Total body irradiation—an attachment free sweeping beam technique

Härtl et al. Radiation Oncology (2016) 11:81 DOI 10.1186/s13014-016-0658-y RESEARCH Open Access Total body irradiation—an attachment free sweeping beam technique Petra M. Härtl†, Marius Treutwein*† , Matthias G. Hautmann, Manuel März, Fabian Pohl, Oliver Kölbl and Barbara Dobler Abstract Introduction: A sweeping beam technique for total body irradiation in standard treatment rooms and for standard linear accelerators (linacs) is introduced, which does not require any accessory attached to the linac. Lung shielding is facilitated to reduce the risk of pulmonary toxicity. Additionally, the applicability of a commercial radiotherapy planning system (RTPS) is examined. Material and Methods: The patient is positioned on a low couch on the floor, the longitudinal axis of the body in the rotational plane of the linac. Eight arc fields and five additional fixed beams are applied to the patient in supine and prone position respectively. The dose distributions were measured in a solid water phantom and in an Alderson phantom. Diode detectors were calibrated for in-vivo dosimetry. The RTPS Oncentra was employed for calculations of the dose distribution. Results: For the cranial 120 cm the longitudinal dose profile in a slab phantom measured with ionization chamber varies between 94 and 107 % of the prescription dose. These values were confirmed by film measurements and RTPS calculations. The transmittance of the lung shields has been determined as a function of the thickness of the absorber material. Measurements in an Alderson phantom and in-vivo dosimetry of the first patients match the calculated dose. Discussion and conclusion: A treatment technique with clinically good dose distributions has been introduced, which can be applied with each standard linac and in standard treatment rooms. Dose calculations were performed with a commercial RTPS and should enable individual dose optimization. Keywords: Total body irradiation, Sweeping beam technique, Lung shielding, Dosimetry, 3D treatment planning Background Total body irradiation (TBI) plays a prominent role especially in the myeloablative conditioning prior to hematopoietic stem cell transplantation [1]. Many different schemes regarding the total dose, the fractionation and the dose rate are reported [2, 3]. However, 12 Gy in 6 fractions on 3 days is a very common myeloablative condition scheme [3, 4]. From 1995 to 2013 at our department different schemes have been applied [5] using a sweeping beam technique as described by Müller [6]. This technique not only used a gravity oriented shaped filter to * Correspondence: † Equal contributors Department of Radiotherapy, Regensburg University Medical Center, Regensburg, Germany compensate the effects of inverse square variation of the fluence with distance as it has later been investigated by Chui et al. [7], but also allowed the application of a set of lung shields close to the collimator. The aim of this study was to establish a treatment procedure with similar parameters, when the linacs which had been employed for this sweeping beam technique had to be replaced [8, 9]. No accessory should be attached directly to the machine to avoid a certification process for in-house developed equipment [10]. Lung shielding should be facilitated to reduce the pulmonary toxicity [2, 11] as it was possible with the former technique. Abandoning of the gravity oriented accessory was a precondition to enable the calculation of the dose distribution with a commercial RTPS in clinical routine [12]. Although the application of a commercial RTPS is © 2016 The Author(s). Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Härtl et al. Radiation Oncology (2016) 11:81 still quite uncommon in TBI with Linacs, as the technical conditions cannot be modelled for many of the applied techniques, single cases have been reported earlier [12–15]. In the recent years further adaptions of RTPS for TBI have been presented [16, 17]. This study presents the measurements required for the introduction of the new technique and the results of the in-vivo dosimetry of the first plans. Methods Linac, couch and patient positioning Two linacs of type Elekta Synergy™ with Agility™ head (Elekta Ltd., Crawley, UK) and photon energies of 6 and 15 MV and electron energies of 6, 8, 10, 12 and 15 MeV were used for this study. Both linacs have been matched [18] and conform the requirement of a back-up concept, guaranteeing the completion of the treatment in time in case of machine breakdown [11, 19]. The patient is positioned on a low couch on the floor in supine and prone position, the longitudinal axis in the rotation plane of the gantry (Fig. 1). The positioning is supported by a soft mask allowing free air flow in the prone direction. The couch top is located 117.5 cm below the isocenter. A plate of Makrolon® polycarbonate of 10 mm thickness is placed on a stand above the patient to reduce the buildup effect in the patient [11]. The distance between the couch and the polycarbonate plate is 33 cm. The plate also serves as tray for the lung shields. The top of the patient’s head is always 60 cm from the vertical isocenter plane and is represented by the longitudinal position l = 0 cm. The dose reference point was defined on the vertical axis through the isocenter in the middle of the diameter of the patient or phantom at l = 60 cm. In most cases this is close to the umbilical transverse plane as a quite common reference Page 2 of 8 point [19–21]. The position of the feet depends on the patient’s body length. The low diameter in the cervical region is partially compensated in prone and supine positioning by a bolus of plastic modelling mass. Treatment fields The photon beam energy chosen for TBI is 6 MV as rather common [11]. The main dose contribution is given by rotational fields (arcs) alternating from 310° to 70° clockwise and reverse. A collimator angle of 90° ensures a constant field width of 10 cm at the isocenter in the sweeping direction limited by the solid jaws. The multileaf collimator is set to an opening of 40 cm to exceed the couch width. A number of eight arcs per patient position and a dose rate of 300 monitor units (MU) per minute has been chosen to achieve a low mean lung dose rate [21] which has been discussed as a parameter to reduce pulmonary toxicity [20, 22–24]. For the compensation of the effects of inverse square variation of the fluence with distance, additional fi (...truncated)