Index of content:
Volume 34, Issue 6, June 2007
- Joint Imaging/Therapy Scientific Session: Room M100J
- Correction Strategies, Interventional Procedures
34(2007); http://dx.doi.org/10.1118/1.2761649View Description Hide Description
Purpose: To investigate the optimum requirements of magnetically shielding a Linac from an MRI coupled to it for the purpose of real‐time image‐guided adaptive radiotherapy. In addition, to investigate the effects on the MRI'smagnetic field homogeneity due to the presence of the required shielding. Method and Materials:Finite element method was used (COMSOL MultiPhysics) to model a Linac coupled to a 0.2 T bi‐planar MRI system, and to calculate the magnetic fields in its vicinity. Associated shielding was optimized by varying sheets of shielding material (eg, iron, Mu Metal) to keep the MRI's magnetic fringe fields within the waveguide to less than 0.5 Gauss. Results: We found that to reduce the MRI's magnetic fringe fields within the waveguide from 20 Gauss (unshielded waveguide) to the specific requirement of less than 0.5 Gauss, we require a 5 cm thick iron plate between the Linac and the MRI, and a 1 mm Mu Metal wrapping around the waveguide. An additional 5 cm plate is placed on the opposing side to improve magnetic field symmetry. The magnet's field inhomogenieties resulting from the shielding were calculated to be less than 142 ppm, and thus easily “shimmable”. Conclusion:Magnetic shielding is needed for operating a Linac coupled to a 0.2T MRI — system. We have optimized the shielding required to reduce the MRI's magnetic fringe fields within the waveguide to less than 0.5 Gauss by using simple and realizable passive shielding. Field inhomogeneities due to the presence of our shielding are sufficiently small that they can be easily corrected though conventional shimming techniques.
34(2007); http://dx.doi.org/10.1118/1.2761650View Description Hide Description
Purpose: Topotherapy is a new delivery technique that uses the binary MLC to implement static beam IMRT. We present a simple approach for real time motion adaptive delivery (MAD) of topotherapy plan using the current delivery system. Methods: Topotherapy uses fixed jaws and binary MLC for intensity modulation and its projection rate is a few Hz. Topotherapy delivery is controlled by a planned projection‐wised leaf sequence, which is optimized assuming stationary target. The couch proceeds in constant speed. To compensate longitudinal tumor motion in real time, instead of sequential execution of the planned leaf sequence, the projections are out of order executed. That is, a previous or future projection that with the same planned target position as the current status, is chosen instead. The transversal target motion is further compensated by shifting and scaling the MLC leaf open time at the chosen projection. Results: Extensive simulations with realistic topotherapy parameters and respiration motions validate MAD technique. The delivered dose conformed well to the target regardless how target moved during delivery. Significant margin reduction could be approached provided that real time tumor localization is achievable. Discussions and Conclusions: We present a novel technique for real time MAD in topotherapy. Unlike the dynamic MLC based tracking method, this technique requires neither the whole target motion trajectory nor the velocity of target motion. Instead, it only requires instantaneous target positions, which greatly simplifies the system implementation. This technique re‐uses the planned leaf sequence by re‐ordering its projection and leaf sequence. It does not involve time consuming re‐optimization. The simulations validated the presented technique. Experimental validation and extension to helical tomotherapy delivery is presented in another paper of this conference.
34(2007); http://dx.doi.org/10.1118/1.2761651View Description Hide Description
Purpose: The purpose of this study was to evaluate the daily setup error of small peripheral lungtumors based on implanted markers and stereoscopic x‐rays. Method and Materials:Twenty‐three patients with small peripheral lungtumors were implanted with radiographic markers for localized radiation therapy. Exhale CT scans were used for planning. At treatment, the patients were aligned to skin marks followed by exhale synchronized stereoscopic x‐rays (Exactrac™, BrainLAB). Setup error is evaluated as the shift from initial skin alignment to the implanted x‐ray alignment. The mean, σ mean, and RMS of σ for this data are reported. Results: A total of 790 daily treatment sessions on 23 patients were recorded. In all patients, an evaluation CT of the implanted marker was performed prior to treatment to verify marker location. The mean and σ for the 23 patients from initial setup to x‐ray localization were 0.2 ± 3.8 mm laterally, 1.6 ± 5.3 mm longitudinally, and −0.7 ± 7.8 mm vertically. The RMS error was Lat 5.5 mm, Long 7.7, and Vert 6.4 mm. Conclusion: A large patient study using implanted markers for lungtumor alignment indicated large initial setup shifts from skin marks. Daily random errors on the order of 8mm were reported. The range of these shifts is as large as 3.0 cm in some instances indicating that a small target would potentially be missed without better setup or with localization. The large systematic error is most likely due to the difference in tumor position from exhale breath hold (CT) relative to free breathing exhale (X‐Ray)..Conflict of Interest: This work is supported in part from a research grant from BrainLAB, Inc.
