Vikren Sarkar

The effect of a limited number of projections and reconstruction algorithms on the image quality of megavoltage digital tomosynthesis.

In order to investigate the effect of the number of projections on digital tomosynthesis image quality, images were acquired over a 40 degree arc and sampled into sets of 2 to 41 projections used as input to three different reconstruction algorithms: the shift‐and‐add, the Feldkamp‐Davis‐Kress filtered back projection algorithms, and the simultaneous algebraic reconstruction technique. The variation of several image characteristics, such as in‐plane resolution, contrast to noise ratio, artifact spread, volumetric accuracy, and dose, are investigated based on the reconstruction algorithms used and also the number of projections used as source data. The results suggest that only 11 projections are required since the various parameters checked do not improve much past that number. As a reconstruction algorithm, SART did best but took much longer to reconstruct images. Thus, if reconstruction time is a determining factor, filtered back‐projection looks like a better compromise. PACS number: 87.57.C‐, 87.57.nf, 87.57.Q‐, 87.59.‐e

Individualized margins for prostate patients using a wireless localization and tracking system.

This study investigates the dosimetric benefits of designing patient‐specific margins for prostate cancer patients based on 4D localization and tracking. Ten prostate patients, each implanted with three radiofrequency transponders, were localized and tracked for 40 fractions. “Conventional margin” (CM) planning target volumes (PTV) and PTVs resulting from uniform margins of 5 mm (5M) and 7 mm (7M) were explored. Through retrospective review of each patients tracking data, an individualized margin (IM) design for each patient was determined. IMRT treatment plans with identical constraints were generated for all four margin strategies and compared. The IM plans generally created the smallest PTV volumes. For similar PTV coverage, the IM plans had a lower mean bladder (rectal) dose by an average of 3.9% (2.5%), 8.5% (5.7%) and 16.2 % (9.8%) compared to 5M, 7M and CM plans, respectively. The IM plan had the lowest gEUD value of 23.8 Gy for bladder, compared to 35.1, 28.4 and 25.7, for CM, 7M and 5M, respectively. Likewise, the IM plan had the lowest NTCP value for rectum of 0.04, compared to 0.07, 0.06 and 0.05 for CM, 7M and 5M, respectively. Individualized margins can lead to significantly reduced PTV volumes and critical structure doses, while still ensuring a minimum delivered CTV dose equal to 95% of the prescribed dose. PACS numbers: 87.53.Kn, 87.55.D

Optimization of Isocenter Location for Intensity Modulated Stereotactic Treatment of Small Intracranial Targets

PURPOSE To quantify the impact of isocenter location on treatment plan quality for intensity-modulated stereotactic treatment of small intracranial lesions. METHODS AND MATERIALS For 18 patients previously treated by stereotactic-intensity modulated radiosurgery (IMRS) or intensity-modulated radiation therapy (IMRT), a retrospective virtual planning study was conducted wherein the impact of isocenter location on plan quality was measured. Treatment indications studied included six arteriovenous malformations, six acoustic neuromas, and six intracranial metastases, ranging in volume from 0.71 to 3.21 cm(3) (mean = 2.26 cm(3)), 1.08 to 2.84 cm(3) (mean = 1.73 cm(3)), and 0.19 to 2.30 cm(3) (mean = 0.79 cm(3)), respectively. Variation of isocenter location causes the geometric grid of pencil beams into which the target is segmented for intensity-modulated treatment to be altered. The impact of this pencil-beam-grid redefinition on achievable conformity index was quantified for three collimators (Varian Millennium 120; BrainLab MM3; Nomos binary Mimic) and three treatment planning systems (TPS; Varian Eclipse v6.5; BrainLab BrainScan v5.31; Best-Nomos Corvus v6.2), resulting in the evaluation of 3,446 treatment plans. RESULTS For all patients, collimator, and TPS combinations studied, a significant variation in plan quality was observed as a function of isocenter and pencil-beam-grid relocation. Optimization of isocenter location resulted in treatment plan conformity variations as large as 109% (min = 15%, mean = 51%, max = 109%). CONCLUSION Optimization of isocenter location for IMRT/IMRS treatment of small intracranial lesions in which pencil-beam dimensions are comparable to target dimensions, can result in significant improvements in treatment plan quality.

