Abstract
BackgroundInternal Target Volume (ITV) is one of the most common strategies to passively manage tumour motion in Radiotherapy (RT).The reliability of this approach is based on the assumption that the tumour motion estimated during pre-treatment 4D Computed Tomography (CT) acquisition is representative of the motion during the whole RT treatment. With the introduction of Magnetic Resonance-guided RT (MRgRT), it has become possible to monitor tumour motion during the treatment and verify this assumption.Aim of this study was to investigate the reliability of the ITV approach with respect to the treatment fraction time (TFT) in abdominal and thoracic lesions.MethodsA total of 12 thoracic and 15 abdominal lesions was analysed. Before treatment, a 10-phase 4DCT was acquired and ITV margins were estimated considering the envelope of the lesion contoured on the different 4DCT phases.All patients underwent MRgRT treatment in free-breathing, monitoring the tumour position on a sagittal plane with 4 frames per second (sec). ITV margins were projected on the tumour trajectory and the percentage of treatment time in which the tumour was inside the ITV (%TT) was measured to varying of TFT.The ITV approach was considered moderately reliable when %TT ≥ 90% and strongly reliable when %TT ≥ 95%. Additional ITV margins required to achieve %TT ≥ 95% were also calculated.ResultsIn the analysed cohort of patients, ITV strategy can be considered strongly reliable only for lung lesions with TFT ≤ 7 min (min). The ITV strategy can be considered only moderately reliable for abdominal lesions, and additional margins are required to obtain %TT ≥ 95%.Considering a TFT ≤ 4 min, additional margins of 2 mm in cranio-caudal (CC) and 1 mm in antero-posterior (AP) are suggested for pancreatic lesions, 3 mm in CC and 2 mm in AP for renal and liver ones.ConclusionsOn the basis of the analysed cases, the ITV approach appears to be reliable in the thorax, while it results more challenging in the abdomen, due to the higher uncertainty in ITV definition and to the observed larger intra and inter-fraction motion variability. The addition of extra margins based on the TFT may represent a valid tool to compensate such limitations.
Highlights
Internal Target Volume (ITV) is one of the most common strategies to passively manage tumour motion in Radiotherapy (RT)
The primary aim of this study was to investigate the reliability of the ITV approach with respect to the treatment fraction time (TFT), using the cine magnetic resonance (MR) data acquired during MR-guided radiotherapy treatment (MRgRT) of patients affected by lesions located in the abdominal and thoracic regions
Lesion 8 did not move as it was fixed to the thoracic wall, so its ITV extension was equal to 1 mm in both the considered directions
Summary
Internal Target Volume (ITV) is one of the most common strategies to passively manage tumour motion in Radiotherapy (RT) The reliability of this approach is based on the assumption that the tumour motion estimated during pre-treatment 4D Computed Tomography (CT) acquisition is representative of the motion during the whole RT treatment. Beyond the respiratory-induced motion, other effects can modify the intra- and inter-fraction tumour position, such as baseline shifts (i.e. sudden changes in the median tumour position) or baseline drifts (i.e. overall slow tumour position change over the course of a treatment fraction) [5,6,7] Due to these sources of variability, a sub-optimal management of the tumour motion can introduce significant dosimetric differences between the planned and delivered dose distribution [8,9,10]. The active techniques, such as real-time tumour tracking or gating, are steadily becoming more prevalent in SBRT, the use of passive techniques is still widespread, as it does not require dedicated treatment machines or additional systems to monitor the patient’s breathing [11, 12]
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