J. Dhont

A comparison of two clinical correlation models used for real-time tumor tracking of semi-periodic motion: A focus on geometrical accuracy in lung and liver cancer patients.

PURPOSE A head-to-head comparison of two clinical correlation models with a focus on geometrical accuracy for internal tumor motion estimation during real-time tumor tracking (RTTT). METHODS AND MATERIALS Both the CyberKnife (CK) and the Vero systems perform RTTT with a correlation model that is able to describe hysteresis in the breathing motion. The CK dual-quadratic (DQ) model consists of two polynomial functions describing the trajectory of the tumor for inhale and exhale breathing motion, respectively. The Vero model is based on a two-dimensional (2D) function depending on position and speed of the external breathing signal to describe a closed-loop tumor trajectory. In this study, 20 s of internal motion data, using an 11 Hz (on average) full fluoroscopy (FF) sequence, was used for training of the CK and Vero models. Further, a subsampled set of 15 internal tumor positions (15p) equally spread over the different phases of the breathing motion was used for separate training of the CK DQ model. Also a linear model was trained using 15p and FF tumor motion data. Fifteen liver and lung cancer patients, treated on the Vero system with RTTT, were retrospectively evaluated comparing the CK FF, CK 15p and Vero FF models using an in-house developed simulator. The distance between estimated target position and the tumor position localized by X-ray imaging was measured in the beams-eye view (BEV) to calculate the 95th percentile BEV modeling errors (ME(95,BEV)). Additionally, the percentage of ME(95,BEV) smaller than 5 mm (P(5mm)) was determined for all correlation models. RESULTS In general, no significant difference (p>0.05, paired t-test) was found between the CK FF and Vero models. Based on patient-specific evaluation of the geometrical accuracy of the linear, CK DQ and Vero correlation models, no statistical necessity (p>0.05, two-way ANOVA) of including hysteresis in correlation models was proven, although during inhale breathing motion, the linear model resulted in a decreased P(5mm) with 5-6% compared to both the DQ CK and Vero models. CONCLUSION Dual-quadratic CyberKnife and 2D Vero correlation models were interchangeable in terms of geometrical accuracy with the CK linear ME(95,BEV)=4.1 mm, CK dual-quadratic ME(95,BEV)=3.9 mm and Vero ME(95,BEV)=3.7 mm, when modeled with FF sequence. CK DQ modeling based on 15p acquired in 20 s may lead to problems for internal motion estimation.

Improving the intra-fraction update efficiency of a correlation model used for internal motion estimation during real-time tumor tracking for SBRT patients: Fast update or no update?

BACKGROUND AND PURPOSE For tumor tracking, a correlation model is used to estimate internal tumor position based on external surrogate motion. When patients experience an internal/external surrogate drift, an update of the correlation model is required to continue tumor tracking. In this study, the accuracy of the internal tumor position estimation for both the clinical available update at discrete points in time (rebuild) and an in-house developed non-clinical online update approach was investigated. METHODS A dynamic phantom with superimposed baseline drifts and 14 SBRT patients, treated with real-time tumor tracking (RTTT) on the Vero system, were retrospectively simulated for three update scenarios, respectively no update, clinical rebuild and 0.5 Hz automated online update of the correlation model. By comparing the target positions based on 0.5 Hz verification X-ray images with the estimated internal tumor positions regarding all three update scenarios, 95th percentile modeling errors (ME95), incidences of full geometrical coverage of the CTV by a 5 mm extended PTV (P₅mm) and population-based PTV margins were calculated. Further, the treatment time reduction was estimated when switching from the clinical rebuild approach to the online correlation model update. RESULTS For dynamic phantom motion with baseline drifts up to 0.4 mm/min, a 0.5 Hz intra-fraction update showed a similar accuracy in terms of ME95 and P5 mm compared to clinical rebuild. For SBRT patients treated on Vero with RTTT, accuracy was improved by 0.5 Hz online update compared to the clinical rebuild protocol, yielding smaller PTV margins (from 3.2 mm to 2.7 mm), reduced ME95,3D (from 4.1 mm to 3.4 mm) and an increased 5th percentile P5 mm (from 90.7% to 96.1%) for the entire patient group. Further, 80% of treatment sessions were reduced in time with on average 5.5 ± 4.1(1 SD)min. CONCLUSION With a fast (0.5 Hz) automated online update of the correlation model, an efficient RTTT workflow with improved geometrical accuracy was obtained.

Feasibility of markerless tumor tracking by sequential dual-energy fluoroscopy on a clinical tumor tracking system

A novel approach to dual-energy imaging for markerless tumor tracking was proposed consisting of sequential dual-energy fluoroscopy, omitting the need for fast-switching kV generators. The implementation of this approach on a clinical tumor tracking system and its efficacy is shown feasible through optimization of the imaging parameters.

Motion management during SBRT for oligometastatic cancer: Results of a prospective phase II trial

PURPOSE To optimize the local control of stereotactic body radiotherapy (SBRT) using the Vero-SBRT system and respiratory motion management in patients with oligometastatic cancer. MATERIALS AND METHODS Patients with five or less metastases were eligible. In metastases with significant motion, a fiducial was implanted for Vero dynamic tracking. For other metastases an internal target volume (ITV) was defined to encompass the respiratory tumor trajectory. A dose of 50Gy in 10 fractions was prescribed on the 80% isodose line. RESULTS We treated 87 metastases in 44 patients, with colorectal cancer as the most common primary origin (65.9%). Metastatic sites were mainly lung (n=62) and liver (n=17). Twenty-seven metastases were treated with dynamic tracking, the remaining 60 using the ITV-concept. Three patients (7%) experienced grade ⩾3 toxicity. After a median follow-up of 12months, the overall one-year local control (LC) amounted to 89% (95% CI 77-95%), with corresponding values of 90% and 88% for the metastases irradiated with the ITV-approach and dynamic tracking, respectively. Median progression-free survival reached 6.5months, one-year overall survival 95%. CONCLUSIONS SBRT with proper respiratory motion management resulted in a high LC and an acceptable toxicity profile in oligometastatic cancer patients.

Treating patients with Dynamic Wave Arc: First clinical experience

BACKGROUND AND PURPOSE Dynamic Wave Arc (DWA) is a system-specific noncoplanar arc technique that combines synchronized gantry-ring rotation with D-MLC optimization. This paper presents the clinical workflow, quality assurance program, and reports the geometric and dosimetric results of the first patient cohort treated with DWA. METHODS AND MATERIALS The RayStation TPS was clinically integrated on the Vero SBRT platform for DWA treatments. The first 15 patients treated with DWA represent a broad range of treatment sites: breast boost, prostate, lung SBRT and bone metastases, which allowed us to explore the potentials and assess the limitations of the current DWA site-specific template solution. For the DWA verification a variety of QA equipment was used, from 3D diode array to an anthropomorphic end-to-end phantom. The geometric accuracy of each arc was verified with an independent orthogonal fluoroscopy method. RESULTS The average beam-on delivery time was 3min, ranging from 1.22min to 8.82min. All patient QAs passed our institutional clinical criteria of gamma index. For both EBT3 film and Delta4 measurements, DWA planned versus delivered dose distributions presented an average agreement above 97%. An overall mean gantry-ring geometric deviation of -0.03°±0.46° and 0.18°±0.26° was obtained, respectively. CONCLUSION For the first time, DWA has been translated into the clinic and used to treat various treatment sides. DWA has been successfully added to the noncoplanar rotational IMRT techniques arsenal, allowing additional flexibility in dose shaping while preserving dosimetrically robust delivery.

The long- and short-term variability of breathing induced tumor motion in lung and liver over the course of a radiotherapy treatment

PURPOSE To evaluate the short and long-term variability of breathing induced tumor motion. MATERIALS AND METHODS 3D tumor motion of 19 lung and 18 liver lesions captured over the course of an SBRT treatment were evaluated and compared to the motion on 4D-CT. An implanted fiducial could be used for unambiguous motion information. Fast orthogonal fluoroscopy (FF) sequences, included in the treatment workflow, were used to evaluate motion during treatment. Several motion parameters were compared between different FF sequences from the same fraction to evaluate the intrafraction variability. To assess interfraction variability, amplitude and hysteresis were compared between fractions and with the 3D tumor motion registered by 4D-CT. Population based margins, necessary on top of the ITV to capture all motion variability, were calculated based on the motion captured during treatment. RESULTS Baseline drift in the cranio-caudal (CC) or anterior-poster (AP) direction is significant (ie. >5 mm) for a large group of patients, in contrary to intrafraction amplitude and hysteresis variability. However, a correlation between intrafraction amplitude variability and mean motion amplitude was found (Pearsons correlation coefficient, r = 0.72, p < 10-4). Interfraction variability in amplitude is significant for 46% of all lesions. As such, 4D-CT accurately captures the motion during treatment for some fractions but not for all. Accounting for motion variability during treatment increases the PTV margins in all directions, most significantly in CC from 5 mm to 13.7 mm for lung and 8.0 mm for liver. CONCLUSION Both short-term and day-to-day tumor motion variability can be significant, especially for lesions moving with amplitudes above 7 mm. Abandoning passive motion management strategies in favor of more active ones is advised.

OC-0500: A comparison of two clinical correlation models for dynamic tumor tracking with a focus on geometrical accuracy

A comparison of two clinical correlation models for dynamic tumor tracking with a focus on geometrical accuracy Jennifer Dhont, Master in Biomedical Engineering, 2014

Predicting tumour motion during the whole radiotherapy treatment: a systematic approach for thoracic and abdominal lesions based on real time MR

INTRODUCTION Aim of this study was to investigate the ability of pre-treatment four dimensional computed tomography (4DCT) to capture respiratory-motion observed in thoracic and abdominal lesions during treatment. Treatment motion was acquired using full-treatment cine-MR acquisitions. Results of this analysis were compared to the ability of 30 seconds (s) cine Magnetic Resonance (MR) to estimate the same parameters. METHODS A 4DCT and 30 s cine-MR (ViewRay, USA) were acquired on the simulation day for 7 thoracic and 13 abdominal lesions. Mean amplitude, intra- and inter-fraction amplitude variability, and baseline drift were extracted from the full treatment data acquired by 2D cine-MR, and correlated to the motion on pre-treatment 30 s cine-MR and 4DCT. Using the full treatment data, safety margins on the ITV, necessary to account for all motion variability from 4DCT observed during treatment, were calculated. Mean treatment amplitudes were 2 ± 1 mm and 5 ± 3 mm in the anteroposterior (AP) and craniocaudal (CC) direction, respectively. Differences between mean amplitude during treatment and amplitude on 4DCT or during 30 s cine-MR were not significant, but 30 s cine-MR was more accurate than 4DCT. Intra-fraction amplitude variability was positively correlated with both 30 s cine-MR and 4DCT amplitude. Inter-fraction amplitude variability was minimal. RESULTS Mean baseline drift over all fractions and patients equalled 1 ± 1 mm in both CC and AP direction, but drifts per fraction up to 16 mm (CC) and 12 mm (AP) were observed. Margins necessary on the ITV ranged from 0 to 8 mm in CC and 0 to 5 mm in AP direction. Neither amplitude on 4DCT nor during 30 s cine MR is correlated to the magnitude of drift or the necessary margins in both directions. CONCLUSION Lesions moving with small amplitude show limited amplitude variability throughout treatment, making passive motion management strategies seem adequate. However, other variations such as baseline drifts and shifts still cause significant geometrical uncertainty, favouring real-time monitoring and an active approach for all lesions influenced by respiratory motion.

PO-0791: Motion management and Vero dynamic tracking for SBRT in oligometastatic disease: a prospective trial

Sant’Orsola-Malpighi HospitalUniversity of Bologna, Radiation Oncology CenterDepartment of ExperimentalDiagnostic and Specialty Medicine DIMES, Bologna, Italy Sant’Orsola-Malpighi HospitalUniversity of Bologna, Department of Medical Physics, Bologna, Italy Fondazione di Ricerca e Cura “Giovanni Paolo II”Catholic University of Sacred Heart, Radiotherapy Unit, Campobasso, Italy Ospedale Bellaria, Radiotherapy Department, Bologna, Italy

PV-0323: Prospective evaluation of markerless tumour tracking using 4D3D registration and dual energy imaging

Purpose or Objective: Four-dimensional cone-beam computed tomography (4D-CBCT) has great capability to provide volumetric and respiratory motion information with one gantry rotation. It is necessary to quantitatively assess, how difference of tumor displacement between actual and 4D-CBCT image exists. In this study, we evaluated the displacement of implanted fiducial markers assumed as tumor on fluoroscopic projection images and reconstructed 4D-CBCT images with different sorting methods.