Naresh Tolani

4D-CT imaging with synchronized intravenous contrast injection to improve delineation of liver tumors for treatment planning

We have developed a tumor-specific protocol for the 4D-CT imaging of liver tumors using synchronized intravenous (IV) contrast injection to improve the accuracy of tumor delineation for treatment planning. Most liver metastases and cholangiocarcinomas can be imaged in the portal venous phase, while hepatocellular carcinomas are most visible in the delayed phase. Combined 4D-CT imaging with synchronized IV contrast allows for both enhancement of tumor contrast and coverage over the entire breathing cycle.

Radiation for Hodgkin's Lymphoma in Young Female Patients: A New Technique to Avoid the Breasts and Decrease the Dose to the Heart

PURPOSE To demonstrate how, in young female patients with Hodgkins lymphoma, using an inclined board technique can further decrease the volume of breasts and heart in the treatment field. METHODS AND MATERIALS An inclined board was constructed with the ability to mount an Aquaplast face mask, a Vacu-Lock, and a hip stopper. Eight female patients with early-stage Hodgkins lymphoma were planned and compared using the conventional flat position and the inclined board position. All patients on the inclined board were planned with 90° degree table position and 15° gantry angle rotation to compensate for the beam divergence resulting from the patients position on the inclined board. Dose-volume histograms were generated, as well as the mean V30 and V5 of both breasts and heart using both treatment positions. RESULTS The mean value of V30 of the right breast, left breast, and heart decreased from 3%, 3%, and 13%, respectively, using the flat position to 0, 0.4%, and 5%, respectively, using the inclined board. The mean value of V5 of the right breast, left breast, and heart decreased from 6%, 13%, and 36%, respectively, using the flat position to 2%, 8%, and 29%, respectively, using the inclined board. CONCLUSIONS Compared with conventional flat positioning, this simple device and technique allows better sparing of the breasts and the heart while maintaining comparable target coverage and total lung dose.

Patient dosimetry for total body irradiation using single‐use MOSFET detectors

We studied the usefulness of a new type of solid‐state detector, the OneDose single‐use MOSFET (metal oxide semiconductor field effect transistor) dosimeter, for entrance dose measurements for total body irradiation (TBI). The factory calibration factors supplied by the manufacturer are applicable to conventional radiotherapy beam arrangements and therefore may not be expected to be valid for TBI dosimetry because of the large field sizes and extended source‐to‐axis distances used. OneDose detectors were placed under a 1‐cm thick bolus at the head, neck, and umbilicus of 9 patients undergoing TBI procedures. Thermoluminescent dosimeters (TLDs) were placed beside the detectors. We found that the OneDose readings differed from the TLD readings by 4.6% at the head, 1.7% at the neck, and 3.9% at the umbilicus, with corresponding standard deviations of 3.9%, 2.2%, and 2.7%. For all patient measurements, 95% of the OneDose readings fell within 3.3%±6.0% of the TLD readings. Anthropomorphic phantom measurements showed differences of −0.1% at the neck and −1.2% midway between the phantoms carina and umbilicus. Our results suggest that these detectors could be used for TBI quality assurance monitoring, although TLDs should remain the standard when critical dose measurements are performed. If OneDose detectors are to be used for TBI, the use of more than one at each location is strongly recommended. Because the detectors are designed for single use, they cannot be individually calibrated. However, to obtain institution‐specific correction factors for better applicability to TBI dosimetry, measurements of several detectors taken from a particular lot could also be obtained in phantom with the TBI geometry configurations used for patient treatment. PACS numbers: 87.53.Bn, 85.30.Tv, 87.55.‐x

Thermoluminescence dosimetry measurements of brachytherapy sources in liquid water.

Radiation therapy dose measurements are customarily performed in liquid water. The characterization of brachytherapy sources is, however, generally based on measurements made with thermoluminescence dosimeters (TLDs), for which contact with water may lead to erroneous readings. Consequently, most dosimetry parameters reported in the literature have been based on measurements in water-equivalent plastics, such as Solid Water. These previous reports employed a correction factor to transfer the dose measurements from a plastic phantom to liquid water. The correction factor most often was based on Monte Carlo calculations. The process of measuring in a water-equivalent plastic phantom whose exact composition may be different from published specifications, then correcting the results to a water medium leads to increased uncertainty in the results. A system has been designed to enable measurements with TLDs in liquid water. This system, which includes jigs to support water-tight capsules of lithium fluoride in configurations suitable for measuring several dosimetric parameters, was used to determine the correction factor from water-equivalent plastic to water. Measurements of several 125I and 131Cs prostate brachytherapy sources in liquid water and in a Solid Water phantom demonstrated a correction factor of 1.039 +/- 0.005 at 1 cm distance. These measurements are in good agreement with a published value of this correction factor for an 125I source.

Comparison of breath-hold and free-breathing positions of an external fiducial by analysis of respiratory traces

An internal target volume (ITV) accounting for respiratory‐induced tumor motion is best obtained using 4DCT. However, when 4DCT is not available, inspiratory/expiratory breath‐hold (BHinsp, BHexp) CT images have been suggested as an alternative. In such cases, an external fiducial on the abdomen can be used as a substitute for tumor motion and CT images are acquired when the marker position matches – as judged by the therapist/physicist ‐ its positions at previously determined free‐breathing (FB) respiratory extrema (FBinsp, FBexp). In this study we retrospectively determined the accuracy of these matches. Free breathing 4DCT images were acquired, followed by BHinsp and BHexp CT images for 25 patients with non‐small‐cell lung cancer. Respiration was monitored using a commercial external fiducial system, which generates positional information while CT studies are conducted. Software was written for statistically analyzing the displacement of the external fiducial during BHinsp and BHexp CT acquisition and comparing these displacements with corresponding mean FB extrema positions (FBinsp and FBexp, respectively) using a Students t‐test. In 72% of patients, mean positions at BHinsp differed significantly from mean positions at FBinsp (p<0.05: 0.13–1.40 cm). In 92% of patients, mean positions at BHexp differed significantly from mean positions at FBexp (p<0.05: 0.03–0.70 cm), although this difference was smaller than 0.5 cm in many cases (median=0.34 cm). Our findings indicate that relying solely on abdominal external markers for accurate BH CT imaging in order to accurately estimate FB extrema positions may be subject to significant error. PACS numbers 87.53.bd, 87.57.C‐, 87.59.Fm, 87.55.Gh

SU-FF-T-203: In Vivo Dosimetry Using Disposable MOSFET Dosimeters for Total Body Irradiation

Purpose: The “OneDose” MOSFET dosimeter, manufactured by Sicel Technologies, is used for individualized patient dosimetry measurements for radiation therapy. These dosimeters, which are pre-calibrated by the manufacturer, are designed for single use for conventional radiation therapy treatments. We tested the usefulness, reliability and applicability of these dosimeters for use in total body irradiation procedures, where the field sizes, distances, and irradiation conditions differ substantially from the standard conditions used for factory calibration Method and Materials: We have compared the response of OneDose dosimeters with that of Thermoluminescent Dosimeters (TLDs) calibrated “in-house”. OneDose dosimeters were paired with two TLDs and were placed beneath bolus to provide adequate build-up. The detectors with build-up were then taped to the skin of the patient at various sites of interest, including the head, neck, umbilicus and lungs. In most cases, cerrobend lung blocks were used. Readings of the OneDose were taken immediately following irradiation. The TLDs were read 2 – 4 days later. Results: Of the four patients studied thus far, the doses measured at the head, neck, and umbilicus fell between 105 and 210 cGy. For these sites agreement with the TLDs was generally within ±5%. Measured doses for the regions of the lung showed greater variability. This may be due to placement errors or the lower doses (< 20 cGy) and steeper dose gradients that occurred when the lungs were shielded with cerrobend blocks. Conclusion: The dose measured by the OneDose detector shows relatively good agreement with that measured by TLDs in total body irradiation. Our research plan includes study of another six patients, with the intent of being more precise in the placement of the dosimeters in the region of the lung. Conflict of Interest Information: ASB has a sponsored research agreement with Sicel Technologies, Inc. for the study of implantable sensors.

SU‐FF‐T‐136: Design of a Jig for Thermoluminescence Dosimetry of Brachytherapy Sources in Liquid Water and the Determination of a Correction Factor for Water‐Equivalent Plastics

Purpose: To develop a method for measuring the characteristics of brachytherapy sources in water, rather than in water‐equivalent plastics, and to use this method to determine the correction factor for water‐equivalent plastic. Method and Materials: Small thermoluminescencedosimeter(TLD) capsules were constructed from capillary tubes to hold 14 mg of lithium fluoride powder. Plastic jigs were designed to hold the capsules in circular pattern around a brachytherapy source, or in a spiral pattern radiating away from the source. The radioactive source was mounted on the tip of a thin graphite rod with its long axis either parallel or perpendicular to the TLD pattern. The jigs were placed in a water phantom to enable measurement of all TG‐43 parameters. A Solid Water™ phantom was constructed to hold the TLD capsules in exactly the same circular pattern around the source. TLD measurements were made in water and Solid Water™ at 1.00 cm distance from a model 6711 125 I source to determine a correction factor for the Solid Water™. Similar measurements were also made with a model CS‐1 131 Cs source. Results: The measured correction factor for Solid Water™ was 1.05 +− 0.02 at a distance of 1.00 cm from the model 6711 125 I source. This value is in good agreement with a Monte Carlo‐based value published previously. Similar measurements with the model CS‐1 131 Cs source produced the same result within experimental uncertainties.Discussion: The preferred medium for therapy dose measurements is liquid water. However, dosimetry measurements reported in the literature are limited to water‐equivalent plastics. The correction factor for plastic phantoms is based on Monte Carlo calculations, and must be validated by actual measurements.

SU‐DD‐A1‐03: Dosimetric Characterization of Model CS‐1 131Cs Source by Thermoluminescence Dosimetry in Liquid Water

Purpose: To determine the dosimetric characteristics of a recently introduced 131 Cs brachytherapy source by performing measurements in liquid water employing thermo‐luminescence dosimeters(TLD).Method and Materials: Small capsules containing 14 mg of lithium fluoride were constructed from capillary tubes and were supported in a water phantom by two plastic jigs. The jigs allowed the capsules to be positioned around a source in circular and spiral patterns designed to permit measurement of dose rate constant, anisotropy function, and radial dose function. The radioactive source was mounted on the tip of a thin graphite rod with its long axis either parallel or perpendicular to the plane of the TLD pattern. To assure confidence in the results, thirteen different seeds were employed, and measurements were performed multiple times. The measureddosimetric parameters were based on the AAPM Task Group 43 formalism. Results: The dose rate constant measured in liquid water was 1.08 cGy/U ± 5%, and was based on the air‐kerma strength standard established by the National Institute of Standards and Technology. Measured values for the anisotropy function F(r,θ) and the radial dose function g(r) also were determined. The results were compared with recently published values. Conclusions: It appears that this is the first time a complete set of dosimetric parameters for a brachytherapy seed has been measured in liquid water. This method avoids the uncertainty introduced by the use of water‐equivalent plastic. Key words: Brachytherapy seed, Solid water, TLD, TG‐43.

SU‐FF‐T‐216: Evaluation and Commissioning of K&S Associates Inc. Diamond Monitor Unit Calculation Software

Purpose: To report on the evaluation and commissioning of the Diamond (K&S Associates Inc., Nashville, TN) monitor unit (MU) calculation software.Method and Materials: Based on clinical dose‐response studies, the ICRU states that dosimetry systems must be capable of delivering dose to an accuracy of 5%. The accurate determination of dose per MU to a point within the patient is an essential part of this process. Good clinical practice dictates that MU obtained from the treatment planning system (TPS) be checked using an independent system. Diamond is a windows‐based computer program for computing beam on time for radiation treatments. The software is used for quality assurance purposes to confirm MU produced by our TPS, Pinnacle3 (Philips Medical Systems, Andover, MA). To ensure that the Diamond MU calculation algorithms are correctly implemented in our clinic, the algorithms were verified by comparing point of interest dose values calculated by Diamond, with dose values calculated using the Pinnacle3 TPS, and in‐house developed photon and electron MU calculation software. The in‐house software, which has been used in our clinic for several years, has been extensively tested against measured data. The tests cover a variety of square, rectangular, and blocked fields at several depths. Standard electron and photon energies for both Varian and Siemens linacs were tested as part of the commissioning process. These comparisons were implemented in a variety of clinically relevant test cases. Results: Results indicate that Diamond calculated MU values are within 2% of the in‐house developed MU calculation algorithms for electron and photons. Only a few calculations showed disparities of greater than 1%. Conclusions: Based on our testing and analysis of calculation methods, we are satisfied with the Diamond MU calculation software. We are currently in the process of evaluating the intensity modulated radiation therapy module of this software.