Synchronized moving aperture radiation therapy (SMART) is a new technique for treating mobile tumours under development at Massachusetts General Hospital (MGH). The basic idea of SMART is to synchronize the moving radiation beam aperture formed by a dynamic multileaf collimator (DMLC) with the tumour motion induced by respiration. SMART is based on the concept of the average tumour trajectory (ATT) exhibited by a tumour during respiration. During the treatment simulation stage, tumour motion is measured and the ATT is derived. Then, the original IMRT MLC leaf sequence is modified using the ATT to compensate for tumour motion. During treatment, the tumour motion is monitored. The treatment starts when leaf motion and tumour motion are synchronized at a specific breathing phase. The treatment will halt when the tumour drifts away from the ATT and will resume when the synchronization between tumour motion and radiation beam is re-established. In this paper, we present a method to derive the ATT from measured tumour trajectory data. We also investigate the validity of the ATT concept for lung tumours during normal breathing. The lung tumour trajectory data were acquired during actual radiotherapy sessions using a real-time tumour-tracking system. SMART treatment is simulated by assuming that the radiation beam follows the derived ATT and the tumour follows the measured trajectory. In simulation, the treatment starts at exhale phase. The duty cycle of SMART delivery was calculated for various treatment times and gating thresholds, as well as for various exhale phases where the treatment begins. The simulation results show that in the case of free breathing, for 4 out of 11 lung datasets with tumour motion greater than 1 cm from peak to peak, the error in tumour tracking can be controlled to within a couple of millimetres while maintaining a reasonable delivery efficiency. That is to say, without any breath coaching/control, the ATT is a valid concept for some lung tumours. However, to make SMART an efficient technique in general, it is found that breath coaching techniques are required.

Recently, at Massachusetts General Hospital (MGH) we proposed a new treatment technique called synchronized moving aperture radiation therapy (SMART) to account for tumour motion during radiotherapy. The basic idea of SMART is to synchronize the moving radiation beam aperture formed by a dynamic multileaf collimator with the tumour motion induced by respiration. The two key requirements for being able to successfully use SMART in clinical practice are the precise and fast detection of tumour position during the simulation/treatment and the good reproducibility of the tumour motion pattern. To fulfil the first requirement, an integrated radiotherapy imaging system is currently being developed at MGH. The results of a previous study show that breath coaching techniques are required to make SMART an efficient technique in general. In this study, we investigate volunteer and patient respiratory coaching using a commercial respiratory gating system as a respiration coaching tool. Five healthy volunteers, observed during six sessions, and 33 lung cancer patients, observed during one session when undergoing 4D CT scans, were investigated with audio and visual promptings, with free breathing as a control. For all five volunteers, breath coaching was well tolerated and the intra- and inter-session reproducibility of the breathing pattern was greatly improved. Out of 33 patients, six exhibited a regular breathing pattern and needed no coaching, four could not be coached at all due to the patient's medical condition or had difficulty following the instructions, 13 could only be coached with audio instructions and 10 could follow the instructions of and benefit from audio-video coaching. We found that, for all volunteers and for those patients who could be properly coached, breath coaching improves the duty cycle of SMART treatment. However, about half of the patients could not follow both audio and video instructions simultaneously, suggesting that the current coaching technique requires improvements.

Analysis of interfraction setup errors, performed with US, CBCT, planar kV images, and electromagnetic transponders, from a large dataset revealed intermodality shifts were comparable (within 3-4 mm). Interfraction planning margins, relative to setup based on skin marks, were generally within the 10 mm prostate-to-planning target volume margin used in our clinic. With image guidance, interfraction residual planning margins were reduced to approximately less than 4 mm. These findings are potentially important for dose escalation studies using smaller margins to better protect normal tissues.