Samuel Goossens

Assessment of tumor motion reproducibility with audio-visual coaching through successive 4D CT sessions

This study aimed to compare combined audio‐visual coaching with audio coaching alone and assess their respective impact on the reproducibility of external breathing motion and, one step further, on the internal lung tumor motion itself, through successive sessions. Thirteen patients with NSCLC were enrolled in this study. The tumor motion was assessed by three to four successive 4D CT sessions, while the breathing signal was measured from magnetic sensors positioned on the epigastric region. For all sessions, the breathing was regularized with either audio coaching alone (AC, n=5) or combined with a real‐time visual feedback (A/VC, n=8) when tolerated by the patients. Peak‐to‐peak amplitude, period and signal shape of both breathing and tumor motions were first measured. Then, the correlation between the respiratory signal and internal tumor motion over time was evaluated, as well as the residual tumor motion for a gated strategy. Although breathing and tumor motions were comparable between AC and AV/C groups, A/VC approach achieved better reproducibility through sessions than AC alone (mean tumor motion of 7.2 mm±1 vs. 8.6 mm±1.8 mm, and mean breathing motion of 14.9 mm±1.2 mm vs. 13.3 mm±3.7 mm, respectively). High internal/external correlation reproducibility was achieved in the superior‐inferior tumor motion direction for all patients. For the anterior‐posterior tumor motion direction, better correlation reproducibility has been observed when visual feedback has been used. For a displacement‐based gating approach, A/VC might also be recommended, since it led to smaller residual tumor motion within clinically relevant duty cycles. This study suggests that combining real‐time visual feedback with audio coaching might improve the reproducibility of key characteristics of the breathing pattern, and might thus be considered in the implementation of lung tumor radiotherapy. PACS number: 87

Impact of motion induced artifacts on automatic registration of lung tumors in Tomotherapy

PURPOSE Tomotherapy MV-CT acquisitions of lung tumors lead to artifacts due to breathing-related motion. This could preclude the reliability of tumor based positioning. We investigate the effect of these artifacts on automatic registration and determine conditions under which correct positioning can be achieved. MATERIALS AND METHODS MV-CT and 4D-CT scans of a dynamic thorax phantom were acquired with various motion amplitudes, directions, and periods. For each acquisition, the average kV-CT image was reconstructed from the 4D-CT data and rigidly registered with the corresponding MV-CT scan in a region of interest. Different kV-MV registration strategies have been assessed. RESULTS All tested registration methods led to acceptable registration errors (within 1.3 ± 1.2 mm) for motion periods of 3 and 6 s, regardless of the motion amplitude, direction, and phase difference. However, a motion period of 5 s, equal to half the Tomotherapy gantry period, induced asymmetric artifacts within MV-CT and significantly degraded the registration accuracy. CONCLUSIONS As long as the breathing period differs from 5 s, positioning based on averaged images of the tumor provides information about its daily baseline shift, and might therefore contribute to reducing margins, regardless of the registration method.

Respiratory motion variations from skin surface on lung cancer patients from 4D CT data

In radiation therapy of thorax and abdomen regions, knowing how respiratory motion modifies tumor position and trajectory is crucial for accurate dose delivery to tumors while avoiding healthy tissue and organs at risk. Three types of variations are studied: motion amplitudes measured from the patients skin surface and internal tumor trajectory, internal/external correlations and tumor trajectory baseline shift. Four male patients with lung cancer with three repeated 4D computed tomography (4DCT) scans, taken on different treatment days, were studied. Surfaces were extracted from 4DCT scans by segmentation. Motion over specific regions of interest was analyzed with respect to the motion of the tumor center of mass and correlation coefficient was computed. Tumor baseline shifts were analyzed after rigid registration based on vertebrae and surface registration. External amplitude variations were observed between fractions. Correlation coefficients of internal trajectories and external distances are greater than 0.6 in the abdomen. This correlation was observable and significant for all patients showing that the external motion is a good surrogate for internal movement on an intra-fraction basis. However for the inter-fraction case, external amplitude variations were observed between fractions and no correlation was found with the internal amplitude variations. Moreover, baseline shifts after surface registration were greater than those after vertebrae registration and the mean distance between surfaces after registration was not correlated to the magnitude of the baseline shift. These two observations show that, with the current representation of the external surface, inter-fraction variations are not detectable on the surface.

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.

- SU-E-T-774: on the Effects of Intrafraction Motion on TomoTherapy SIB Treatments of Lung Tumors: A 4D Monte Carlo Study including Non-Rigid Registration and Beamlet-Breathing Phase Correlation

Purpose: In the context of lungtumor TomoTherapy treatments including a boost region based on a PET scan distribution, the clinical impact of neglecting intra‐fraction motion during dose computation was evaluated with 4D Monte Carlo(MC) simulations. Methods: The study includes ten lungtumor patients including a boost region defined using PET data and treated simultaneously with the remaining of the target volume (simultaneous integrated boost (SIB)) and planned according to the guidelines of ICRU Report 83. In this communication, we focus on one patient and vary temporal parameters to evaluate the potential effect of intrafraction motion interplay with sliding jaw and leaf motions. The 4D‐CT scan was acquired and temporally distributed over ten phases according to a breathing signal delivered by a motion detector, providing also an estimation of the breathing period. Dose was computed with the MC model TomoPen. for every projection, simulations were performed on a CT phase selected according to breathing period and initial phase. The 10 dose maps were accumulated using non‐rigid registration and then compared to the approved treatment plan. Parameters were varied as follows 1) variation of breathing period (from 3 to 8 sec) (motion interplay study) 2) variation in the initial phase (multiple fraction averaging effect) and 3) calculation with all the projections for every phase (only intra‐fraction motion).Results: Depending on the breathing period, all the simulations showed acceptable dose distributions on target volumes of the ten patients (0.3 to 1% for gEUDs of the boost tumor volume and the clinical target volume) indicating potential but minimal interplay effect Conclusions: For the patients studied, assuming regular breathing, the results showed that SIB treatments for a lungtumor with TomoTherapy including intra‐fraction motion delivered acceptable dose distributions. This ongoing study will be extended to more patients, including hypo‐fractionated stereotactic treatments. Tomotherapy Inc. has partially supported this research