily doses. Additionally, the risk of normal tissue damage with therapeutic doses of stan- dard radiation is particularly prominent in the context of GBM treatment, since large volumes of normal brain parenchyma are necessarily included in radiation treatment fields to account for gross or microscopic residual disease. The need to increase the therapeutic ratio when treating GBM has prompted an investiga- tion into alternative radiation fractionation schedules. In vitro studies have shown that rapidly proliferating cells exhi- bit a relatively higher probability of cell death per unit dose in re- sponse to radiation doses equal to or less than 0.2 Gy, an effect referred to as low-dose hyper-radiosensitivity [6]. At or below this threshold 0.2 Gy dose, insufficient numbers of DNA double strand 0167-8140/$ - see front matter Ó 2013 Elsevier Ireland Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radonc.2013.05.022 ⇑ Corresponding author. Address: Department of Radiation Oncology, William Beaumont Health System, 3601 West 13 Mile Road, Royal Oak, MI 48073, USA. E-mail address: Brian.Marples@beaumont.edu (B. Marples). Radiotherapy and Oncology 108 (2013) 149–154 Contents lists available at SciVerse ScienceDirect Radiotherapy and Oncology journal homepage: www.thegreenjournal.com