34(2007); http://dx.doi.org/10.1118/1.2761652View Description Hide Description
Purpose: For on‐line positioning corrections based on fiducial markers we have developed Stereographic Targeting (SGT). SGT is described and clinical results for prostate patients are reported. Method and Materials: SGT entails (1) acquisition of a MV and orthogonal kV image in rapid succession without gantry rotation, (2) marker detection and calculation of center of mass (COM) translation and rotations, (3) correction execution by remote couch control and (4) optional repetition of previous steps for verification. MV images are acquired with 2–6 MU of a treatment beam and kV images with Elekta Synergy®. Performance was measured for 10 prostate patients with 3–4 markers who had daily SGT with verification imaging plus an extra kV image at the end of each fraction to study intrafraction motion. Results: Compared with manual analysis, automatically determined marker positions in SGT were accurate within 0.5, 0.2, and 0.3 mm (SD) for the lateral (LR), cranial‐caudal (CC), and anteriorposterior (AP) directions respectively and within 1°(SD) for rotations. SGT added < 1 min to treatment time while it reduced COM systematic errors (Σ) from 4 mm to 0.5 mm (SD) and random errors (σ) from 3 mm to < 0.8 mm on all axes. The largest prostate rotations were about the LR axis: Σ = 3.4° and σ = 4.5°. The start‐to‐end intrafraction motion was 0.5 mm (Σ) and 1.5 mm (σ) for AP and 0.5 mm (Σ) and 1.4 mm (σ) for CC. Conclusion: SGT allows for fast, precise positioning (3D deviation < 3 mm within one minute) and is now routinely used for our prostate treatments. Taking into account rotations and the mainly random intrafraction motion, planning margins <=5 mm are obtained. While intrafraction motion can be reduced by repetitive SGT within one fraction, we develop a strategy to counter rotations as well.
34(2007); http://dx.doi.org/10.1118/1.2761653View Description Hide Description
Purpose: While ART has been studied for years, the specific quantitative implementation details have not. In order for this new scheme of radiation therapy to reach its potential, an effective ART planning strategy capable of taking into account the dosedelivery history and patient's on‐treatment geometric model must be in place. This work performs a study of dynamic closed‐loop control algorithms for ART and demonstrates their utilities with data from phantom and clinical cases with on‐treatment cone‐beam CTimages.Method: In closed‐loop control, the controller is not run just once but repeatedly, each time receiving the current state of the system as its input. To meet the requirements of different clinical applications, two classes of algorithms are developed: those Adapting to Changing Geometry (ACG) and those Adapting to Geometry and DeliveredDose (AGDD). The former takes into account organ deformations found just before treatment. The latter optimizes the dose distribution accumulated over the entire course of treatment by adapting at each fraction not only to the anatomic information just before treatment but also to the dosedelivery history. The closed‐loop algorithms are showcased by phantom and clinical cases. Results: A comparison of the approaches with conventional open‐loop IMRT without adaptively incorporating feedback information indicates that closed‐loop ART may significantly improve the current practice. In both phantom and clinical studies, AGDD outperforms ACG algorithms in three aspects: target dose coverage, sensitive‐structure sparing, and steeper gradients around the tumor. Within the AGDD formalism, it is beneficial not to correct all the previous dosimetric errors at once right after the information is available but over a number of fractions until next set of feedback data is available. Conclusion:ART with closed‐loop dynamic algorithms substantially improve the dose distribution. In addition, the differing performance of the specific implementations shows that the algorithmic details matter.
