Biomedical subjects

Jatinder R Palta

Publications and source records attributed to Jatinder R Palta.

5 recordsLinked to original sources

PubMed2003

Validation of dynamic MLC-controller log files using a two-dimensional diode array.

Intensity-modulated radiation therapy (IMRT) delivered with multi-leaf collimator (MLC) in the step-and-shoot mode uses multiple static MLC segments to achieve intensity modulation. For typical IMRT treatment plans, significant numbers of segments are delivered with monitor units (MUs) of much less than 10. Verification of the ability of the linear accelerator (linac) to deliver small MU segments accurately is an important step in the IMRT commissioning and quality assurance (QA) process. Recent studies have reported large discrepancies between the intended and delivered segment MUs. These discrepancies could potentially cause large errors in the delivered patient dose. We have undertaken a systematic study to evaluate the accuracy of the dynamic MLC log files, which are created automatically by our commercial MLC workstation after each delivery, in recording the fractional MU delivered in the step-and-shoot mode. Two linac models were evaluated with simple-geometry leaf sequences and delivered with different total MUs and different nominal dose rates. A commercial two-dimensional diode array was used for the measurement. Large discrepancies between the intended and delivered segment MUs were found. The discrepancies were larger for small MU segments at higher dose rate, with some small MU segments completely undelivered. The recorded fractional MUs in the log files were found to agree with what was delivered within the limits of our experimental uncertainty. Our results indicate that it is important to verify the delivery accuracy of small MU segments that could potentially occur in a patient treatment and that the log files are useful in checking the integrity of the linac delivery once validated. Thus validated log files can be used as a QA tool for general IMRT delivery and patient-specific plan verification.

Documentation↗

PubMed2003

Dosimetric characteristics of Thermo-Shield material for orthovoltage photon beams.

Conventionally, lead has been used for field shaping in orthovoltage radiation therapy. Recently, a compensator material named Thermo-Shield was presented for field shaping in electron beams. Thermo-Shield is composed of nontoxic, high atomic weight metal particles dispersed in a thermoplastic matrix. It is manually moldable and conforms to human anatomy or any shape at temperatures of 108-132 degrees F. It is reusable and can be continuously reshaped to better fit the treatment field. Dosimetric characteristics of thermoplastic material were studied for Philips RT250 orthovoltage photon beams ranging from 75 to 250 kVp. It was found that Thermo-Shield should be four to five times thicker than lead to achieve the same transmission (less than 5%). However, it did not cause significant degradation in penumbra. Clinical procedures for use are discussed.

Equipment Design↗

PubMed2003

Generalized monitor unit calculation for the Varian enhanced dynamic wedge field.

The generalized monitor unit (MU) calculation equation for the Varian enhanced dynamic wedge (EDW) is derived. The assumption of this MU calculation method is that the wedge factor of the EDW at the center of the field is a function of field size, the position of the center of the field in the wedge direction, and the final position of the moving jaw. The wedge factors at the center of the field in both symmetric and asymmetric fields are examined. The difference between calculated and measured wedge factors is within 1.0%. The method developed here is easy to implement. The only datum required in addition to the standard set of conventional physical wedge implementation data is the off-axis output factor for the open field in the reference condition. The off-center point calculation is also examined. For the off-center point calculation, the dose profile in the wedge direction for the largest EDW field is used to obtain the relative off-center ratio in any smaller wedge field. The accuracy of the off-center point calculation decreases when the point of calculation is too close to the field edge.

Algorithms↗

PubMed2003

Guidance document on delivery, treatment planning, and clinical implementation of IMRT: report of the IMRT Subcommittee of the AAPM Radiation Therapy Committee.

Intensity-modulated radiation therapy (IMRT) represents one of the most significant technical advances in radiation therapy since the advent of the medical linear accelerator. It allows the clinical implementation of highly conformal nonconvex dose distributions. This complex but promising treatment modality is rapidly proliferating in both academic and community practice settings. However, these advances do not come without a risk. IMRT is not just an add-on to the current radiation therapy process; it represents a new paradigm that requires the knowledge of multimodality imaging, setup uncertainties and internal organ motion, tumor control probabilities, normal tissue complication probabilities, three-dimensional (3-D) dose calculation and optimization, and dynamic beam delivery of nonuniform beam intensities. Therefore, the purpose of this report is to guide and assist the clinical medical physicist in developing and implementing a viable and safe IMRT program. The scope of the IMRT program is quite broad, encompassing multileaf-collimator-based IMRT delivery systems, goal-based inverse treatment planning, and clinical implementation of IMRT with patient-specific quality assurance. This report, while not prescribing specific procedures, provides the framework and guidance to allow clinical radiation oncology physicists to make judicious decisions in implementing a safe and efficient IMRT program in their clinics.

Algorithms↗

PubMed2002

Application of imaging-derived parameters to dosimetry of intravascular brachytherapy sources: perturbation effects of residual plaque burden.

The dosimetric effect of geometric and material heterogeneities on intravascular brachytherapy dose delivery has been studied recently. Residual plaque within the coronary vessel appears to have an impact on the uniform delivery of radiation dose to the arterial tissue. In this study, we have examined the effect of residual plaque burden and post-PCI (percutaneous coronary intervention) plaque configuration on the dose to the arterial wall from clinical intravascular brachytherapy beta-emitting sources containing 32P and 90Sr/90Y. Monte Carlo simulations using the MCNP4B code were performed for these catheter-based sources with residual plaque burden ranging between 25% and 50%. The residual plaque burden values were derived from post-PCI data provided in several recent clinical studies. Dose calculations were performed for three different values of plaque density (1.45 g cm(-3), 2.20 g cm(-3), and 3.1 g cm(-3)) and three different plaque morphologies for the same residual plaque burden. The dose perturbation factor (DPF), defined as the ratio of dose at 2 mm radial distance for a given case to the dose at the same radial distance in homogeneous water medium, was determined for each of the three different plaque densities. The range of DPF values was 0.81-1.01, 0.62-0.99, and 0.41-0.97 for different plaque densities for the 32P source. Corresponding DPF values for the 90Sr/90Y source were 0.90-1.01, 0.84-1.01, and 0.62-1.01. The results indicate the need for accurate assessment of post-PCI clinical measurements such as minimal lumen diameter and residual plaque burden and incorporation of these values into dose calculations.

Brachytherapy↗