K M Ayyangar

University of Nebraska at Omaha, Omaha, NE, United States

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Publications (71)146.01 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: This work was undertaken with the intention of investigating the possibility of clinical use of commercially available self-developing radiochromic film - Gafchromic EBT film - for IMRT dose verification. The dose response curves were generated for the films using VXR-16 film scanner. The results obtained with EBT films were compared with the results of Kodak EDR2 films. It was found that the EBT film has a linear response between the dose ranges of 0 and 600 cGy. The dose-related characteristics of the EBT film, like post-irradiation color growth with time, film uniformity and effect of scanning orientation, were studied. There is up to 8.6% increase in the color density between 2 and 40 h after irradiation. There was a considerable variation, up to 8.5%, in the film uniformity over its sensitive region. The quantitative difference between calculated and measured dose distributions was analyzed using Gamma index with the tolerance of 3% dose difference and 3 mm distance agreement. EDR2 films showed good and consistent results with the calculated dose distribution, whereas the results obtained using EBT were inconsistent. The variation in the film uniformity limits the use of EBT film for conventional large field IMRT verification. For IMRT of smaller field size (4.5 × 4.5 cm), the results obtained with EBT were comparable with results of EDR2 films.
    Journal of Medical Physics 04/2006; 31(2):78-82.
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    ABSTRACT: In ultrasound-guided intensity-modulated radiation therapy (IMRT) of prostate cancer, ultrasound imaging ascertains the anatomical position of patients during x-ray therapy delivery. The ultrasound transducers are made of piezoelectric ceramics. The same crystal is used for both ultrasound production and reception. Three-dimensional (3D) ultrasound devices capture and correlate series of 2-dimensional (2D) B-mode images. The transducers are often arranged in a convex array for focusing. Lower frequency reaches greater depth, but results in low resolution. For clear image, some gel is usually applied between the probe and the skin contact surface. For prostate positioning, axial and sagittal scans are performed, and the volume contours from computed tomography (CT) planning are superimposed on the ultrasound images obtained before radiation delivery at the linear accelerator. The planning volumes are then overlaid on the ultrasound images and adjusted until they match. The computer automatically deduces the offset necessary to move the patient so that the treatment area is in the correct location. The couch is translated as needed. The currently available commercial equipment can attain a positional accuracy of 1-2 mm. Commercial manufacturer designs differ in the detection of probe coordinates relative to the isocenter. Some use a position-sensing robotic arm, while others have infrared light-emitting diodes or pattern-recognition software with charge-couple-device cameras. Commissioning includes testing of image quality and positional accuracy. Ultrasound is mainly used in prostate positioning. Data for 7825 daily fractions of 234 prostate patients indicated average 3D inter-fractional displacement of about 7.8 mm. There was no perceivable trend of shift over time. Scatter plots showed slight prevalence toward superior-posterior directions. Uncertainties of ultrasound guidance included tissue inhomogeneities, speckle noise, probe pressure, and inter-observer variation. Some published studies detected improvement in treatment based on gastrointestinal toxicity and the reduction of prostate movement.
    Medical Dosimetry 02/2006; 31(1):20-9. · 1.01 Impact Factor
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    ABSTRACT: The quantitative dose validation of intensity-modulated radiation therapy (IMRT) plans require 2-dimensional (2D) high-resolution dosimetry systems with uniform response over its sensitive region. The present work deals with clinical use of commercially available self-developing Radio Chromic Film, Gafchromic EBT film, for IMRT dose verification. Dose response curves were generated for the films using a VXR-16 film scanner. The results obtained with EBT films were compared with the results of Kodak extended dose range 2 (EDR2) films. The EBT film had a linear response between the dose range of 0 to 600 cGy. The dose-related characteristics of the EBT film, such as post irradiation color growth with time, film uniformity, and effect of scanning orientation, were studied. There was up to 8.6% increase in the color density between 2 to 40 hours after irradiation. There was a considerable variation, up to 8.5%, in the film uniformity over its sensitive region. The quantitative differences between calculated and measured dose distributions were analyzed using DTA and Gamma index with the tolerance of 3% dose difference and 3-mm distance agreement. The EDR2 films showed consistent results with the calculated dose distributions, whereas the results obtained using EBT were inconsistent. The variation in the film uniformity limits the use of EBT film for conventional large-field IMRT verification. For IMRT of smaller field sizes (4.5 x 4.5 cm), the results obtained with EBT were comparable with results of EDR2 films.
