Organ motion Clinical use of a novel in vivo 4D monitoring system for simultaneous patient motion and dose measurements Amanda J. Cherpak a,b,⇑ , Joanna E. Cygler a,b , Steve Andrusyk a , Jason Pantarotto a , Robert MacRae a , Gad Perry a a The Ottawa Hospital Cancer Centre, Canada; b Department of Physics, Carleton University, Ottawa, Canada article info Article history: Received 10 February 2011 Received in revised form 24 August 2011 Accepted 25 August 2011 Available online 30 September 2011 Keywords: RADPOS Breathing motion In vivo dosimetry MOSFET abstract Purpose: A new 4D in vivo dosimetry tool, RADPOS, has been used on lung cancer patients to evaluate the feasibility of using the detectors to characterize variations in patient breathing patterns as well as to monitor daily variations in dose. Methods and materials: The RADPOS system combines a MOSFET dosimeter with an electromagnetic posi- tioning sensor for simultaneous measurement of real-time dose and spatial coordinates. Three RADPOS sensors were placed on patients’ chest and abdomen during a 4DCT and daily treatments. A fourth detec- tor was also placed on the couch as reference. Position data were collected in real-time and total dose was read at the end of each fraction. Results: Significant deviations in surface motion have been found between the day of 4DCT and treatment fractions in 9 of 10 patients. Variations in daily dose ranged from 2.5 to 13.7 cGy (2.8–14.0%) and results agreed with treatment plan values for all but three points. Conclusions: Changes in breathing motion have been found that emphasize a need for continued position monitoring. RADPOS measurements can be used to monitor such variations as well as to measure surface dose without any disruption to the treatment schedule or discomfort to patients. Ó 2011 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 102 (2012) 290–296 Respiratory-induced tumor motion is a main concern in radia- tion treatment planning of lung cancer patients [1]. One method to address this concern is to plan according to a maximum-inten- sity projection of the target from a 4DCT scan [2]. Respiratory cor- related 4DCT is used during treatment planning to define treatment volumes based on the displacement of the tumor during the scan. While this process assumes that the patient’s breathing pattern (and therefore tumor motion) measured on the day of the planning CT remains relevant throughout the entire treatment process, it has been shown that this assumption is not always valid [3–5]. Tumor motion can be unpredictable and vary over the course of treatment as well as during a single fraction [3] resulting in increased irradiation of healthy tissue, a geographic miss of the target or both [6]. Image-guidance systems such as CBCT (cone- beam CT) can be used to verify patient position before treatment begins [7]. This reduces the risk of set-up errors, which can have large consequences on delivered dose [8]. It does not, however, ac- count for intra-fraction motion throughout the duration of the treatment, which can be between 15 and 45 min for some im- age-guided IMRT treatments [9]. Tumor motion during treatment can be directly measured by implanting fiducial markers into the tumor. The movement of these fiducials can then be tracked using fluoroscopy, which deliv- ers extra dose to the patient. The implantation of fiducials is also associated with higher levels of patient morbidity [10]. It is more practical to measure a surrogate for tumor motion in the clinic. Commonly used methods of external motion measurement during treatment include spirometry [11], infrared markers [12], and elec- tromagnetic positioning systems [3]. A spirometer is a device that measures air flow in and out of a patient’s mouth. This provides an indirect measure of lung volume but no quantitative measure- ments of position changes. Infrared markers can be placed on a pa- tient’s chest and are used in conjunction with cameras inside the treatment room to measure external surface motion. Qualitative information can be extracted; however the markers must be con- tinuously visible to numerous cameras to enable 3D data collec- tion. Electromagnetic positioning systems measure a detector’s response to a transmitted magnetic field to calculate 3D position displacements. They provide similar data to optical tracking sys- tems, however they benefit from several advantages such as low cost, ease of calibration, and quick set-up [3]. The major disadvan- tage of such systems is the interference caused by metals. Special corrections have to be applied when metallic materials are in close vicinity to the transmitter or position sensor [13,14]. Optical 0167-8140/$ - see front matter Ó 2011 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.radonc.2011.08.021 ⇑ Corresponding author at: The Ottawa Hospital Cancer Centre, 501 Smyth Rd., Box 927, Ottawa, Ontario, Canada K1H 8L6. E-mail address: acherpak@ottawahospital.on.ca (A.J. Cherpak). Radiotherapy and Oncology 102 (2012) 290–296 Contents lists available at SciVerse ScienceDirect Radiotherapy and Oncology journal homepage: www.thegreenjournal.com