34(2007); http://dx.doi.org/10.1118/1.2761654View Description Hide Description
Purpose: A prototype mobile C‐arm capable of real‐time fluoroscopy,tomosynthesis, and cone‐beam CT(CBCT) has been developed for intraoperative guidance of minimally invasive procedures. This work investigates the imaging performance, radiation dose, and image acquisition / reconstruction time associated with a range of 3D imaging techniques. In addition, protocols for intraoperative use are developed for implementation in clinical research trials. Method and Materials:Imaging performance was assessed in terms of spatial resolution as well as soft‐tissue and bony detail visualization in phantoms and cadavers. Image reconstruction via 3D filtered backprojection was performed across a range of tomosynthesis angle (10°–178°) and number of projections (5–200). The corresponding radiation dose was evaluated analytically and experimentally (using a Farmer chamber in cylindrical head and body phantoms) to quantify peripheral, central, and organ dose (e.g., dose to the eyes). Acquisition / reconstruction times were benchmarked in relation to operational time constraints. Results:CBCTimages exhibited sub‐mm spatial resolution and soft‐tissue visibility (∼20 HU) sufficient for interventional guidance at a central dose of 10 mGy. Tomosynthesis offered a useful, high‐speed adjuvant to CBCT, providing visualization of high‐contrast features at a fairly limited arc — e.g., visualization of the clivus (skull base) achieved at angles down to ∼30° (central dose ∼1.6 mGy). Reconstruction times were ∼62 sec (full CBCT volume, 0.8 mm voxels) and ∼20 sec (tomosynthesis “slab,” 0.4 mm voxels), respectively.Conclusion:C‐arm tomosynthesis and CBCT provide a valuable addition to the image guidance arsenal. The former offers fast, low‐dose 3D images sufficient for guidance with respect to high‐contrast bony features. The latter offers sub‐mm spatial resolution and soft‐tissue visibility at the cost of time and dose. Deployment of the C‐arm prototype in clinical research trials promises increased surgical precision, with protocols for intraoperative imaging consistent with operational time and dose constraints.
34(2007); http://dx.doi.org/10.1118/1.2761655View Description Hide Description
Purpose: Intra‐operative dosimetryoptimization of TRUS‐guided prostate brachytherapy requires localization of seeds relative to the prostate [Todor et al. PMB 48(9):1153–71]. Method and Materials: Seeds were reconstructed from C‐arm fluoroscopy, registered to TRUS, and exported to commercial brachytherapy system for dosimetryoptimization. Technical obstacles included pose‐dependant C‐arm calibration; distortion correction; pose estimation of C‐arm images; registering C‐arm to TRUS; and seed reconstruction. We proved that calibration is not critical and distortion correction in AP suffices. A radiographic tracking fiducial was attached in known pose over the template to recover the C‐arm pose from a single image, relative to the template. The 3D coordinates of the segmented seeds are calculated upon resolving the correspondence of seeds in the multiple C‐arm images, by formalizing seed‐matching as a network‐flow problem [Jain et al. Med Phys, 32(11):3475–92]. The seed locations are exported in template coordinates to the Interplant® commercial system for dosimetry analysis and optimization.Results: In precision‐machined hard phantoms with 40–100 seeds, we correctly reconstructed 98.5% seeds with a mean 3D accuracy of 0.63mm (0.91mm error for mismatched seeds). In soft tissue phantoms with 45–87 seeds and clinically realistic 15° C‐arm motion, we correctly reconstructed 100% seeds with 1.5mm absolute accuracy (0.25mm relative accuracy), and registered them to the prostate segmented from TRUS with an accuracy of 3.4mm (0.82mm relative). In a Phase‐1 clinical trial, so far on 4 patients with 66–84 seeds, we achieved intra‐operative monitoring of seed distribution and dosimetry. We optimized the V100 dose by inserting 3–9 additional seeds. Conclusion: Intra‐operative doseoptimization is possible with an average C‐arm, at negligible additional cost to existing clinical installations. Conflict of Interest: The work has been done in collaboration with Acoustic MedSystems for seed import into the Interplant®. Keywords: Prostate Brachytherapy, Intra‐operative Dosimetry.Support: DoD/PC‐050170, DoD/PC‐050042, NIH‐1R43CA099374, NSF‐9731478.
TH‐C‐M100J‐08: Dosimetric Effects of a 4D Magnetic Localization System for LINAC Beam Gating On Prostate and Lung Radiation Therapy34(2007); http://dx.doi.org/10.1118/1.2761656View Description Hide Description
Purpose: The Calypso® Medical 4D Localization System is capable of tracking continuous dynamic motion, and has been FDA cleared for use in the prostate. To date, automatic intervention for the measured motion is not a function of the system. We investigated use of the system to gate radiation therapydelivery on a motion phantom for both a lung and prostate fraction Materials and Methods: A Calypso gating prototype system with an optically isolated relay was connected to the BEAM_HOLD interface of a Varian Trilogy linac. A sample 3D lung plan and SMLC prostate plan were randomly selected from the active patient list. The Washington University 4D phantom was programmed to move a film box through 2 patient‐measured prostate and lung trajectories. Radiation therapy was delivered to the static phantom, to the phantom undergoing each motion trajectory without gating, and to the phantom undergoing motion trajectories while the Calypso System was gating the linac. A 4×4×4 mm gating window centered on isocenter was used for the prostate, while a linear gating window corresponding to exhalation (ie 2 mm<y<5 mm) was used for the lung.Results: Without gating beam delivery, prostate motion during treatment delivery caused approximately 10% underdosing and overdosing due to the superior/inferior and anterior/posterior prostate motion. The Calypso System triggering gated therapy delivery reduced these areas significantly, as seen in difference images using film dosimetry. Likewise, lungtumor motion caused a geometric mismatch between static and motion delivery. The Calypso System with prototype gating showed a decrease in this mismatch. Conclusions: A wireless electromagnetic implanted transponder system for linac gating was effective in reducing dosimetric errors caused by prostate and lung motion. More work is needed to define optimal gating windows to provide maximal clinical efficiency with minimal delivery error.