    Medical Dosimetry 02/2006; 31(4):273-82. · 1.01 Impact Factor
  • International Journal of Radiation OncologyBiologyPhysics 10/2005; 63(1):310-1; author reply 311. · 4.52 Impact Factor
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    ABSTRACT: Accurate reporting of personal dose is required by regulation for hospital personnel that work with radioactive material. Pocket dosimeters are commonly used for monitoring this personal dose. We show that operating a cell phone in the vicinity of a pocket dosimeter can introduce large and erroneous readings of the dosimeter. This note reports a systematic study of this electromagnetic interference. We found that simple practical measures are enough to mitigate this problem, such as increasing the distance between the cell phone and the dosimeter or shielding the dosimeter, while maintaining its sensitivity to ionizing radiation, by placing it inside a common anti-static bag.
    Physics in Medicine and Biology 06/2005; 50(9):N93-9. · 2.70 Impact Factor
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    ABSTRACT: Purpose: The three‐source model proposed by Yang et al.1 when applied to Siemens PRIMUS 6 MV beam over‐estimated head scatter by 10–200% for field sizes less than 2 cm × 2 cm and elongated narrow beams. This necessitated the development of a modified approach Method and Materials: The complete theoretical background for the three‐source model can be found in the published literature1 in which the total energy fluence at the point of calculation can be divided into three components. The primary component Cp has been chosen around 90% in the model. A modified approach is proposed to the three‐source model with the primary source component Cp having growth function that grows exponentially with radius of the beam in the Sp plane. The primary source function was integrated in the Sp plane using the formula, Cp = Cp3∗(exp (−rs∗Cp2); rs < r3 where Cp2, Cp3 and r3 are fitting coefficients. Results: The measured head scatter factors for smaller field sizes including the rectangular fields where the exchanged collimator jaw positions have been compared with both three‐source model and modified three‐source model for Siemens PRIMUS 6 MV beam. It was observed that the accuracy of the modified model is improved and is within 10% of the measurements for small field sizes. Conclusion: In routine IMRT treatments, about 10–15% of the segmented fields use small or elongated fields. The modified approach to the three‐source model improves the accuracy of the head scatter factors calculation significantly for field sizes below 2 cm × 2cm.
    Medical Physics 05/2005; 32(6):2023-2023. · 2.91 Impact Factor
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    ABSTRACT: Purpose: The Novalis body system, ExacTrac®, was used to verify target localization and to improve patient positioning for daily treatment with patient pre‐positioning by mask and head frame in intensity modulated radio‐surgery(IMRS). This study is to present preliminary results of quantitative measurements from the ExacTrac system. Method and Materials: Accuracy of image fusion and correct calibration of ExacTrac system were investigated using the head section of anthropomorphic phantom. The phantom was randomly moved away from the target position manually and re‐positioned using the ExacTrac system. Couch final position was then compared with target position to determine positioning errors. ExacTrac recorded data from 12 patients was used for this study. A verification plan for exporting DRRs to ExacTrac was created for each patient using reference point as localizer. The patient was placed on the couch with mask and isocenter was aligned with target positioner. Two x‐ray images were taken and registered to DRRs using automatic 3D fusion. After visual examination of the match of bony structures surrounding the isocenter, the necessary couch movements were performed based on shifts computed from 3D fusion. Results: For 7 cranial patients with total 106 treatments, the lateral, longitudinal and vertical average shifts were 0.80, 1.91, and 0.99 mm respectively; for 4 orbital patients with total 103 treatments, the shifts were 0.6, 1.4, and 0.6mm respectively; for one C‐spine patient with total 28 treatments, shifts were 1.5, 1.6, and 2.3 mm respectively. According to 11 phantom measurements, our ExacTrac system had accuracy of 0.27, 0.64, and 0.55 mm respectively. Conclusion: Target shifts in patient positioning by mask and head frame could be more than 1.0 mm and was larger in longitudinal direction for treatment of cranial and orbital tumors. With x‐ray image guidance, ExacTrac system localizes target with accuracy of less than 1.0 mm.
    Medical Physics 05/2005; 32(6):2045-2045. · 2.91 Impact Factor
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    ABSTRACT: To study prostate motion from 4,154 ultrasound alignment fractions on 130 prostate patients treated with conformal radiotherapy or intensity-modulated radiation therapy at the University of Nebraska Medical Center. Each prostate patient was immobilized in a vacuum cradle. Daily treatment was verified by ultrasound scan after laser setup with skin marks and before radiation delivery by the same physician responsible for anatomic delineation during planning. Directional statistics were employed to test the significance of shift directions. Polar histograms showed the prevalence of prostate motion in superior-posterior directions. The average direction was about 27 degrees from the superior axis. The average changes of prostate position in superior to inferior (SI), anterior-posterior (AP), and left to right (LR) directions and in radial distance were 0.25, -0.13, 0.03, and 0.92, cm respectively. Our data indicated that prostate motion was not patient specific, and its average magnitude remained virtually unchanged over time. Recommended planning target volume (PTV) margins for use without ultrasound localization were 0.90 cm in SI, 1.02 cm in AP, and 0.80 cm in LR directions. Ultrasound localization revealed a predominance of prostate shift from planning position in the superior-posterior direction, with an average closer to the superior axis. The motion data provides recommended margins for PTV.
