Balancing dose and image registration accuracy for cone beam tomosynthesis (CBTS) for breast patient setup.

PURPOSE: To balance dose reduction and image registration accuracy in breast setup imaging. In particular, the authors demonstrate the relationship between scan angle and dose delivery for cone beam tomosynthesis (CBTS) when employed for setup verification of breast cancer patients with surgical clips. METHODS: The dose measurements were performed in a female torso phantom for varying scan angles of CBTS. Setup accuracy was measured using three registration methods: Clip centroid localization accuracy and the accuracy of two semiautomatic registration algorithms. The dose to the organs outside of the ipsilateral breast and registration accuracy information were compared to determine the optimal scan angle for CBTS for breast patient setup verification. Isocenter positions at the center of the patient and at the breast-chest wall interface were considered. RESULTS: Image registration accuracy was within 1 mm for the CBTS scan angles theta above 20 degrees for some scenarios and as large as 80 degrees for the worst case, depending on the imaged breast and registration algorithm. Registration accuracy was highest based on clip centroid localization. For left and right breast imaging with the isocenter at the chest wall, the dose to the contralateral side of the patient was very low (<0.5 cGy) for all scan angles considered. For central isocenter location, the optimal scan angles were 30 degrees - 50 degrees for the left breast imaging and 40 degrees - 50 degrees for the right breast imaging, with the difference due to the geometric asymmetry of the current clinical imaging system. CONCLUSIONS: The optimal scan angles for CBTS imaging were found to be between 10 degrees and 50 degrees, depending on the isocenter location and ipsilateral breast. Use of the isocenter at the breast-chest wall locations always resulted in greater accuracy of image registration (<1 mm) at smaller angles (10 degrees - 20 degrees) and at lower doses (<0.1 cGy) to the contralateral organs. For chest wall isocenters, doses delivered to organs outside of the target breast were much smaller than the scattered and leakage doses of the treatment beams. The complete volumetric information of all clips in the region of interest, combined with the small dose to the contralateral organs and the small scan angle, could result in an advantage for small angle CBTS with off center isocenters over simple orthogonal pairs.

Online adaption approaches for intensity modulated proton therapy for head and neck patients based on cone beam CTs and Monte Carlo simulations.

To develop an online plan adaptation algorithm for intensity modulated proton therapy (IMPT) based on fast Monte Carlo dose calculation and cone beam CT (CBCT) imaging. A cohort of ten head and neck cancer patients with an average of six CBCT scans were studied. To adapt the treatment plan to the new patient geometry, contours were propagated to the CBCTs with a vector field (VF) calculated with deformable image registration between the CT and the CBCTs. Within the adaptive planning algorithm, beamlets were shifted following the VF at their distal falloff and raytraced in the CBCT to adjust their energies, creating a geometrically adapted plan. Four geometric adaptation modes were studied: unconstrained geometric shifts (Free), isocenter shift (Iso), a range shifter (RS), or isocenter shift and range shifter (Iso-RS). After evaluation of the geometrical adaptation, the weights of a selected subset of beamlets were automatically tuned using MC-generated influence matrices to fulfill the original plan requirements. All beamlet calculations were done with a fast Monte Carlo running on a GPU (graphics processing unit). Geometrical adaptation alone only worked with small anatomy changes. The weight-tuned adaptation worked for every fraction, with the Free and Iso modes performing similarly and being superior than the two range shifters modes. The mean V95 and V107 were 99.4 ± 0.9 and 6.4% ± 4.7% in the Free mode with weight tuning. The calculation time per fraction was ~5 min, but further task parallelization could reduce it to ~1-2 min for delivery adaptation right after patient setup. An online adaptation algorithm was developed that significantly improved the treatment quality for inter-fractional geometry changes. Clinical implementation of the algorithm would allow delivery adaptation right before treatment and thus allow planning margin reductions for IMPT.

SU-E-J-61: Multi-Criteria Optimization for IGRT Decision Processes.