This work was supported by Calypso Medical Technologies.
TH‐C‐M100J‐09: MR Guided Focused Ultrasound (MRgFU) For The Treatmentment of Prostate Cancer: A Feasibility Study of Increasing Cellular Uptake of AS‐MDM2 in Vivo34(2007); http://dx.doi.org/10.1118/1.2761657View Description Hide Description
Purpose: MDM2 is an oncogene and overexpressed in 30–40% of prostate cancer. Antisense MDM2 oligonucleotide (AS‐MDM2) inhibits MDM2 expression, and enhances the effects of radiation and chemotherapy on prostate cancer. The purpose of this study is to investigate the feasibility of increasing the cellular uptake of AS‐MDM2 using MR guided High Intensity Focused Ultrasound (MRgFU). Materials and Methods: A HIFU system (InSightec ExAblate 2000) and a 1.5T MR scanner(GE) were used for this study. Human prostate cancer cells LNCaP 105, were grown orthotopically in the prostates of 11 nude mice. Extensive experiments were performed to determine the optimal MR parameters for target delineation and the optimal ultrasound parameters for animal treatment studies using an acoustic phantom. The mice bearing implanted prostate tumors (61±22mm3) were treated under general anesthesia using pulsed focused ultrasound with the output acoustic power of 4W, pulse width of 100msec and either 300 or 900 pulses in one sonication. The focal region is cigar shaped, about 2mm in diameter and 10mm in focal length. The focal peak was set within the target under the MR guidance. Two to four sonications were used to cover the whole tumor. Immediately after the treatment 0.1ml of AS‐MDM2, dissolved in PBS, was given by tail vein injection at doses 25mg/kg. After 24hr, the animals were sacrificed and tumors were removed. The expression levels of p53 proteins were analyzed by immunohistochemical staining. Results: Our preliminary results showed that the animals tolerated well the HIFU treatment. With 300 pulses per site in each sonication blood cell extravasation on H&E staining and an increase in p53 expression (5.0±2.4%) in the treatedtumor bearing LNCaP cells were observed as compared to the control group (1.3 ± 0.5%). Conclusion: MRgFU can be used an alternative treatment modality for prostate cancertreatment.
34(2007); http://dx.doi.org/10.1118/1.2761658View Description Hide Description
Purpose: To develop and employ novel image guidance methods for targeting in the stereotactic functional procedure of deep brain stimulation (DBS) in regions that are poorly defined with anatomic imaging using a deformable atlas and functional imaging.Method and Materials: An image guidance system was developed to enhance targeting for stereotactic DBS surgeries. An atlas of the structures in the basal ganglia was created from the Schaltenbrand‐Bailey series of histologically stained sagittal and axial sections. By defining a surface that connects each plane, a voxelized binary atlas was created and smoothed to reduce inconsistencies. A set of programs were created using Matlab to allow for user driven linear atlas deformation to match the atlas with patient specific anatomy and landmarks. An additional set of programs were created to record intraoperative microelectrode recording (MER) maps and to visualize these maps through sagittal and coronal cuts in the patient deformed atlas. To add additional functional information, a high resolution functional MRI (fMRI) protocol was developed that allows for localization of motor, sensory, language, and emotional regions in the basal ganglia. Software to visualize the deformed atlas, MRI and fMRI all together was created to allow for target definition and planning based off multiple sources of information simultaneously. Results: The developed atlas‐based image guidance system has been used as a clinical tool for several months and now allows physicians the ability to deform an anatomic atlas to patient specific anatomy and also obtain and view electrode tracks through the atlas in oblique angles. fMRI data on initial subjects has shown good qualitative agreement with expected physiological locations and MER maps in patients. Conclusion: This work allows for improved targeting in DBS based off the simultaneous usage of a 3D deformed atlas, microelectrode recording maps, and fMRI data.
