Real-time 4DMRI-based internal target volume definition for moving lung tumors.

In photon radiotherapy, respiratory-induced target motion can be accounted for by internal target volumes (ITV) or mid-ventilation target volumes (midV) defined on the basis of four-dimensional computed tomography (4D-CT). Intrinsic limitations of these approaches can result in target volumes that are not representative for the gross tumor volume (GTV) motion over the course of treatment. To address these limitations, we propose a novel patient-specific ITV definition method based on real-time 4D magnetic resonance imaging (rt-4DMRI). Three lung cancer patients underwent weekly rt-4DMRI scans. A total of 24 datasets were included in this retrospective study. The GTV was contoured on breath-hold MR images and propagated to all rt-4DMRI images by deformable image registration. Different targets were created for the first (reference) imaging sessions: ITVs encompassing all GTV positions over the complete (ITV 80 s ) or partial acquisition time ( ITV 10 s ), ITVs including only voxels with a GTV probability-of-presence (POP) of at least 5% ( ITV 5 % ) or 10% ( ITV 10 % ), and the mid-ventilation GTV position. Reference planning target volumes ( PTV r ) were created by adding margins around the ITVs and midV target volumes. The geometrical overlap of the PTV r with ITV n 5 % from the six to eight subsequent imaging sessions on days n was quantified in terms of the Dice similarity coefficient (DSC), sensitivity [SE: ( PTV r ∩ ITV n 5 % )/ ITV n 5 % ] and precision [PRE: ( PTV r ∩ ITV n 5 % )/ PTV r ] as surrogates for target coverage and normal tissue sparing. Patient-specific analysis yielded a high variance of the overlap values of PTV r 10 s , when different periods within the reference imaging session were sampled. The mid-ventilation-based PTVs were smaller than the ITV-based PTVs. While the SE was high for patients with small breathing pattern variations, changes of the median breathing amplitudes in different imaging sessions led to inferior SE values for the mid-ventilation PTV for one patient. In contrast, PTV r 5 % and PTV r 10 % showed higher SE values with a higher robustness against interfractional changes, at the cost of larger target volumes. The results indicate that rt-4DMRI could be valuable for the definition of target volumes based on the GTV POP to achieve a higher robustness against interfractional changes than feasible with today's 4D-CT-based target definition concepts.

Rabe M ，Thieke C ，Düsberg M ，Neppl S ，Gerum S ，Reiner M ，Nicolay NH ，Schlemmer HP ，Debus J ，Dinkel J ，Landry G ，Parodi K ，Belka C ，Kurz C ，Kamp F ... -  《-》

Clinical evaluation of 4D MRI in the delineation of gross and internal tumor volumes in comparison with 4DCT.

To evaluate clinical utility of respiratory-correlated (RC) four-dimensional magnetic resonance imaging (4DMRI) for lung tumor delineation and motion assessment, in comparison with the current clinical standard of 4D computed tomography (4DCT). A prospective T2-weighted (T2w) RC-4DMRI technique was applied to acquire coronal 4DMRI images for 14 lung cancer patients (16 lesions) during free breathing (FB) under an IRB-approved protocol, together with a breath-hold (BH) T1w 3DMRI and axial 4DMRI. Clinical simulation CT and 4DCT were acquired within 2 h. An internal navigator was applied to trigger amplitude-binned 4DMRI acquisition whereas a bellows or real-time position management (RPM) was used in the 4DCT reconstruction. Six radiation oncologists manually delineated the gross and internal tumor volumes (GTV and ITV) in 399 3D images using programmed clinical workflows under a tumor delineation guideline. The ITV was the union of GTVs within the breathing cycle without margin. Average GTV and motion range were assessed and ITV variation between 4DMRI and 4DCT was evaluated using the Dice similarity index, mean distance agreement (MDA), and volume difference. The mean tumor volume is similar between 4DCT (GTV4DCT  = 1.0, as the reference) and T2w-4DMRI (GTVT2wMR  = 0.97), but smaller in T1w MRI (GTVT1wMR  = 0.76), suggesting possible peripheral edema around the tumor. Average GTV variation within the breathing cycle (22%) in 4DMRI is slightly greater than 4DCT (17%). GTV motion variation (-4 to 12 mm) and ITV variation (∆VITV =-25 to 95%) between 4DCT and 4DMRI are large, confirmed by relatively low ITV similarity (Dice = 0.72 ± 0.11) and large MDA = 2.9 ± 1.5 mm. Average GTVs are similar between T2w-4DMRI and 4DCT, but smaller by 25% in T1w BH MRI. Physician training and breathing coaching may be necessary to reduce ITV variability between 4DMRI and 4DCT. Four-dimensional magnetic resonance imaging is a promising and viable technique for clinical lung tumor delineation and motion assessment.

Zhang J ，Srivastava S ，Wang C ，Beckham T ，Johnson C ，Dutta P ，Shepherd A ，Mechalakos J ，Hunt M ，Wu A ，Rimner A ，Li G ... -  《Journal of Applied Clinical Medical Physics》

ITV versus mid-ventilation for treatment planning in lung SBRT: a comparison of target coverage and PTV adequacy by using in-treatment 4D cone beam CT.

Bellec J ，Arab-Ceschia F ，Castelli J ，Lafond C ，Chajon E ... -  《Radiation Oncology》

Dosimetric comparison of stereotactic body radiotherapy using 4D CT and multiphase CT images for treatment planning of lung cancer: evaluation of the impact on daily dose coverage.

Wang L ，Hayes S ，Paskalev K ，Jin L ，Buyyounouski MK ，Ma CC ，Feigenberg S ... -  《-》

ITV, mid-ventilation, gating or couch tracking - A comparison of respiratory motion-management techniques based on 4D dose calculations.

Respiratory motion-management techniques (MMT) aim to ensure tumor dose coverage while sparing lung tissue. Dynamic treatment-couch tracking of the moving tumor is a promising new MMT and was compared to the internal-target-volume (ITV) concept, the mid-ventilation (MidV) principle and the gating approach in a planning study based on 4D dose calculations. For twenty patients with lung lesions, planning target volumes (PTV) were adapted to the MMT and stereotactic body radiotherapy treatments were prepared with the 65%-isodose enclosing the PTV. For tracking, three concepts for target volume definition were considered: Including the gross tumor volume of one phase (single-phase tracking), including deformations between phases (multi-phase tracking) and additionally including tracking latencies of a couch tracking system (reliable couch tracking). The accumulated tumor and lung doses were estimated with 4D dose calculations based on 4D-CT datasets and deformable image registration. Single-phase tracking showed the lowest ipsilateral lung Dmean (median: 3.3Gy), followed by multi-phase tracking, gating, reliable couch tracking, MidV and ITV concepts (3.6, 3.8, 4.1, 4.3 and 4.8Gy). The 4D dose calculations showed the MidV and single-phase tracking overestimated the target mean dose (-2.3% and -1.3%), while it was slightly underestimated by the other MMT (<+1%). The ITV concept ensures tumor coverage, but exposes the lung tissue to a higher dose. The MidV, gating and tracking concepts were shown to reduce the lung dose. Neglecting non-translational changes of the tumor in the target volume definition for tracking results in a slightly reduced target coverage. The slightly inferior dose coverage for MidV should be considered when applying this technique clinically.

Ehrbar S ，Jöhl A ，Tartas A ，Stark LS ，Riesterer O ，Klöck S ，Guckenberger M ，Tanadini-Lang S ... -  《-》