SU-E-J-195: Dosimetric Consequences of Metal Prosthesis in Prostate 3D CRT by Using Monte Carlo Dose Calculation Method.

PURPOSE: To investigate dosimetric consequences of patient metal prosthesis in prostate radiotherapy, considering the body heterogeneity, metal artifacts, prostate daily displacements and target delineation. METHODS: Three prostate patients with metal prosthesis were simulated with CT images and 3D ultrasound images. Conformal RT treatment plans were generated based on target volumes delineated on CT images without artifact correction (PTVa). The patients' treatments were aided with an ultrasound (US) localization system for daily PTV setup correction. We retrospectively reevaluated the delineation of PTVs by 1) using simulation CT images artifact-corrected by using an algorithm developed by M. Bazalova et al (2007) (PTVc) and 2) using the 3D ultrasound simulation images as guide (PTVus). Daily setup corrections to the PTVs were incorporated to calculate composite delivered dose by using XVMC simulation on the patient phantom derived from the artifact-corrected CT images. DVHs and dose distributions for different PTVs were then compared with the reference treatment plans (XVMC-calculated on the artifact-degraded CT images). RESULTS: The PTVa volume was the largest, about 1.3% larger than artifact-corrected PTVc and 5.2% larger than PTVus. Adapting artifact-corrected CT images can improve the DVH curves of PTVc and increase the D95% and V95% for PTVc by more than 5% while D50% and V50% for rectum and bladder are raised by up to 41.6%. DVH analysis on PTVa and PTVus shows a small difference in the changes of their DVH indices, less than 4% for the studied cases. CONCLUSIONS: The strike artifacts from metal prosthesis will increase the volume of PTVa, and affect the patient dose calculation. Original patient plan did not accurately predict the dose degradation. Artifact correction may be necessary for some cases having severe metal artifacts. Using US images to help delineating PTV makes a negligible clinical significance.

A method incorporating 4DCT data for evaluating the dosimetric effects of respiratory motion in single-arc IMAT.

This study introduces a method incorporating 4DCT data to determine the impact of respiratory motion in single-arc intensity-modulated arc therapy (IMAT). Simulation was done by re-warping the static dose distribution of all phases of a 4DCT image set with a 3D deformation map to reference CT images at end-inspiration and end-expiration. To calculate the dose received during respiration under IMAT, the control points were interpolated and re-distributed into separate IMAT plans corresponding to each respiratory phase. This study also investigated the role that plan complexity may play in the dosimetric impact of the respiratory motion in the delivery of IMAT. The dosimetric impact of organ motion was evaluated by analyzing the degradation of D(95,) D(50) and D(05) of the CTV and PTV. From the results shown for the patients in this study who had maximum organ motion displacement approximately 15 mm, the dosimetric impact is rather small. Therefore, our preliminary results suggest that respiratory motion of less than 1.5 cm may be ignored for both moderately and highly modulated IMAT, irrespective of the number of fractions. Specifically, highly modulated plans only increased the degradation of D(95) of the DVH curves for a single fraction by 2% in the CTV and 9% in the PTV compared to the expected value of the multi-fraction plan.

Placement of MammoSite brachytherapy catheter under computed-tomography scan guidance.

Placement of the MammoSite breast brachytherapy catheter is most commonly performed either intraoperatively or under ultrasound-guided technique. Below, we present a case report of an alternate approach utilizing CT-scan guidance. This is the first reported case of a balloon brachytherapy catheter placement with this technique.

Improved dose homogeneity in scalp irradiation using a single set-up point and different energy electron beams.

Homogeneous irradiation of the entire or a large portion of the superficial scalp poses both technical and dosimetric challenges. Some techniques will irradiate too much of the underlying normal brain while other techniques are either complex and involve field matching problems or may require sophisticated linear accelerator (linac) add-ons such as intensity modulated radiation therapy (IMRT)/electron multileaf collimation. However, many radiotherapy facilities are not equipped with such treatment modalities. We propose a practical treatment technique that can be delivered with a standard linac capable of producing high energy electrons. The proposed technique offers a simple alternative for achieving results equivalent to IMRT. Dose homogeneity throughout the treatment volume is achieved by aiming different energy electron beams at differential areas of the treatment surface to achieve improved dosimetry and rapid treatment delivery, while using a single set-up point. We introduced this treatment technique at our institution to treat superficial cancers of the scalp and other irregular surfaces.