    International Journal of Radiation OncologyBiologyPhysics 04/2005; 61(4):984-92. · 4.52 Impact Factor
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    ABSTRACT: This paper compares dose volume histograms (DVHs) generated by the ADAC Pinnacle and the Nomos Corvus planning systems. Seven prostate cases and seven head and neck cases were selected for review. Plans computed on both systems possessed exactly the same anatomical contours and IMRT segments. The Pinnacle system used the collapsed cone convolution superposition, while Corvus employed a finite size pencil beam (FSPB) convolution. Prostate DVH results demonstrated similar DVH curves from both systems. For each structure, the ratio of Pinnacle dose value divided by Corvus value was calculated. The high dose structures (which might contain tumour) had ratios close to unity, while the low dose structures (the critical organs) had ratios farther away from unity. Almost all ratios were less than unity, indicating a systematic difference that Pinnacle calculated doses were lower than Corvus ones. Head and neck data provided similar findings. A possible cause for this discrepancy could be the beam modelling. The difference in DVH parameters that we discovered between the two systems was about the same order of magnitude as the measurement-computation difference. When low dose is critical, such difference may affect the clinical planning decision.
    Australasian physical & engineering sciences in medicine / supported by the Australasian College of Physical Scientists in Medicine and the Australasian Association of Physical Sciences in Medicine 04/2005; 28(1):1-7. · 0.89 Impact Factor
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    ABSTRACT: Traditional external beam radiotherapy of gynecological cancer consists of a 3D, four-field-box technique. The radiation treatment area is a large region of normal tissue, with greater inhomogeneity over the treatment volume, which could benefit more with intensity-modulated radiation therapy (IMRT). This is a case report of IMRT planning for a patient with endometrial cancer. The planning target volume (PTV) spanned the intrapelvic and periaortic lymph nodes to a 33-cm length. Planning and treatment were accomplished using double isocenters. The IMRT plan was compared with a 3D plan, and the effects of field parameters were studied. Delineated anatomical contours included the intrapelvic nodes (PTV), bone marrow, small bowel, bladder, rectum, sigmoid colon, periaortic nodes (PTV), spinal cord, left kidney, right kidney, large bowel, liver, and tissue (excluding the PTVs). Comparisons were made between IMRT and 3D plans, 23-MV and 6-MV energies, zero and rotated collimator angles, different numbers of segments, and opposite gantry angle configurations. The plans were evaluated based on dosevolume histograms (DVHs). Compared with the 3D plan, the IMRT plan had superior dose conformity and spared the bladder and sigmoid colon embedded in the intrapelvic nodes. The higher energy (23 MV) reduced the dose to most critical organs and delivered less integral dose. Zero collimator angles resulted in a better plan than "optimized" collimator angles, with lower dose to most of the normal structures. The number of segments did not have much effect on isodose distribution, but a reasonable number of segments was necessary to keep treatment time from being prohibitively long. Gantry angles, when evenly spaced, had no noticeable effect on the plan. The patient tolerated the treatment well, and the initial complete blood count was favorable. Our results indicated that large-volume tumor sites may also benefit from precise conformal delivery of IMRT.
    Journal of Applied Clinical Medical Physics 02/2005; 6(3):46-62. · 0.96 Impact Factor
  • S. Pillai, K Ayyangar, S Li, R Nehru, W Zhen, C Enke
    Medical Physics 01/2005; 32(6). · 2.91 Impact Factor
  • K. Ayyangar, D Djajaputra, W Zhen, C Enke
    Medical Physics 01/2005; 32(6). · 2.91 Impact Factor
  • Medical Physics 01/2005; 32(6). · 2.91 Impact Factor
  • Medical Physics 01/2005; 32(6). · 2.91 Impact Factor
  • Y. Fu, S Li, K Ayyangar, C Enke
    Medical Physics 01/2005; 32(6). · 2.91 Impact Factor
  • International Journal of Radiation Oncology Biology Physics - INT J RADIAT ONCOL BIOL PHYS. 01/2004; 60.
  • International Journal of Radiation Oncology Biology Physics - INT J RADIAT ONCOL BIOL PHYS. 01/2004; 60(1).