- Imaging for Therapy Assessment
34(2007); http://dx.doi.org/10.1118/1.2761754View Description Hide Description
Purpose: To develop a reproducible, tissue equivalent, deformable lung phantom for verification of 4D‐CT scanning procedures, deformable image registration (DIR) and 4D dose calculation in moving/deformable anatomies.Methods and Materials: The phantom consists of a Lucite cylinder filled with water containing a latex balloon filled with dampened natural sponges. The balloon is attached to a piston that mimics the human diaphragm. Nylon wires and Lucite beads, emulating vascular and bronchial bifurcations, were glued at various locations, uniformly, throughout the sponges. The phantom is capable of simulating programmed irregular breathing patterns with varying periods and amplitudes. A deformable, tissue equivalent tumour holding radiochromic film was embedded in the sponge. Eight 3D‐CT datasets (0.7×0.7×1.25 mm) of the phantom in eight static positions of the piston were acquired. 3D trajectories of 12 anatomical point landmarks as well as the tumour center‐of‐mass were studied. Results: Reproducible lung deformation is achieved by piston‐provoked pressure changes in water surrounding the deforming balloon. The resulting mean density for the artificial lung was 0.243 g/cm3 comparable to 0.252 g/cm3 for a real lung. A truly 3D, non‐isotropic deformation of the balloon similar to a real lung has been obtained. The SI displacement of the landmarks varies between 94% and 3% of the diaphragm excursion for positions closer and farther away from the diaphragm, respectively. Reproducibility in the deformed phantom, established by seven repeat scans at the same phantom compression state, was within image resolution. The accuracy of DIR of the extreme phases was 0.7(±0.7) mm. Conclusions: Our novel phantom is tissue‐equivalent, deformable, and can reproducibly emulate 3D non‐isotropic lung deformations. The presence of vascular and bronchial bifurcations allows verification of DIR of 4D‐CT images of the phantom. Registered phases of the phantom can be used in 4D dose calculations that can be validated by comparison with dose measurements.
TH‐E‐M100J‐02: Potential of Dynamic Contrast Enhanced‐Magnetic Resonance Imaging (DCE‐MRI) Extracted Parameters to Estimate Treatment Response in Locally Advanced Head and Neck (LAHN) Cancer Patients34(2007); http://dx.doi.org/10.1118/1.2761755View Description Hide Description
Purpose: To evaluate the use of DCE‐MRI to measure changes in tumorphysiology in LAHN cancer patients receiving TT and cisplatin based concurrent chemoradiation (ChemoRT). Material and Methods: Eligible patients with LAHN were enrolled on an IRB approved clinical trial to establish the safety and efficacy of adding TT (bevacizumab and erlotinib) to ChemoRT. To quantify this efficacy, DCE‐MR images were acquired on a 1.5T GE Signa Exite scanner before treatment started, at the end of the lead‐in phase (2 weeks of TT alone), at the end of week 1 of ChemoRT, and at the end of the ChemoRT (70 Gy). The images were analyzed using a full Time Point (fTP) pharmacokinetic analysis implemented by CAD Sciences® (White Plains, NY) that measures the vascular permeability (PERM) and extracellular volume fraction (EVF). The T10 for the primary and nodes was determined from series acquired with varying TRs. A dynamic 3D spoiled gradient echo sequence was used before and after bolus injection of Gd DTPA (Magnevist®). Regions of interest (ROIs) were defined over the entire extent of the tumor and LN, respectively. Enhancement curve analysis, PERM and EVF statistics and ROI volume comparisons were performed. Results: The T10 for tumor was 1500 msec, and 2000 msec for LN. All the fTP analyses used these values. Fifteen patients have been imaged to date. We found that coronal imaging allows better ROI selection without vascular averaging for this type of subjects.. Detailed analyses of all time points have been completed on three patients, all with clinical complete response. Combined PERM and EVF analyses showed marked decreases with treatment (p<0.014). Conclusions: Preliminary results demonstrate the feasibility of using DCE‐MRI to measure treatment induced changes in tumorphysiology. Correlations between these changes and treatment outcome will be determined as data from the remaining patients is analyzed.
34(2007); http://dx.doi.org/10.1118/1.2761756View Description Hide Description
Purpose: This study investigates whether changes in dynamic contrast enhanced (DCE) MR imaging during the early course of radiation therapy (RT) are correlated with local tumorcontrol in patients with advanced head and neck (H/N) cancer. We hypothesize that an early decrease in tumor perfusion or vascular permeability would be an indicator of early tumor response to RT. Methods: Patients who had newly diagnosed extracranial head and neck cancer and underwent a 7‐week course of definitive chemo‐RT participated in an IRB approved pilot study. Patients had three DCE MRI scans prior to RT, after 2 weeks of RT, and 3 months after the completion of RT. Perfusion parameters of the Gd‐DTPA transfer constant (K represents a combination of blood flow and vascular permeability) and the fractional blood volume (Vp) were estimated using the modified Toft model. Local tumor response was assessed clinically. The association between the changes in perfusion parameters of the tumor during the early course of RT and local tumor responses was analyzed. Results: For the patients in whom complete response was achieved, K in the tumor had the largest reduction, approximately 30%, after 2 weeks of RT compared to prior to RT. However, for the patients whose tumors failed to respond to or partially responded to RT, K in the tumor either increased by ∼80%, or decreased slightly. In contrast, changes in tumor Vp after 2 weeks of radiation were less specific for non‐responders compared to the changes in the tumor K. Conclusion: Preliminary data of this on‐going study suggest that DCE might have potential for early assessment of tumor response to radiation and chemo therapy in advance head and neck cancers.