Methods of bolusing the tracheostomy stoma.

PURPOSE: The tracheostomy stoma is a potential site of recurrence for patients who have subglottic cancer or subglottic spread of cancer. In these patients, it is important that the anterior supraclavicular field does not underdose the posterior wall of the tracheostomy stoma when using a 6-MV anterior photon field. Conventionally, this problem is surmounted with placement of a plastic tracheostomy tube, which is uncomfortable for the patient, potentially traumatic, and can interfere with vocalization via a tracheal esophageal puncture. Our study was designed to investigate the dosimetry of this region and see if alternate methods would be effective. METHODS AND MATERIALS: A phantom was constructed using a No. 6 tracheostomy tube as the model for the tracheostomy curvature and size. Using the water-equivalent phantom, film dosimetry, and films oriented parallel to the en face field, we investigated the dose at the depth of the surface of the posterior wall of the phantom's tracheostomy stoma. Dose was measured both in space and at the tissue interface by scanning points of interest both horizontally and vertically. We measured doses with a No. 6 and No. 8 plastic tracheostomy tube, either 0.5 cm and 1.0 cm of bolus (1-cm airhole) with no tracheostomy tube, as well as 0.3 cm and 0.6 cm tissue-equivalent Aquaplast (Med-Tec Co., Orange City, Iowa) over the tracheostomy. Dosimetry at the posterior interface was confirmed using thermoluminescent dosimeters. RESULTS: Three mm and 6 mm of Aquaplast produced a posterior tracheal dose of 93% and 100%. CONCLUSION: There is no need for these patients to wear a temporary plastic tracheostomy tube during their external radiation therapy. Aquaplast should allow better position reproducibility, reduce trauma, not interfere with patient respiratory efforts, and be compatible with vocalization via a tracheal esophageal puncture.

Clinical experience with routine diode dosimetry for electron beam radiotherapy.

PURPOSE: Electron beam radiotherapy is frequently administered based on clinical setups without formal treatment planning. We felt, therefore, that it was important to monitor electron beam treatments by in vivo dosimetry to prevent errors in treatment delivery. In this study, we present our clinical experience with patient dose verification using electron diodes and quantitatively assess the dose perturbations caused by the diodes during electron beam radiotherapy. METHODS AND MATERIALS: A commercial diode dosimeter was used for the in vivo dose measurements. During patient dosimetry, the patients were set up as usual by the therapists. Before treatment, a diode was placed on the patient's skin surface and secured with hypoallergenic tape. The patient was then treated and the diode response registered and stored in the patient radiotherapy system database via our in-house software. A customized patient in vivo dosimetry report showing patient details, expected and measured dose, and percent difference was then generated and printed for analysis and record keeping. We studied the perturbation of electron beams by diodes using film dosimetry. Beam profiles at the 90% prescription isodose depths were obtained with and without the diode on the beam central axis, for 6-20 MeV electron beams and applicator/insert sizes ranging from a 3-cm diameter circular field to a 25 x 25 cm open field. RESULTS: In vivo dose measurements on 360 patients resulted in the following ranges of deviations from the expected dose at the various anatomic sites: Breast (222 patients) -20.3 to +23.5% (median deviation 0%); Head and Neck (63 patients) -21.5 to +14.8% (median -0.7%); Other sites (75 patients) -17.6 to +18.8% (median +0.5%). Routine diode dosimetry during the first treatment on 360 patients (460 treatment sites) resulted in 11.5% of the measurements outside our acceptable +/-6% dose deviation window. Only 3.7% of the total measurements were outside +/-10% dose deviation. Detailed investigations revealed that the dose discrepancies, overwhelmingly, were due to inaccurate diode orientation and positioning, especially in the regions with rapidly changing contours and/or sloping surfaces. The presence of a diode in the treatment field was found, in some cases, to cause significant dose reduction, most noticeable with smaller fields and lower energy beams. The reduction in dose ranged from 16% (for a 6 MeV beam and a 3 cm diameter circular field) to 4% (for a 12 MeV beam and a 10 x 10 cm field). CONCLUSIONS: Diode dosimetry was found to be convenient and valuable for verifying in real time the dose delivery accuracy of electron beam treatments, but with some caveats. When treating a small field by low energy electrons, frequent use of diodes is undesirable, because it might result in appreciable reduction of dose to the target volume.