Planning for mARC treatments with the Eclipse treatment planning system

While modulated arc (mARC) capabilities have been available on Siemens linear accelerators for almost two years now, there was, until recently, only one treatment planning system capable of planning these treatments. The Eclipse treatment planning system now offers a module that can plan for mARC treatments. The purpose of this work was to test the module to determine whether it is capable of creating clinically acceptable plans. A total of 23 plans were created for various clinical sites and all plans delivered without anomaly. The average 3%/3 mm gamma pass rate for the plans was 98.0%, with a standard deviation of 1.7%. For a total of 14 plans, an equivalent static gantry IMRT plan was also created to compare delivery time. In all but two cases, the mARC plans delivered significantly faster than the static gantry plan. We have confirmed the successful creation of mARC plans that are deliverable with high fidelity on an ARTISTE linear accelerator, thus demonstrating the successful implementation of the Eclipse mARC module. PACS numbers: 87.55.D‐, 87.55.ne, 87.57.uq,

Four-dimensional tissue deformation reconstruction (4D TDR) validation using a real tissue phantom

Calculation of four‐dimensional (4D) dose distributions requires the remapping of dose calculated on each available binned phase of the 4D CT onto a reference phase for summation. Deformable image registration (DIR) is usually used for this task, but unfortunately almost always considers only endpoints rather than the whole motion path. A new algorithm, 4D tissue deformation reconstruction (4D TDR), that uses either CT projection data or all available 4D CT images to reconstruct 4D motion data, was developed. The purpose of this work is to verify the accuracy of the fit of this new algorithm using a realistic tissue phantom. A previously described fresh tissue phantom with implanted electromagnetic tracking (EMT) fiducials was used for this experiment. The phantom was animated using a sinusoidal and a real patient‐breathing signal. Four‐dimensional computer tomography (4D CT) and EMT tracking were performed. Deformation reconstruction was conducted using the 4D TDR and a modified 4D TDR which takes real tissue hysteresis (4D TDRHysteresis) into account. Deformation estimation results were compared to the EMT and 4D CT coordinate measurements. To eliminate the possibility of the high contrast markers driving the 4D TDR, a comparison was made using the original 4D CT data and data in which the fiducials were electronically masked. For the sinusoidal animation, the average deviation of the 4D TDR compared to the manually determined coordinates from 4D CT data was 1.9 mm, albeit with as large as 4.5 mm deviation. The 4D TDR calculation traces matched 95% of the EMT trace within 2.8 mm. The motion hysteresis generated by real tissue is not properly projected other than at endpoints of motion. Sinusoidal animation resulted in 95% of EMT measured locations to be within less than 1.2 mm of the measured 4D CT motion path, enabling accurate motion characterization of the tissue hysteresis. The 4D TDRHysteresis calculation traces accounted well for the hysteresis and matched 95% of the EMT trace within 1.6 mm. An irregular (in amplitude and frequency) recorded patient trace applied to the same tissue resulted in 95% of the EMT trace points within less than 4.5 mm when compared to both the 4D CT and 4D TDRHysteresis motion paths. The average deviation of 4D TDRHysteresis compared to 4D CT datasets was 0.9 mm under regular sinusoidal and 1.0 mm under irregular patient trace animation. The EMT trace data fit to the 4D TDRHysteresis was within 1.6 mm for sinusoidal and 4.5 mm for patient trace animation. While various algorithms have been validated for end‐to‐end accuracy, one can only be fully confident in the performance of a predictive algorithm if one looks at data along the full motion path. The 4D TDR, calculating the whole motion path rather than only phase‐ or endpoints, allows us to fully characterize the accuracy of a predictive algorithm, minimizing assumptions. This algorithm went one step further by allowing for the inclusion of tissue hysteresis effects, a real‐world effect that is neglected when endpoint‐only validation is performed. Our results show that the 4D TDRHysteresis correctly models the deformation at the endpoints and any intermediate points along the motion path. PACS numbers: 87.55.km, 87.55.Qr, 87.57.nf, 87.85.Tu

The effect of pH and viscosity on bovine spermatozoa motility under controlled conditions

Infertility in a large proportion of infertile couples is due, in part, to the male factor. Spermatozoa must survive the unique environment of the female reproductive tract in their path to fertilize the ovum. This fact is even more problematic for men with oligospermia. Of the many environmental factors that effect sperm motility in the female reproductive tract, we have decided to concentrate on the effect of pH and viscosity on bovine spermatozoa in the laboratory setting. Follicular fluid was harvested from heifer ovaries to serve as a chemo-attractant. Through image analysis, our data shows trends of sperm motility as a function of pH and viscosity. There is a significant increase in the number of immotile sperm seen as pH decreases from 6.5 to 6.0. Furthermore, an exponential relationship between sperm speed and environmental viscosity was observed in vitro. This suggests that modulating vaginal pH and vaginal secretion viscosity could greatly affect spermatozoa motility and therefore male fertility.

Discovering Related Clinical Concepts Using Large Amounts of Clinical Notes.

The ability to find highly related clinical concepts is essential for many applications such as for hypothesis generation, query expansion for medical literature search, search results filtering, ICD-10 code filtering and many other applications. While manually constructed medical terminologies such as SNOMED CT can surface certain related concepts, these terminologies are inadequate as they depend on expertise of several subject matter experts making the terminology curation process open to geographic and language bias. In addition, these terminologies also provide no quantifiable evidence on how related the concepts are. In this work, we explore an unsupervised graphical approach to mine related concepts by leveraging the volume within large amounts of clinical notes. Our evaluation shows that we are able to use a data driven approach to discovering highly related concepts for various search terms including medications, symptoms and diseases.