PURPOSE: To develop a multi-criteria optimization framework for image guided radiotherapy decision processes. METHODS: An algorithm is proposed for a multi-criteria framework for the purpose of patient setup verification decision processes. Optimal patient setup shifts and rotations are not always straightforward, particularly for deformable or moving targets of the spine, abdomen, thorax, breast, head and neck and limbs that change as a Result of treatment. The algorithm relies upon dosimetric constraints and objectives to aid in the patient setup and plan delivery such that the patient is positioned or the plan is optimized to maximize tumor dose coverage and minimize dose to organs at risk while allowing for daily clinical changes. A simple 1D model, a lung lesion are presented and a spine lesion. RESULTS: The algorithm delivers a multi-criteria optimization framework allowing for clinical decisions to accommodate patient target variation which make setup decisions less straightforward. With dosimetric considerations, optimal patient positions and plan parameters can be derived. CONCLUSIONS: A multi-criteria framework is demonstrated to aid in the patient setup and determine the most appropriate daily position considering dosimetric goals. Future implementations include optimizations relying upon multiple plans, field parameters, and other dose metrics (TCP, NTCP, EUD, etc).

MO-E-BRCD-01: Proton Treatment Planning Issues.

Proton treatment planning involves many issues that affect the accuracy and robustness of treatment planning and delivery. Some issues such as patient setup uncertainty and CT number calibration are common with photon planning but have potentially greater effects on the treatment plan simulation and delivery for proton. Other issues such as range uncertainty and LET and RBE variations are unique to particle therapy. The complications of proton treatment planning have been well documented in the literature but there are multiple planning methods developed by clinics to reduce or avoid proton dosimetry errors in treatment delivery. Additionally, error reduction methods are dependent upon the delivery Method: scattering or scanning, single-field optimization or multi-field optimization. This educational session will discuss the documented proton treatment planning issues and the methods developed in three different clinical centers to minimize or eliminate the errors associated with the issues for various treatment sites and proton treatment modalities. LEARNING OBJECTIVES: 1. Understand the issues associated with proton treatment planning and the effects of range uncertainty, LET variations, and setup uncertainties. 2. Understand the differences of photon and proton treatment planning issues. 3. Understand the methods developed to reduce errors in proton treatment planning and delivery at three different centers.

MO-A-213AB-07: Evaluation of Distal Dose Surface with In-Room PET for Proton Therapy Monitoring.

PURPOSE: The objective of this study is to evaluate the feasibility of proton beam treatment verification using in-room PET. As of February 2012, four patients have been studied in a clinical trial. In addition, we suggest a new method comparing the distal surface of the measured and simulated PET activities to verify the location of the distal dose surface. METHODS: Patients were scanned for 20 minutes with an in-room PET positioned next to the proton treatment head in a gantry room for beam delivery using passive scattering. The time between end of treatment and the start of the scan was within about 2 minutes. The predicted distribution of the PET activities and the proton dose distributions in the patients were also calculated using Monte Carlo (MC). Along the beam direction, the 50% fall-off positions of the maximum PET activity at each line profile were compared with the MC simulated and the measured PET images, and then the differences were assessed with root-mean-square deviation (RMSD) and mapped in the beam's eye view. RESULTS: The measured PET images showed a good spatial correlation with the simulated PET images and the proton dose distributions even though the treated volumes and locations varied between patients. The RMSD values, representing the surface differences between the measured and simulated PET, were assessed to be 4.3-5.1 mm for four patients. Some region including the penumbra showed larger differences but was excluded. CONCLUSIONS: We have explored the potential of the in-room PET for proton therapy monitoring through a clinical trial. The PET image analysis method based on MC simulations showed that the distal dose surface could be determined within a few millimeters but not within the aimed accuracy of 2-3 mm. Improvements in PET-CT image registration and biological washout modeling will most likely increase the accuracy further. NIH/NCI P01 CA021239.