TH‐E‐M100J‐04: Quantitative Assessment of Tissue Toxicity in Breast Cancer Radiation Therapy Using Spectrophotometry and Ultrasonic Tissue Characterization Imaging34(2007); http://dx.doi.org/10.1118/1.2761757View Description Hide Description
Purpose: To investigate a novel combination of two non‐invasive techniques, spectrophotometry and ultrasoundtissue characterization (UTC) imaging, to quantitatively evaluate breast tissue toxicity in radiation treatment.Method and Materials:Skin and soft tissue injury are the most common toxicities of breast cancerradiation therapy. There is currently no objective means of measuring breast tissue injury in the clinic. We investigated the combination of Spectrophotometry and UTC imaging to examine radiation toxicity. A spectrophotometer (Mexameter® MX) was used to measure the radiation damage to the skin surface and an ultrasound scanner (Ultrasonix® with 14‐MHz probe) was utilized to measure the soft tissue changes. Six imaging parameters were computed to quantitatively measure toxicity: melanin, erythema, skin thickness, UTC slope, intercept and midband value. Subjective clinical assessment of toxicity was done by a radiation oncologist. Statistical analysis was performed to correlate spectrophotometer and UTC findings with clinical assessments (RTOG clinical toxicity scale). To date, twelve breast cancer patients were enrolled. All patients received a standard course of radiation: the whole breast received 50–50.4 Gy followed by an electron boost of 10–16 Gy at the lumpectomy site. Each patient received a series of spectrophotometer and ultrasound scans prior to, during and post radiation treatment.Results: Twenty‐eight spectrophotometer and ultrasound scans were performed. The contra‐lateral (untreated) breast scans showed good accuracy and reproducibility. Our spectrophotometer and UTC evaluations were consistent with the clinical breast toxicity assessments. We observed significant patient variations. During six‐week radiation treatment, patient melanin increases were between 5 to 20%, erythema increases varied from 0% to 100%. Conclusion: The combination of spectrophotometer and UTC provides effective means of assessing radiation damage to the skin and the underlying breast tissue respectively. This tool becomes increasingly valuable as we evaluate new strategies for breast cancerradiation therapy, such as partial breast IMRT and MammoSite.
34(2007); http://dx.doi.org/10.1118/1.2761758View Description Hide Description
Purpose: Capability to assess treatment response early could dramatically change the practice of radiation therapy as it is performed today. Our aim was to investigate feasibility of repeat 3′‐Deoxy‐3′‐fluorothymidine ([F‐18]‐FLT) PET/CT imaging as a surrogate for early treatment response. Materials and Methods: Multiple patients with different tumor types — head and neck, lung, esophagus and CNS — underwent either a radical (6 weeks) or palliative (2 weeks) radio‐ or chemoradiotherapy. Each patient was imaged twice with FLT‐PET/CT, a marker of cell proliferation. The baseline FLT‐PET/CT scan was acquired up to a week before the start of radiotherapy or chemoradiotherapy. The follow‐up FLT‐PET/CT scan was acquired in the first half of therapy, typically after one to two‐weeks of radiotherapy. The FLT‐PET image acquisition was dynamic over 90 minutes after the injection of FLT. The CT data between the imaging sessions was co‐registered and the corresponding PET data compared and analyzed.Results: The FLT uptake has proved to be high compared to the normal background activity for all investigated tumor types. The absolute FLT uptake varied significantly for different tumor types, with standardized uptake values (SUV) ranging from less than 1 in CNS tumors to over 10 in head and neck and lungtumors. Detectable change in proliferative activity (>20%) was observed within a week of the initiation of therapy. Significant inter‐ and intra‐patient, and high tumor heterogeneity was observed. Conclusions: FLT‐PET/CT imaging is an extremely powerful tool for early assessment of treatment response for a variety of tumor types. While the absolute FLT uptake varied significantly between different tumor types, the signal to background ratio was high. Early treatment assessment is possible as early as one week after the start of therapy. High inter‐ and intra‐patient tumor response variability offers unprecedented possibilities for treatment adaptation.
