Localization and quantification of the delivered dose to the spinal cord
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1 · T. Schuster2 · N. Aswathanarayana3 · L.-S. Fromm3 · M. Molls1 · H. Geinitz4 · S. Kampfer1 1 Department of Radiation Oncology, Klinik und Poliklinik für Strahlentherapie und Radiologische
Onkologie, Technische Universität München, Klinikum rechts der Isar, Munich 2 Institute of Medical Statistics and Epidemiology, Technische Universität München, Klinikum rechts der Isar, Munich 3 Medical School, Technische Universität München, Munich 4 Abteilung für Radio-Onkologie, Krankenhaus der Barmherzigen Schwestern Linz
Localization and quantification of the delivered dose to the spinal cord Predicting actual delivered dose during daily MVCT image-guided tomotherapy
Our understanding of spinal cord tolerance to radiation is mainly based on animal models combined with some clinical data, mostly from the 2D or 3D conformal radiotherapy (3D-CRT) period [1, 2, 6]. Intensity-modulated radiotherapy (IMRT) has allowed a reduction of dose to the spinal cord during treatment of head and neck (H&N) cancer [7], but raised several issues. Due to the steep dose gradients to normal tissue, setup error reduction has become extremely important in IMRT treatments. Thus, reliance on image-guided radiotherapy (IGRT) by CT has increased. Furthermore, IMRT introduces inhomogeneous dose distributions outside the planning target volume. Preclinical studies have shown that in the presence of inhomogeneous dose distributions, the spinal cord tolerance of relatively small volumes is strongly affected by low-dose irradiation of adjacent tissue [1, 2]. The lateral white matter is more radiosensitive than the central part of the white matter and as the grey matter [11]. In order to correlate this information to the possible toxicities in cervical spinal cord of humans, we need to exactly locate the actual delivered dose in H&N cancer patients. In a previous study, we analyzed whether daily IGRT is necessary in H&N heli-
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cal tomotherapy [3]. The aim of this study was to quantify and localize the changes in the actual delivered dose to the spinal cord, relative to the planned dose, during daily IGRT and to find predictive measurements that would allow an estimation of the actual delivered dose.
Methods and materials Patient characteristics We retrospectively analyzed 20 H&N cancer patients treated with helical tomotherapy (HT; Accuray Incorporated, Sunnyvale, CA, USA): 10 nasopharynx cancer patients (definitive radiotherapy) and 10 tonsil cancer patients (postoperative radiotherapy). Sixteen patients underwent concomitant platinum-based chemoradiotherapy.
Treatment planning and delivery All patients underwent bilateral nodal radiotherapy (including the supraclavicular nodes) five fractions per week, with a boost to the high-risk regions (tumor and involved lymph nodes regions plus safety margins). The prescribed dose to the planning tumor volume (PTV) was 50 Gy
(2 Gy/day), followed by a boost up to 64 Gy (postoperative case) or 70 Gy (definitive case). Two patients were treated definitively with a
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