The development of a 4D treatment planning methodology to simulate the tracking of central lung tumors in an MRI‐linac

Abstract

PurposeTargeting and tracking of central lung tumors may be feasible on the Elekta MRI‐linac (MRL) due to the soft‐tissue visualization capabilities of MRI. The purpose of this work is to develop a novel treatment planning methodology to simulate tracking of central lung tumors with the MRL and to quantify the benefits in OAR sparing compared with the ITV approach.MethodsFull 4D‐CT datasets for five central lung cancer patients were selected to simulate the condition of having 4D‐pseudo‐CTs derived from 4D‐MRI data available on the MRL with real‐time tracking capabilities. We used the MRL treatment planning system to generate two plans: (a) with a set of MLC‐defined apertures around the target at each phase of the breathing (“4D‐MRL” method); (b) with a fixed set of fields encompassing the maximum inhale and exhale of the breathing cycle (“ITV” method). For both plans, dose accumulation was performed onto a reference phase. To further study the potential benefits of a 4D‐MRL method, the results were stratified by tumor motion amplitude, OAR‐to‐tumor proximity, and the relative OAR motion (ROM).ResultsWith the 4D‐MRL method, the reduction in mean doses was up to 3.0 Gy and 1.9 Gy for the heart and the lung. Moreover, the lung's V12.5 Gy was spared by a maximum of 300 cc. Maximum doses to serial organs were reduced by up to 6.1 Gy, 1.5 Gy, and 9.0 Gy for the esophagus, spinal cord, and the trachea, respectively. OAR dose reduction with our method depended on the tumor motion amplitude and the ROM. Some OARs with large ROMs and in close proximity to the tumor benefited from tracking despite small tumor amplitudes.ConclusionsWe developed a novel 4D tracking methodology for the MRL for central lung tumors and quantified the potential dosimetric benefits compared with our current ITV approach.

Synthetic 4D-CT of the thorax for treatment plan adaptation on MR-guided radiotherapy systems

“…In this article, three methods to calculate 4D-sCT were introduced and validated in MidP against 4D-CT. Employing 4D/MidP-sCT on hybrid MRgRT systems would enable plans to be adapted for anatomical differences and changes in respiratory pattern throughout the course of radiotherapy treatment, which might permit target dose boosting and sparing OARs (Al-Ward et al 2018 ), whilst mitigating the risk of registration errors between CT and MRI. Moreover, 4D-sCT could be combined with motion information from fast 2D cine MRI to obtain a patient-specific motion-model (Stemkens et al 2016 ), which might be applied to generate low-latency volumetric sCT.…”

“…Patient specific lung electron density values should therefore be implemented to account for underlying lung pathology (Rosenblum et al 1980 , Soejima et al 2000 , Durham and Adcock 2015 ). Furthermore, four-dimensional (4D) or midposition (MidP) (time-weighted mean position of the respiratory cycle) (Wolthaus et al 2008a ) sCT might be employed to account for respiratory motion in dose reconstruction (Al-Ward et al 2018 ).…”

Movement assessment of breast and organ-at-risks using free-breathing, self-gating 4D magnetic resonance imaging workflow for breast cancer radiation therapy

“…Organ motion was quantified by measuring the minimum distance between heart (OAR) and chest wall at end of exhale and end of inhale, and in DIBH [15] . In addition, the anterior-posterior and lateral relative OAR motion (OAR movement between inhale and exhale) for heart and chest wall were assessed [16] .…”

MRI-guidance for motion management in external beam radiotherapy: current status and future challenges

“…However, for advanced delivery strategies, such as multi-leaf-collimator tracking and active scanning proton therapy (Chang et al 2017), the role of 4D planning may become more important. In these cases, 4D planning can be used to generate a motion-robust plan, providing a better estimate of the delivered dose (Bernatowicz et al 2017, Zhang et al 2014, Al-Ward et al 2018.…”