TH‐E‐M100J‐06: Parotid Dose Deviation Related to Parotid Volume Reduction During Radiation Therapy: An Analysis Based On TomoTherapy Adaptive Tool34(2007); http://dx.doi.org/10.1118/1.2761759View Description Hide Description
Purpose: To determine the parotid dose deviation from the planned dosimetry of oropharyngeal patients treated by Helical Tomotherapy as a consequence of patient deformation. Methods and materials: MVCTs of 14 oropharyngeal patients who underwent tomotherapy treatment to 50–56 Gy/25–28 fractions were used to calculate the actual delivered summation dose and dose volume histogram (DVH). The mean parotid dose (MPD) was analyzed as a function of the body and local weight loss, which was calculated using MVCT electron density in a range 2.5 cm superior/inferior of the parotids and interfield distance (IFD) change at the midlevel of the parotid (Dp ). The deformable registration and automatically generated contours were reviewed by the radiation oncologist.Results: The automatically generated contours were considered to be adequate by the physician for dosimetric analysis. MPD changes between 0 to 2.5 Gy were observed, with accelerating changes after fraction 15. The MPD change was found to correlate to with local anatomic change as opposed to total body weight. The dependency on the local body weight calculated by MVCT electron density is positive but not significant. However, a strong correlation between MPD and Dp was observed (R2 >0.9). Dp also linearly decreased with the number of fractions with patient specific slopes due mainly to parotid volume reduction. Conclusions: We evaluated the Tomotherapy adaptive radiotherapy(ART) tool's deformable registration capability based on daily MVCT of 14 oropharyngeal patients underwent 5–5.5 weeks of Tomotherapy treatment. The mean parotid dose (MPD) change correlated with local anatomy changes with reduced parotid volume resulting in a reduction in a mid‐parotid level IFD change in Dp . The patient specific slope determined by Dp and MPD may be used to monitor and predict the parotid dose error due to patient deformation.
TH‐E‐M100J‐07: Changes in Standardized Uptake Value of Brain Tissues in Pediatric Brain Tumor Patients Treated with Chemoradiotherapy34(2007); http://dx.doi.org/10.1118/1.2761760View Description Hide Description
Purpose: Survivors of pediatric braintumors who received radiation and chemotherapy may develop late effects involving neurologic, endocrine, and cognitive function. Objective measures demonstrating normal tissue effects may allow for early intervention. In this study, we investigate the feasibility of using 18F‐flurodeoxyglucose (FDG) PET to monitor changes in glucose metabolism of normal braintissues in children with braintumorstreated with combined modality therapy. Methods: Nine children with brainstem glioma (BSG) treated with 55.8Gy and concurrent chemotherapy were evaluated with pre‐ and post‐irradiation (2–3 months) FDG CT‐PET (GE Medical Systems, Milwaukee, WI). CT‐PET data were registered with T1‐weighted MR images. Volumes of interest were contoured from which standardized uptake values (SUV) were calculated using commercially available software (HERMES Medical Solutions, Stockholm). For comparison, nine children with Hodgkin's diseases (HD) were analyzed. The HD patients had no evidence of CNS involvement at the time of their evaluation and served as a control group with normal brain metabolism. Results: The cerebellar SUV of BSG patients showed a statistically significant increase comparing mean values pre‐ and post‐therapy (3.2±1.0 vs. 5.9±1.3, p=0.0005). No difference was observed comparing pre‐ and post‐therapy values in patients with HD (6.1±1.6 vs. 5.8±1.2). For BSG patients, the SUV of basal ganglia, medial temporal lobe, and frontal/occipital cortex showed a similar increase as those observed for the cerebellum despite differences in radiationdoses.Conclusion: Children with BSG demonstrate a reduction in cerebral and cerebellar glucose uptake. The low SUV of these braintumor patients prior to treatment may be due to decreased glucose metabolism resulting from the regional effects of tumor, increased intracranial pressure with diminished blood flow or corticosteroids taken before and during therapy. Better understanding of this phenomenon will be required in order to incorporate FDG‐PET into the assessment of radiation effects on the brain.