Percent depth-dose distribution discrepancies from very small volume ion chambers.

As very small ion chambers become commercially available, medical physicists may be inclined to use them during the linear accelerator commissioning process to better characterize the beam in steep dose gradient areas. For this work, a total of eight different ion chambers (volumes from 0.007 cc to 0.6 cc) and four different scanning systems were used to scan PDDs at both +300V and −300V biases. We observed a reproducible, significant difference (overresponse with depth) in PDDs acquired when using very small ion chambers, with specific bias/water tank combinations — up to 5% at a depth of 25 cm in water. This difference was not observed when the PDDs were sampled using the ion chamber in static positions in conjunction with an external electrometer. This suggests noise/signal interference produced by the controller box and cable system assemblies, which can become relatively significant for the very small current signals collected by very small ion chambers, especially at depth as the signal level is even further reduced. Based on the results observed here, the use of very small active volume chambers under specific scanning conditions may lead to collection of erroneous data, introducing systematic errors into the treatment planning system. In case the use of such a chamber is required, we recommend determining whether such erroneous effect exists by comparing the scans with those obtained with a larger chamber. PACS numbers: 87.56.bd, 87.56.Fc, 87.56.Da

Dosimetric impact of the 160 MLC on head and neck IMRT treatments.

The purpose of this work is to investigate if the change in plan quality with the finer leaf resolution and lower leakage of the 160 MLC would be dosimetrically significant for head and neck intensity‐modulated radiation therapy (IMRT) treatment plans. The 160 MLC consisting of 80 leaves of 0.5 cm on each bank, a leaf span of 20 cm, and leakage of less than 0.37% without additional backup jaws was compared against the 120 Millennium MLC with 60 leaves of 0.5 and 1.0 cm, a leaf span of 14.5 cm, and leakage of 2.0%. CT image sets of 16 patients previously treated for stage III and IV head and neck carcinomas were replanned on Prowess 5.0 and Eclipse 11.0 using the 160 MLC and the 120 MLC. IMRT constraints for both sets of 6 MV plans were identical and based on RTOG 0522. Dose‐volume histograms (DVHs), minimum dose, mean dose, maximum dose, and dose to 1 cc to the organ at risks (OAR) and the planning target volume, as recommended by QUANTEC 2010, were compared. Both collimators were able to achieve the target dose to the PTVs. The dose to the organs at risk (brainstem, spinal cord, parotids, and larynx) were 1%–12% (i.e., 0.5–8 Gy for a 70 Gy prescription) lower with the 160 MLC compared to the 120 MLC, depending on the proximity of the organ to the target. The large field HN plans generated with the 160 MLC were dosimetrically advantageous for critical structures, especially those located further away from the central axis, without compromising the target volume. PACS number: 87.55 D‐

SU‐E‐U‐08: Presentation of a New Intrafractional Prostate Monitoring Method with Ultrasound Image Guidance During Radiotherapy Treatment

Purpose: Clinical presentation of a new intrafractional ultrasound image guidance (USIG) prostate monitoring/tracking method. Methods: Clarity ultrasound system has recently released a new feature to monitor/track intrafractional prostate motion using a trans‐perineal image acquisition position. After initial localization and image guidance correction, the ultrasound probe remains fixed in the sagittal‐plane, trans‐perineal imaging position via a couch‐mounted arm‐support system. The software then enters a live monitoring mode, where the scanning ultrasound probe continuously acquires fanned sagittal images for tracking of during‐treatment prostate position. Compared to other tracking technologies (e.g. RF implanted beacon tracking, or real‐time fluoro monitoring), the ultrasound system has the unique advantages of live 3D image display, without invasive procedure or imaging radiation dose. We present our initial experience using this monitoring feature for 8 patients and 218 treatment sessions. Results: The monitoring software functioned as expected during treatment with consistent reporting of prostate deviation from isocenter location, and notification of instances where positional error exceeded our tolerance of 3 mm for 5 second. Comparisons of prostate tracking data streams obtained from Clarity USIG with data streams obtained in our clinic from Calypso RF tracking of a similar patient population show similarity of recorded motion information for both methods. Average during‐treatment prostate motion for both tracking methods was on the order of 1–2 mm in 3 cardinal directions, with roughly 5% of instances where motion exceeded 3 mm, 5 second tolerance level. When Monitoring reported that tolerance was exceeded we confirmed this by performing redundant, static imaging Alignment, with confirmation that target position had, indeed, changed as reported. Conclusion: The newly released trans‐perineal ultrasound Monitoring approach was confirmed to function well clinically, and to report accurate intrafractional monitoring data. Acquired motion streams were consistent with our Calypso monitoring experience obtained on a similar patient population in our own clinic.