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    ABSTRACT: The dose linearity and uniformity of a linear accelerator designed for multileaf collimation system-(MLC) based IMRT was studied as a part of commissioning and also in response to recently published data. The linear accelerator is equipped with a PRIMEVIEW, a graphical interface and a SIMTEC IM-MAXX, which is an enhanced autofield sequencer. The SIMTEC IM-MAXX sequencer permits the radiation beam to be " ON" continuously while delivering intensity modulated radiation therapy subfields at a defined gantry angle. The dose delivery is inhibited when the electron beam in the linear accelerator is forced out of phase with the microwave power while the MLC configures the field shape of a subfield. This beam switching mechanism reduces the overhead time and hence shortens the patient treatment time. The dose linearity, reproducibility, and uniformity were assessed for this type of dose delivery mechanism. The subfields with monitor units ranged from 1 MU to 100 MU were delivered using 6 MV and 23 MV photon beams. The doses were computed and converted to dose per monitor unit. The dose linearity was found to vary within 2% for both 6 MV and 23 MV photon beam using high dose rate setting (300 MU/min) except below 2 MU. The dose uniformity was assessed by delivering 4 subfields to a Kodak X-OMAT TL film using identical low monitor units. The optical density was converted to dose and found to show small variation within 3%. Our results indicate that this linear accelerator with SIMTEC IM-MAXX sequencer has better dose linearity, reproducibility, and uniformity than had been reported.
    Medical Physics 09/2003; 30(8):2253-6. · 2.91 Impact Factor
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    ABSTRACT: Current standards of practice are based on the use of an independent calculation to validate the monitor units (MUs) derived from a treatment planning system. The ADAC PINNACLE treatment planning system has shown discrepancies of 10% or more compared to simple independent calculations for highly contoured areas such as tangential breast and chest wall irradiation. The ADAC treatment planning system generally requires more MUs to deliver the same prescribed dose. Independent MU calculation methods are based on full phantom conditions. On the other hand, the MUs from the ADAC treatment planning system are derived using realistic phantom scatter. As such, differences exist in TMR factors, off-axis wedge factors, and the phantom scatter factor. To systematically study the discrepancies due to phantom conditions, experimental measurements were performed with various percentages of tissue missing. The agreement between the experimental measurements and ADAC calculations was found to be within 2%. Using breast field geometry, a relationship between missing tissue and the dosimetric parameters used by ADAC was developed. This relationship, when applied, yielded independent MU calculations whose values closely matched those from the ADAC treatment planning system.
    Medical Dosimetry 02/2003; 28(2):79-83. · 1.01 Impact Factor
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    ABSTRACT: This study is an attempt to compare the dosimetric parameters of intensity-modulated radiation therapy (IMRT) and stereotactic radiosurgery (SRS) using patient data. Radiosurgery was delivered through circular tertiary collimators attached to a linear accelerator. Six patients who were treated with SRS were replanned and evaluated with the IMRT planning system. Contouring of all structures, including target volume, was done on the IMRT system to closely match the SRS system. Treatment plans were generated after specifying the goals in the prescription module. The NOMOS BEAK collimator attached to the NOMOS MIMiC delivery device was chosen for treatment delivery. Various parameters such as conformity index, homogeneity index, target volume coverage, nontarget tissue, and brainstem doses were calculated and compared between the IMRT and SRS systems. Patient data were divided into 2 groups based on the complexity of the lesion and the number of isocenters used for radiosurgery. Analysis was done for each group and for the cumulative data. Superior conformality and homogeneous dose distribution in IMRT for multiple isocenter cases were observed. In addition, critical structure volumes for 50%, 70%, and 90% of the prescribed dose were lower in IMRT compared to SRS treatment. However, nontarget tissue received significantly higher doses with IMRT plans. Results show that IMRT treatment modality produces similar results as radiosurgery for small, spherical lesions, whereas it is found to be superior to SRS for irregular lesions in terms of critical structure sparing and better dose homogeneity.
    Medical Dosimetry 02/2003; 28(2):85-90. · 1.01 Impact Factor

Publication Stats

500 Citations
146.01 Total Impact Points

Institutions

  • 2001–2006
    • University of Nebraska at Omaha
      • Department of Radiation Oncology
      Omaha, NE, United States
    • Baylor College of Medicine
      • Department of Radiology
      Houston, TX, United States
  • 2000–2006
    • The Nebraska Medical Center
      Omaha, Nebraska, United States
    • University of Nebraska Medical Center
      • Department of Radiation Oncology
      Omaha, Nebraska, United States
    • Christian Medical College Vellore
      Velluru, Tamil Nādu, India
  • 2003
    • Apollo Hospitals
      Chennai, Tamil Nādu, India
  • 1995–2000
    • Medical University of Ohio at Toledo
      Toledo, Ohio, United States
  • 1999
    • Case Western Reserve University
      • Department of Radiology (University Hospitals Case Medical Center)
      Cleveland, OH, United States
  • 1987–1994
    • Thomas Jefferson University
      • Department of Radiation Oncology
      Philadelphia, PA, United States
  • 1979–1993
    • Thomas Jefferson University Hospitals
      • Department of Radiation Oncology
      Philadelphia, Pennsylvania, United States
  • 1991
    • Kansas City VA Medical Center
      Kansas City, Missouri, United States