- Motion Modeling
34(2007); http://dx.doi.org/10.1118/1.2761332View Description Hide Description
Purpose: To examine the influence of phase delays and baseline drifting on residual motion in gated radiotherapy.Method and Materials: Four patients with hepatocellular carcinoma were studied within a phase I clinical trial for gated proton beam radiotherapy. Between 3 and 5 gold fiducial markers were implanted in or near the tumor site to serve as internal surrogates for the treatment target. The Varian Real‐time Position Management (RPM) system was used as an external surrogate for respiration. The RPM was synchronized to an orthogonal bi‐plane fluoroscopic imaging system, and simulation sessions were conducted under free breathing. The three‐dimensional coordinates of each marker were tracked retrospectively, and compared with the synchronized RPM signal. The phase delay from internal position to external surrogate, and baseline drifting of the exhale position over the course of several minutes were measured, and their impact on gated treatment was assessed. Results: Phase delays were found, ranging from −0.02 seconds to +0.22 seconds, indicating that motion inside the liver may not be seen immediately on the external surface. Phase delays varied from marker to marker, and also over time during the course of 4 to 6 minutes. Baseline drifting was also found in two of the four patients, indicating that the exhale position of the liver may not be stable within the first several minutes after lying on the treatment couch. The baseline drift was usually from inferior to superior, with values ranging from −1 mm to 6 mm. The worst‐case residual motion of an amplitude‐gated treatment was estimated to be 7.4 mm. Conclusion: We observed phase delays of more than 100 ms in two of four patients, and baseline drifting of 5 mm or more in two of four patients. The utility of external surrogates for gating should be studied in more detail.
34(2007); http://dx.doi.org/10.1118/1.2761333View Description Hide Description
Purpose: To develop and evaluate a template matching algorithm for direct detection of a lungtumor trajectory in the raw projections acquired from a cone beam CT.Method and Materials: A planning respiration‐correlated CT (pRCCT) was used to construct digitally‐reconstructed radiographic(DRR) templates for registration with raw projection data of a free‐breathing cone beam CT (FB‐CBCT). The pRCCT and FB‐CBCT were first registered to obtain the approximate mean 3D position of the tumor. This position was applied as a shift to the planning isocenter in the pRCCT. Next, 72 DRRs were generated at 5 degree angular increments around this shifted position, for each phase in the pRCCT. The appropriate phase DRR template for registration was selected using the measured phase of the FB‐CBCT projection. This phase was measured by tracking the diaphragm apex position in the projections. Two template matching algorithms were compared: normalized cross correlation and a block normalized cross correlation. A dynamic phantom was induced to move reproducibly 2cm during planning and 3cm during online CBCTimaging. The phantom was imaged at isocenter and with a 1cm translational shift from isocenter. Results: The tumor excursion measured with the normalized cross correlation method was 28.7 mm, and 28.5 mm with the block normalized cross correlation. The error in detecting the daily standard deviation of tumor position was 0.1mm for both methods. The mean absolute difference in detected tumor position was 1.07 mm, with a 95% confidence value of 2.43 mm. Block normalized cross correlation reduced the maximum error in tumor position detection slightly, in relation to normalized cross correlation. Conclusion: Direct detection of the tumor position from cone beam CT projections is feasible with template matching, provided that appropriate and accurate templates are used. Conflict of Interest: Supported in part by NIH R01 CA116249 and Elekta, Inc.
TU‐C‐M100J‐03: Objective Assessment of Deformable Image Registration in Radiotherapy — a Multi‐Institution Study34(2007); http://dx.doi.org/10.1118/1.2761334View Description Hide Description
Purpose: The looming potential of deformable alignment tools to play an integral role in adaptive radiotherapy suggests a need for objective assessment of these complex algorithms. Current studies of accuracy require analytically generated deformations applied to sample image data, or use of contours or bifurcations, with inherent uncertainty as well as potential bias, as these visible features are likely to dominate the local deformation parameters. Method and Materials: A deformable phantom was embedded with 48 small radiopaque markers, placed in regions varying from high contrast to roughly uniform regional intensity, and small to large regional discontinuities in movement. CT volumes of this phantom were acquired in two different deformation states. After manual localization of marker coordinates, images were edited to remove the markers. The resulting image volumes were sent to 4 collaborating institutions, each of which has developed previously published deformable alignment tools routinely in use. Alignments were done, and applied to the list of reference coordinates. The transformed coordinates were compared to the actual marker locations. Results: 5 alignment techniques were tested from the 4 institutions. All algorithms performed generally well, as compared to previous publications. Average errors in predicted location ranged from 1.7–3.9 mm, depending on technique. No algorithm was uniformly accurate across all regions of the phantom, with maximum errors ranging from 5.5 – 15.4 mm. Larger errors were seen in regions near significant shape changes, as well as areas with uniform contrast but large local motion discontinuity.Conclusion: An objective test of deformable alignment is feasible. Reasonable accuracy was achieved, although variable errors in different regions suggest caution in globally accepting the results from deformable alignment. Valuable feedback from investigators is being applied to development of further generations of deformable phantoms for alignment experiments.
Sponsored by NIH P01‐CA59827.