Marie Wanet

Gradient-based delineation of the primary GTV on FDG-PET in non-small cell lung cancer: A comparison with threshold-based approaches, CT and surgical specimens.

PURPOSE The aim of this study was to validate a gradient-based segmentation method for GTV delineation on FDG-PET in NSCLC through surgical specimen, in comparison with threshold-based approaches and CT. MATERIALS AND METHODS Ten patients with stage I-II NSCLC were prospectively enrolled. Before lobectomy, all patients underwent contrast enhanced CT and gated FDG-PET. Next, the surgical specimen was removed, inflated with gelatin, frozen and sliced. The digitized slices were used to reconstruct the 3D macroscopic specimen. GTVs were manually delineated on the macroscopic specimen and on CT images. GTVs were automatically segmented on PET images using a gradient-based method, a source to background ratio method and fixed threshold values at 40% and 50% of SUV(max). All images were finally registered. Analyses of raw volumes and logarithmic differences between GTVs and GTV(macro) were performed on all patients and on a subgroup excluding the poorly defined tumors. A matching analysis between the different GTVs was also conducted using Dices similarity index. RESULTS Considering all patients, both lung and mediastinal windowed CT overestimated the macroscopy, while FDG-PET provided closer values. Among various PET segmentation methods, the gradient-based technique best estimated the true tumor volume. When analysis was restricted to well defined tumors without lung fibrosis or atelectasis, the mediastinal windowed CT accurately assessed the macroscopic specimen. Finally, the matching analysis did not reveal significant difference between the different imaging modalities. CONCLUSIONS FDG-PET improved the GTV definition in NSCLC including when the primary tumor was surrounded by modifications of the lung parenchyma. In this context, the gradient-based method outperformed the threshold-based ones in terms of accuracy and robustness. In other cases, the conventional mediastinal windowed CT remained appropriate.

Validation of the mid-position strategy for lung tumors in helical TomoTherapy

PURPOSE To compare the mid-position (MidP) strategy to the conventional internal target volume (ITV) for lung tumor management in helical TomoTherapy, using 4D Monte Carlo (MC) plan simulations. MATERIALS AND METHODS For NSCLC patients treated by SBRT (n = 8) or SIB-IMRT (n = 7), target volumes and OARs were delineated on a contrast-enhanced CT, while 4D-CT was used to generate either ITV or MidP volumes with deformable registrations. PTV margins were added. Conformity indexes, volumetric and dosimetric parameters were compared for both strategies. Dose distributions were also computed using a 4D MC model (TomoPen) to assess how intra-fraction tumor motion affects tumor coverage, with and without interplay effect. RESULTS PTVs derived from MidP were on average 1.2 times smaller than those from ITV, leading to lower doses to OARs. Planned dose conformity to TVs was similar for both strategies. 4D MC computation showed that ITV ensured adequate TV coverage (D95 within 1% of clinical requirements), while MidP failed in 3 patients of the SBRT group (D95 to the TV lowered by 4.35%, 2.16% and 2.61%) due to interplay effect in one case and to breathing motion alone in the others. CONCLUSIONS Compared to the ITV, the MidP significantly reduced PTV and doses to OARs. MidP is safe for helical delivery except for very small tumors (<5 cc) with large-amplitude motion (>10mm) where the ITV might remain the most adequate approach.

Metabolic imaging in non-small-cell lung cancer radiotherapy

Metabolic imaging by positrons emission tomography (PET) offers new perspectives in the field of non-small-cell lung cancer radiation therapy. First, it can be used to refine the way nodal and primary tumour target volumes are selected and delineated, in better agreement with the underlying tumour reality. In addition, the non-invasive spatiotemporal mapping of the tumour biology and the organs at risk function might be further used to steer radiation dose distribution. Delivering higher dose to low responsive tumour area, in a way that better preserves the normal tissue function, should thus reconcile the tumour radiobiological imperatives (maximising tumour local control) with dose related to the treatment safety (minimising late toxicity). By predicting response early in the course of radiation therapy, PET may also participate to better select patients who are believed to benefit most from treatment intensification. Altogether, these technological advances open avenues to in-depth modify the way the treatment plan is designed and the dose is delivered, in better accordance with the radiobiology of individual solid cancers and normal tissues.

OC-0378 helical tomotherapy dynamics are not an issue to treat lung tumors for patients coached to ensure regular breathing

Purpose/objective: To evaluate helical tomotherapy (HT) treatment delivery and breathing motion interplay effect on dose distributions using a comprehensive 4D Monte Carlo (MC) dose engine including non-rigid deformation of dose maps computed on a 4D CT scan. Material/methods: Treatment planning is usually performed on a 3DCT image, even though lung tumors may move during delivery. To mitigate this issue, 4DCT-based delineation and appropriate margin definition are used to ensure good tumor coverage. However, treatment beam and breathing-induced motions interplay may lead to clinically unacceptable delivered doses, irrespective of the margin definition technique used. The particular delivery scheme of HT, with synchronous motion of both the gantry and the treatment couch, raise concerns specific to this modality for treating lung tumors. To evaluate the effect of motion interplay on dose distributions, a 4D MC dose computation engine was devised: 1) the MC core was TomoPen, a previously validated MC model of tomotherapy; 2) dose maps were computed on a 4D CT scan (10 phases); 3) resulting dose maps were accumulated through non-rigid deformation. Regular breathing was ensured with patient audio coaching. The treatment sinogram was correlated to the measured breathing period. 4D dose distributions (“interplay simulated”) were computed for 7 patients with a large range of motion amplitudes (up to 11.4 mm in the superior-inferior direction). Those were compared to MC dose distributions calculated on the 3DCT (“planned” dose distributions) and also on the 4DCT with the entire sinogram computed on every single phase, thus accounting only for breathing motion assuming an infinitely slow treatment (“no interplay”). Generalized equivalent uniform doses (gEUDs) and typical DVHs metrics (D95, D2, V95, Dmean, …) were computed for the tumor volumes (CTV and GTV) and compared for all simulation modalities. Results: For all modalities and all 7 patients, dose distributions complied with local clinical requirements (reported as in ICRU 83, RTOG 0618 and 0236 depending on the cases studied). Between “interplay simulated” and “no interplay”, D95 and D2 for tumor volumes were within 2.2% whereas Dmean and gEUDs were within 1% . The maximum difference between planned dose and “interplay simulated” was a 3% increase in Dmean and gEUD (see figure (b)). Since the “no interplay” effect was in agreement within 0.1 % with “interplay simulated”, the difference was attributed to the different quality of images of the 3D and 4D CT data sets. Conclusions: For the patients included in this study, coached to ensure regular breathing, HT delivery and breathing motion interplay was found clinically negligible, confirming in practical clinical routine previous theoretical analysis. Thus, as with 3D conformal static delivery, the problem of covering moving lung tumors may be restricted to the sole definition of margins if gating or tracking technologies are not available.