Parotid gland tumors: a comparison of postoperative radiotherapy techniques using three dimensional (3D) dose distributions and dose-volume histograms (DVHS).

PURPOSE: To compare different treatment techniques for unilateral treatment of parotid gland tumors. METHODS AND MATERIALS: The CT-scans of a representative parotid patient were used. The field size was 9 x 11 cm, the separation was 15.5 cm, and the prescription depth was 4.5 cm. Using 3D dose distributions, tissue inhomogeneity corrections, scatter integration (for photons) and pencil beam (for electrons) algorithms and dose-volume histogram (DVH), nine treatment techniques were compared. [1] unilateral 6 MV photons [2] unilateral 12 MeV electrons [3] unilateral 16 MeV electrons [4] an ipsilateral wedge pair technique using 6 MV photons [5] a 3-field AP (wedged), PA (wedged) and lateral portal technique using 6 MV photons [6] a mixed beam technique using 6 MV photons and 12 MeV electrons (1:4 weighting) [7] a mixed beam technique using 6 MV photons and 16 MeV electrons (1:4 weighting) [8] a mixed beam technique using 18 MV photons and 20 MeV electrons (2:3 weighting) [9] a mixed beam technique using 18 MV photons and 20 MeV electrons (1:1 weighting). RESULTS: Using dose-volume histograms to evaluate the dose to the contralateral parotid gland, the percentage of contralateral parotid volume receiving > or = 30% of the prescribed dose was 100% for techniques [1], [8] and [9], and < 5% for techniques [2] through [7]. Evaluating the "hottest" 5 cc of the ipsilateral mandible and temporal lobes, the hot spots were: 152% and 150% for technique [2], 132% and 130% for technique [6]. Comparing the exit doses, techniques [1], [8] and [9] contributed to > or = 50% of the prescribed dose to the contralateral mandible and the temporal lobes. Only techniques [2] and [6] kept the highest point doses to both the brain stem and the spinal cord below 50% of the prescribed dose. CONCLUSION: The single photon lateral field [1] and the mixed electron-photon beams [8] and [9] are not recommended treatment techniques for unilateral parotid irradiation because of high doses delivered to the contralateral parotid gland and high exit doses which are associated with Xerostomia. The en face electron beam technique [2] and the mixed electron-photon beam technique [6] are unacceptable due to the excessive dose heterogeneity to the contiguous normal structures. In spite of optimal dose fall-off achieved using the en face technique [3], most patients cannot tolerate the resulting high skin doses. We conclude that the ipsilateral wedge pair [4], the 3-field [5], and the mixed electron-photon beam [7] techniques are optimal techniques in providing relatively homogeneous dose distributions within the target area and for minimizing dose to the relevant normal structures.

Customization of a radiation management system to support in vivo patient dosimetry using diodes.