Auto-masked 2D/3D image registration and its validation with clinical cone-beam computed tomography.

Image-guided alignment procedures in radiotherapy aim at minimizing discrepancies between the planned and the real patient setup. For that purpose, we developed a 2D/3D approach which rigidly registers a computed tomography (CT) with two x-rays by maximizing the agreement in pixel intensity between the x-rays and the corresponding reconstructed radiographs from the CT. Moreover, the algorithm selects regions of interest (masks) in the x-rays based on 3D segmentations from the pre-planning stage. For validation, orthogonal x-ray pairs from different viewing directions of 80 pelvic cone-beam CT (CBCT) raw data sets were used. The 2D/3D results were compared to corresponding standard 3D/3D CBCT-to-CT alignments. Outcome over 8400 2D/3D experiments showed that parametric errors in root mean square were <0.18° (rotations) and <0.73 mm (translations), respectively, using rank correlation as intensity metric. This corresponds to a mean target registration error, related to the voxels of the lesser pelvis, of <2 mm in 94.1% of the cases. From the results we conclude that 2D/3D registration based on sequentially acquired orthogonal x-rays of the pelvis is a viable alternative to CBCT-based approaches if rigid alignment on bony anatomy is sufficient, no volumetric intra-interventional data set is required and the expected error range fits the individual treatment prescription.

Efficacy of prostate stabilizing techniques during brachytherapy procedure.

During the prostate brachytherapy procedure, multiple needles are inserted into the prostate and radioactive seeds are deposited. Stabilizing needles are first inserted to provide some rigidity and support to the prostate, ideally this will provide better seed placement and an overall improved treatment. However, there is much speculation regarding the effectiveness of using regular brachytherapy needles as stabilizers. In this study, we explored the efficacy of (1) two types of needles--18 gauge brachytherapy needle vs. 18 gauge hooked needle; and (2) parallel vs. angulated needle configurations to stabilize the prostate. Prostate phantom movement and needle insertion progression were imaged using ultrasound (US). The recorded images were analyzed and prostate displacement was computed from images using implanted artifacts. Experimentation allowed us to further understand the mechanics behind prostate stabilization. We observed superior stabilization by the hooked needles compared to the regular brachytherapy needles (more than 40% for parallel stabilization). Prostate movement was also reduced significantly when regular brachytherapy needles were in an angulated configuration as compared to the parallel configuration (approximately 40%). When the hooked needles were angled for stabilization, further improvement in decreased displacement was observed. In general, for convenience of dosimetric planning, all needles are desired to be in parallel and in this case, hooked needles are better suited to improve stabilization of the prostate. On the other hand, both regular and hooked needles appear to be equally effective in reducing prostate movement when they are in angulated configurations, which will be useful in robotic permanent seed implantation (PSI).

Tumor Detection in vivo with Optical Spectroscopy.

Ultrasound induced blood stasis has been observed for a long time, but to date most experimental observations have been in vitro. In this paper we discuss a possible diagnostic use for this previously undesirable effect of ultrasound - tumor detection in vivo. We demonstrate that, using optical spectroscopy, effects of ultrasound can be used to differentiate tumor from non-tumor in murine tissue. Finally, we propose a novel diagnostic algorithm that quantitatively differentiates tumor from non-tumor with maximum specificity 0.83, maximum sensitivity 0.79, and area under ROC curve 0.90.

In vivo Optical Spectroscopy of Acoustically Induced Blood Stasis.

Ultrasound-induced blood stasis has been observed for more than thirty years. Most of the literature has been focused on the health risks associated with this phenomenon and methods employed to prevent stasis from occurring during ultrasound imaging. To date, experimental observations have been either in vitro or invasive. The current work demonstrates ultrasound- induced blood stasis in murine tumor and nontumor tissue, observed through noninvasive measurements of optical spectroscopy, and discusses possible diagnostic uses for this previously undesirable effect of ultrasound.