Considerable effort is generally made in the quality control of radiation therapy units and in patient chart checking to ensure accurate delivery of treatment to the patients. Record and verify (RV) systems have been implemented which will inhibit the beam from being turned ON if the parameters set on the medical linear accelerators do not match a preprogrammed file. It should be emphasized, however, that RV systems are designed only to ensure proper setup of parameters on the therapy units and bear no direct link with proper patient position. The only real link between treatment planning and delivery to the patient is in-vivo dosimetry. Diodes provide a convenient way of measuring the dose received by the patient in real time and, after the initial tedious calibrations, require very little additional effort. In order to facilitate diode use by the therapists in our institution, the radiation management system (RMS) database in use with our linear accelerators was interfaced with the output obtained from a commercially available diode dosimetry system. The chart printout format was altered to read this value from the RMS database allowing a near real-time recording of the actual dose received by the patient for up to three sites concurrently. This provides for an immediate additional check to the planned and received doses already recorded by the RMS RV software. This procedure, if carried out during the first treatment and subsequently on a weekly basis, would ensure that differences of the order of 5% or larger between planned and delivered dose would not go undetected.

Radiation therapy of breast carcinoma: confirmation of prescription dose using diodes.

PURPOSE: To quantitate the dose delivered during tangential breast radiation therapy and measure the scatter dose to the contralateral breast for three different breast setup techniques. METHODS AND MATERIALS: A commercial semiconductor diode system is used for dose measurements. The diode characteristics were studied by comparing the diode response against a standard ionization chamber response in a reference configuration. In vivo dose measurements on 11 patients undergoing tangential breast radiation therapy with 6 MV photons were performed. Medial and lateral field entrance and exit doses were measured and compared with the expected values from the treatment planning system. Scatter doses to the contralateral breast for three breast setup techniques were measured and documented as a function of distance from the field edge and various beam modifiers commonly used in breast radiation therapy. RESULTS: The diodes used in this study exhibited excellent linearity, dose reproducibility, and minimal anisotropy. The in-phantom measurements resulted in dose accuracy within +/- 1.5%. Dose measurements on patients resulted in standard deviations of 1.2 and 2.3% for the medial entrance and exit doses and 1.7 and 2.2% for the lateral entrance and exit doses, respectively. In patients, the scatter doses to the opposite breast at a 5 cm perpendicular distance from the medial field edge resulted in cumulative scatter doses of 2.47 to 5.30 Gy from the tangential fields and an additional 0.50 Gy from the supraclavicular or axillary field, if included. CONCLUSION: Quantitative verification of the prescribed daily dose is important in breast radiation therapy to ensure precision in patient setup and accuracy in dose delivery. Diodes provide a convenient way of real-time patient dose verification and are easy to use by the therapists.

The use of diode dosimetry in quality improvement of patient care in radiation therapy.

The purpose of this work is to improve the quality of patient care in radiation therapy by implementing a comprehensive quality assurance (QA) program aiming to enhance patient in vivo dosimetry on a routine basis. The characteristics of two commercially available semi-conductor diode dosimetry systems were evaluated. The diodes were calibrated relative to an ionization chamber-electrometer system with calibrations traceable to the National Institute of Standards and Technology (NIST). Correction factors of clinical relevance were quantified to convert the diode readings into patient dose. The results of dose measurements on 6 patients undergoing external beam radiation therapy for carcinoma of the prostate on three different therapy units are presented. Field shaping during treatments was accomplished either by multileaf collimation or by cerrobend blocking. A deviation of less than +/-4% between the measured and prescribed patient doses was observed. The results indicate that the diodes exhibit excellent linearity, dose reproducibility, minimal anisotropy, and can be used with confidence for patient dose verification. Furthermore, diodes render real time verification of dose delivered to patients.

Production of spherical ZrO2-Y2O3 and ZnO particles.

In recent years substantial effort has been focused on the use of engineered ceramics for biomedical applications. To produce ceramic components with reliable and reproducible properties for such speciality applications, it is necessary to use high-purity raw material powders with specific properties. Fine ceramic particles having specific shapes and sizes are also required for conducting biocompatibility experiments. This article reports on the laboratory scale production, in an aerosol reactor, of spherical, submicron zirconia (partially stabilized by yttria; ZrO2-Y2O3) and zinc oxide (ZnO) particles by the thermal decomposition of mists generated from aqueous solutions of inorganic metal salt precursors. The particles produced were characterized by scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy.