Source-skin Distance Research Articles

Uncertainties of “in vivo” Dosimetry Using Semiconductors

The purpose of the present study is to evaluate the uncertainties of in vivo dosimetry measured with diode detectors for conformal radiation therapy techniques in order to define appropriate tolerance levels for pelvis and breast treatment in MEIH (Middle East Institute of Health). The present work is carried out on 30-472 and 30-473 diode detectors irradiated by 4 and 15MV photon beams of a medical linear accelerator Synergy from ELEKTA. Dose computation is performed with Pinnacle 7.4 k treatment planning system. First, an estimation of the uncertainties in a simple geometric case, using a water-equivalent solid phantom is done. Secondly, each treatment parameter such as field size, beam angle, beam modifiers and source-skin distance is evaluated in order to simulate the conformal radiation treatments used in the present institution for the main anatomical sites. Interpretation of entrance dose in-vivo measurements requires the determination of appropriate tolerance levels. Indeed, the authors found that the 5% action level proposed in the literature underestimates the uncertainties in the treatment process. A method for the evaluation of tolerance/action levels related to the different anatomical sites is developed. By the end of the present study the authors have developed an integrated monitoring system that offers accurate information about the dose received by patients.

Treatment planning validation for symmetric and asymmetric motorized wedged fields

Purpose: Wedged beam are often used in clinical radiotherapy to compensate missing tissues and dose gradients. The Elekta Precise linear accelerator supports an internal motorized wedge, which is a single large, physical wedge on a motorized carriage. In this study, the dosimetric performance of Elekta precise three dimensional treatment planning system (3DTPS) is evaluated by comparing the calculated and measured doses. Methods: The calculations were performed by the 3DTPS for symmetric as well as asymmetric fields in a source to skin distance (SSD) setup at the depth of maximum dose (d max ) as well as at 5, 10, and 20 cm depths in water phantom using 60° motorized wedges for field sizes of 4 × 4, 10 × 10, and 20 × 20 cm 2 for 6 and 15 MV photon beams. Measurements were produced by Elekta Precise linear accelerator using 0.125 cc volume ionization chamber. Results: Good agreement between the measured and calculated isodose lines were found, with the maximum difference not exceed 5%. The difference between the calculated and measured data increases as the field size decreases, and the deviation in symmetric setting was less than that of asymmetric setting. The increase in wedge angle led to increase in the difference between calculated and measured data. Conclusion: The results from this study showed that the accuracy of Elekta Precise 3DTPS used with the motorized wedges for symmetric and asymmetric fields is adequate for the clinical applications under the studied experimental conditions.

Evaluation of testicular dose and associated risk from common pelvis radiation therapy in Iran

This study aimed to investigate testicular dose (TD) and the associated risk of heritable disease from common pelvis radiotherapy of male patients in Iran. In this work, the relation between TD and changes in beam energy, pelvis size, source to skin distance (SSD) and beam directions (anterior or posterior) was also evaluated. The values of TDs were measured on 67 randomly selected male patients during common pelvis radiotherapy using 1.17 and 1.33 MeV, Theratron Cobalt-60 unit at SSD of 80 cm and 9 MV, Neptun 10 PC and 18 MV, GE Saturne 20 at SSD of 100 cm at Seyed-Al Shohada Hospital, Isfahan, Iran. Results showed that, the maximum TD was up to 12% of the tumor dose. Considering the risk factor for radiation-induced heritable disorders of 0.1% per Sv, an excess risk of hereditary disorders of 72 per 10,000 births was conservatively calculated. There was a significant difference in the measured TD using different treatment machines and energies (P < 0.001). The Pearson Correlation test showed that, as expected, there was a correlation between TD and patient's pelvis size (r = 0.275, P < 0.001). Using the student's t-tests, it was found that, there was not a significant difference between TD and beam direction (P = 0.231). Iranian male patients undergoing pelvic radiotherapy have the potential of receiving a TD of more than 1 Gy which might result in temporary azoospermia. The risk for induction of hereditary disorders in future generations should be considered as low but not negligible in comparison with the correspondent nominal risk.

SU-E-T-623: Delivery of 3D Conformal Proton-Therapy Fields at Extended Source- To-Axis Distances

Purpose: To evaluate the dosimetric properties of proton dose distributions delivered at extended source-to-skin distances (SSD). Methods: Radiation was delivered with a gantry-mounted proton double scattering system (Proteus 230, IBA). This system has a maximum field diameter of 24 cm at isocenter and a nominal source-to-axis distance of 230 cm. Dose was measured at nominal SSD as well as at -10, +10, +25, +50, +75, and +100 cm for several range and modulation width combinations. Depth dose distributions were measured with a multi-layer ionization chamber (MLIC), and lateral dose distributions with a 2D ionization chamber array as well as with a diode in a water phantom. Results: The maximum field diameter was found to increase from 24.0 cm at nominal SSD to 29.1 cm and 33.3 cm at +50 cm and +100 cm respectively. Field flatness remained below 3% for all SSD. Tilt in the spread-out Bragg peak depth dose distribution increased with SSD up to 0.4 %-per-g/cm2 at +100 cm. The measured range decreased with 1.1×10-3 g/cm2 per centimeter shift in SSD due to proton energy loss in air, very close to the theoretically calculated value of 1.06×10-3 g/cm3. The output and dose rate decrease with the inverse of the SSD squared as expected. Conclusion: Extending the SSD up to 100 cm increases the maximum field diameter from 24.0 cm to 33.3 cm while the dose uniformity remains acceptable.

A 3D quantitative evaluation for assessing the changes of Treatment Planning System and irradiation techniques in radiotherapy

Purpose: This work proposes and compares two 3D global evaluation methods for assessing the alteration of calculated dose distributions when treatment planning system algorithms or irradiation techniques is modified in radiation therapy. Methods : The global analysis is based on gamma index (γ) proposed by Low et al . 1 and Chi (χ) index proposed by Bakai et al . 2 . The γ and χ values are signed in order to identify the over and under estimating dosage. The 3D maps, the cumulative Gamma Voxels Histograms (GVHs) and Chi Voxels Histograms (CVHs) were generated using two software. The γ and χ criteria were set to 3 mm for the distance to agreement and 3% for dose. Pearson's Chi-squared test was applied to assess the statistically significance between GVHs and CVHs. We illustrated this method for the change of dose calculation algorithms for lung cancer, and the change of irradiation techniques for breast cancer. For each patient, 2 treatment plans were generated. For the example of change of dose calculation algorithms, a plan 1 was calculated using Pencil Beam Convolution (PBC) algorithm and a plan 2 was calculated using Modified Batho method (PBC-MB). For the example of change of irradiation technique, a plan 1 was calculated using Source Skin Distance SSD technique and a plan 2 was calculated using a single isocenter technique. Results: The 3D analysis based on γ and χ indexes showed a significant effect on the dosimetric representation in the lung cancer when we change the PBC algorithm to PBC-MB method. The comparison between the two irradiation techniques showed that the single isocenter technique produces a better dose distribution for the treatment of breast cancer. Pearson's Chi-squared test showed that there was no statistically significance between GVHs and CVHs generated by γ and χ indexes, ( p &gt; 0.05). The global analysis using 3D for γ and χ indexes confirmed the results obtained from dosimetric analysis. Conclusion : The methods proposed in this study provide useful tools for radiotherapy to compare two dose distributions obtained using different algorithms or different irradiation techniques. The χ-index was (~190) times faster than γ-index. The χ-index is thus a valuable and more convenient method for 3D global analysis compared with γ-index. ------------------------ Cite this article as: Chaikh A, Giraud JY, Balosso J. A 3D quantitative evaluation for assessing the changes of treatment planning system and irradiation techniques in radiotherapy. Int J Cancer Ther Oncol 2014; 2 (3):02033. DOI : 10.14319/ijcto.0203.3

Characterization of GaN dosimetry for 6 MV photon beam in clinical conditions

We characterized a recently proposed implantable GaN-based dosimeter in clinical conditions, for its application in external radiotherapy according to ESTRO (European Society for Radiotherapy & Oncology) practical guidelines. Our studies were carried out using a 6 MV photon beam with the dosimeter under test in a water tank or a PMMA phantom. They were focused on evaluating short term and long term reproducibility of measurements, and assessing the effects of parameters such as field size, source-skin distance, use of wedge filter, beam incidence, dose rate, accumulated dose, GaN-induced dose perturbation, air cavities and temperature. The estimated repeatability and reproducibility are better than 0.5% and 2% at 1σ respectively. There are no significant effects of the parameters under our studies, apart from field size and temperature. The field-size dependence is due to over-compensation of the GaN response method of the dosimeter, the resulting errors remain lower than 5% for field sizes up to 10 × 10 cm2. The temperature dependence mainly results from the GaN luminescence properties, and causes the GaN response to decrease steadily when increasing temperature, with a sensitivity of −1.4%/°C. The observed quasi-linear temperature dependence may facilitate the correction to improve the accuracy of measurements.

Impact of various color LED flashlights and different lighting source to skin distances on the manual and the computer‐aided detection of basal cell carcinoma borders

Quantitative analysis based on digital skin image has been proven to be helpful in dermatology. Moreover, the borders of the basal cell carcinoma (BCC) lesions have been challenging borders for the automatic detection methods. In this work, a computer-aided dermatoscopy system was proposed to enhance the clinical detection of BCC lesion borders. Fifty cases of BCC were selected and 2000 pictures were taken. The lesion images data were obtained with eight colors of flashlights and in five different lighting source to skin distances (SSDs). Then, the image-processing techniques were used for automatic detection of lesion borders. Further, the dermatologists marked the lesions on the obtained photos. Considerable differences between the obtained values referring to the photographs that were taken at super blue and aqua green color lighting were observed for most of the BCC borders. It was observed that by changing the SSD, an optimum distance could be found where that the accuracy of the detection reaches to a maximum value. This study clearly indicates that by changing SSD and lighting color, manual and automatic detection of BCC lesions borders can be enhanced.

SU‐E‐T‐219: Illustration of Electron Beam Loss of Field Edge Fidelity (Beam Shape) and Uniformity with Air Gap

Purpose: Clinical use of electron beams frequently involves elaborate field shapes and/or matching of edges with adjacent poton fields. Treatment on accasion requires an extended Source Skin Distance (SSD). The optical projection of the field shape on the patient at the extended SSD does not depict the impact of the increased distance on shape and uniformity. The purpose of this presentation is to illustrate some examples of the impact Methods: Shape and dosimetric quantities measured on films exposed cross‐beam at 1 cm depth in plastic water at standard and extended SSD are compared with optical projections of shape and TPS calculations of dosimetric quantities. Results: Small field sizes of low energy electron beams have a small Effective SSD or f‐air and show a greater impact of extended distance on field shape and uniformity. For example, an f‐air value of 0.9 indicates that a net 10 percent of the beam directed towards the measurement point at the standard SSD is scattered away at the extended SSD. The scattering causes an outward bowing of straight edges, a rounding of sharp corners, loss of shape detail and a loss of uniformity of dose. Conclusion: When substantial departure from Inverse Square Law (ISL) is evident in extended SSD Electron Beam calculations it is advisable to evaluate the altered shape and uniformity so that clinical relevance can be judged. Substantial departure from ISL may mean more than a 5 or 10 percent departure from ISL or an air gap of more than 5 or 6 cm. Facilities ought to establish acceptable f‐air and air gap limits or action levels. Careful attention to minimizing air gaps as much as possible minimizes discrepancy between optically projected and actual shape and uniformity.

Developing an Adaptive Radiotherapy Technique for Virally Mediated Head and Neck Cancer

BackgroundVirally mediated head and neck cancers (VMHNC) often present with nodal involvement and are highly radio responsive, meaning that treatment plan adaptation during radiotherapy (RT) in a subset of patients is required. This study sought to determine potential risk profiles and a corresponding adaptive treatment strategy for these patients. MethodologyOne hundred twenty-one patients with virally mediated, node positive nasopharyngeal (Epstein-Barr virus positive) or oropharyngeal (human papillomavirus positive) cancers who were receiving curative intent RT were reviewed. The type, frequency, and timing of adaptive interventions, including source-to-skin distance (SSD) corrections, rescanning, and replanning, were evaluated. Patients were reviewed based on the maximum size of the dominant node to assess the need for plan adaptation. ResultsForty-six patients (38%) required plan adaptation during treatment. The median fraction at which the adaptive intervention occurred was 26 for SSD corrections and 22 for replanning CTs. A trend toward three risk profile groupings was discovered: (1) low risk with minimal need (<10%) for adaptive intervention (dominant pretreatment nodal size of ≤35 mm), (2) intermediate risk with possible need (<20%) for adaptive intervention (dominant pretreatment nodal size of 36–45 mm), and (3) high risk with increased likelihood (>50%) for adaptive intervention (dominant pretreatment nodal size of ≥46 mm). ConclusionsIn this study, patients with VMHNC and a maximum dominant nodal size of >46 mm were identified at a higher risk of requiring replanning during a course of definitive RT. Findings will be tested in a future prospective adaptive RT study.

Monte Carlo optimization of total body irradiation in a phantom and patient geometry

The objective of this work is to apply a Monte Carlo (MC) accelerator model, validated by experimental data at isocentre distances, to a large-field total body irradiation (TBI) technique and to develop a strategy for individual patient treatment on the basis of MC dose distributions. Calculations are carried out using BEAMnrc/DOSXYZnrc code packages for a 15 MV Varian accelerator. Acceptable agreement is obtained between MC data and measurements in a large water phantom behind a spoiler at source–skin distances (SSD) = 460 cm as well as in a CIRS® thorax phantom. Dose distributions in patients are studied when simulating bilateral beam delivery at a distance of 480 cm to the patient central sagittal plane. A procedure for individual improvement of the dose uniformity is suggested including the design of compensators in a conventional treatment planning system (TPS) and a subsequent update of the dose distribution. It is demonstrated that the dose uniformity for the simple TBI technique can be considerably improved. The optimization strategy developed is straightforward and suitable for clinics where the TPS available is deficient to calculate 3D dose distributions at extended SSD.

Mono-Isocentric Technique in the Breast Cancer and Organ at Risk Tolerance

Purpose: To investigate the impact of the single-isocentre technique on the volumetric dose of lung and heart for adjuvant radiation in breast cancer with regional nodal. Methods and materials: Thirty patients treated for breast cancer with supraclavicular fossa irradiation; two techniques of treatment TMT (Traditional Matching Technique) and MIT (Mono-Isocentric Technique) are compared, TMT (tangents in SAD and supraclavicular (SCL) in SSD: Source Skin Distance) and MIT (the all fields in SAD: Source Axe Distance) (Chart 1). Techniques were compared according to dose volume histograms (DVHs) analysis in terms of PTV homogeneity and as OARs (Organs at Risk) dose and volume parameters. Results: The dose distribution in PTV is similar in the both techniques TMT and MIT but with hot spots in the junction of the three fields for the TMT (average 120% for TMT and 110% for MIT). The analysis of DVHs shows a decrease in the mean OARs. Lung and heart dose is improved using the MIT and with significant difference in the V20 and V30 for the lung and in the V10 and V40 for the heart. Conclusions: The results of our study demonstrated that the target volumes were sufficiently irradiated with the MIT and the lung and heart volumes irradiated were small. Furthermore, it should not be over or under dose in the supraclavicular and tangential junction.

Characterization of a cable‐free system based on p‐type MOSFET detectors for “in vivo” entrance skin dose measurements in interventional radiology

During radiological interventional procedures (RIP) the skin of a patient under examination may undergo a prolonged x-ray exposure, receiving a dose as high as 5 Gy in a single session. This paper describes the use of the OneDose(TM) cable-free system based on p-type MOSFET detectors to determine the entrance skin dose (ESD) at selected points during RIP. At first, some dosimetric characteristics of the detector, such as reproducibility, linearity, and fading, have been investigated using a C-arc as a source of radiation. The reference setting (RS) was: 80 kV energy, 40 cm × 40 cm field of view (FOV), current-time product of 50 mAs and source to skin distance (SSD) of 50 cm. A calibrated PMX III solid state detector was used as the reference detector and Gafchromic(®) films have been used as an independent dosimetric system to test the entire procedure. A calibration factor for the RS and correction factors as functions of tube voltage and FOV size have been determined. Reproducibility ranged from 4% at low doses (around 10 cGy as measured by the reference detector) to about 1% for high doses (around 2 Gy). The system response was found to be linear with respect to both dose measured with the PMX III and tube voltage. The fading test has shown that the maximum deviation from the optimal reading conditions (3 min after a single irradiation) was 9.1% corresponding to four irradiations in one hour read 3 min after the last exposure. The calibration factor in the RS has shown that the system response at the kV energy range is about four times larger than in the MV energy range. A fifth order and fourth order polynomial functions were found to provide correction factors for tube voltage and FOV size, respectively, in measurement settings different than the RS. ESDs measured with the system after applying the proper correction factors agreed within one standard deviation (SD) with the corresponding ESDs measured with the reference detector. The ESDs measured with Gafchromic(®) films were in agreement within one SD compared to the ESDs measured using the OneDose(TM) system, as well. The global uncertainty associated to the OneDose(TM) system established in our experiments, ranged from 7% to 10%, depending on the duration of the RIP due to fading. These values are much lower than the uncertainty commonly accepted for general diagnostic practices (20%) and of about the same size of the uncertainty recommended for practices with high risk of deterministic side effects (7%). The OneDose(TM) system has shown a high sensitivity in the kV energy range and has been found capable of measuring the entrance skin dose in RIP.

SU-E-I-108: Simple Approach to Determine Patient's Entrance Skin Exposure for Radiographic.

To evaluate a model which can predict patient's Entrance Skin Exposure (ESE) from height and weight and a single kVp mR/mAs measurement for radiographic. Patient height, weight and post exposure mAs are obtained to assess ESE based on IAEA model. Patient's thickness is obtained by assuming a cylindrical geometry. The validity of this assumption is tested against actual measurements. Source to Skin Distance (SSD) is computed from patient's thickness and the Source-Image receptor Distance (SID) used during exposure. Entrance Skin Exposure is calculated as a product of post exposure mAs and a single 80 kVp mR/mAs obtained at a reference distance corrected inversely for SSD. Values of mR/mAs at other kVp's are obtained by quadratic variation of output with kVp. The validity of predicting patient thickness will be presented based on numerous measurements performed on test subjects. Deviation between the ESE values using of single kVp, mR/mAs measurement and the ones obtained by the conventional multiple kVp mR/mAs will be presented as well. This work indicates a simple approach to determine the patient's ESE based on the patient's height and weight for radiographic units, which will be very useful for institutions that do not have a dedicated physicist.

Evaluation of off-axis wedge correction factor using diode dosimeters for estimation of delivered dose in external radiotherapy.

An in vivo dosimetry system, using p-type diode dosimeters, was characterized for clinical applications of treatment machines ranging in megavoltage energies. This paper investigates two different models of diodes for externally wedged beams and explains a new algorithm for the calculation of the target dose at various tissue depths in external radiotherapy. The values of off-axis wedge correction factors were determined at two different positions in the wedged (toward the thick and thin edges) and in the non-wedged directions on entrance and exit surfaces of a polystyrene phantom in 60Co and 6 MV photon beams. Depth transmission was defined on the entrance and exit surfaces to obtain the off-axis wedge correction factor at any depth. As the sensitivity of the diodes depends on physical characteristics [field size, source–skin distance (SSD), thickness, backscatter], correction factors were applied to the diode reading when measuring conditions different from calibration situations. The results indicate that needful correction factors for 60Co wedged photons are usually larger than those for 6 MV wedged photon beams. In vivo dosimetry performed with the proposed algorithms at externally wedged beams has negligible probable errors (less than 0.5&) and is a reliable method for patient dose control.

Clinical validation of the Acuros XB photon dose calculation algorithm, a grid-based Boltzmann equation solver

Introduction. A new algorithm that uses a grid-based technique to solve the linear Boltzmann transport equation (LBTE) has been developed to improve the accuracy and speed of external photon beam treatment planning calculations. The aim of this study was to test the accuracy of this algorithm in both heterogeneous and homogeneous media. Material and methods. Output factors, depth dose curves and profiles for symmetric fields were measured in water using diamond and ionization chamber detectors. Furthermore, asymmetric fields, fields collimated with the multi-leaf collimator, enhanced dynamic wedge fields as well as fields with different source-skin distances were measured. Various test plans were created on a CIRS thorax phantom including tissue-equivalent inserts and corresponding dose distributions within the phantom were measured with radiochromic films. The new grid-based LBTE solver, Acuros XB (Eclipse version 10.0, Varian Medical Systems, CA, USA) was used to calculate dose distributions for all field configurations and plans, for both 6 MV and 15 MV photons. Calculations were also performed with AAA, a standard convolution algorithm. Results. Compared to measurements, the output factors were within 1% for Acuros XB. For the depth doses, the average deviations were within 1% in dose and 1 mm in distance to agreement (DTA). For the profiles, the deviations were within 2%/1 mm except near the penumbra. Similar results were obtained for the other field configurations. Good agreement with AAA was also found. For the plans calculated on the CIRS phantom, the number of points meeting the gamma criterion of 3% in dose and 3 mm in DTA was higher with Acuros XB (98% for 6 MV; 100% for 15 MV) than with AAA (94% for 6 MV; 96% for 15 MV). Conclusion. Dose calculations with the Acuros XB algorithm in homogeneous media are in good agreement with both measurements and the AAA algorithm. In heterogeneous media, the Acuros XB algorithm is superior to AAA in both lung and bony material.

Modern three-dimensional conformal craniospinal radiotherapy

The main problem of craniospinal irradiation (CSI) is the matching of the fields. The use of a suitable technique is very important because matching of the fields is necessary to use for the optimal cancer irradiation of the long planning target volume (PTV). Since 2007, 8 patients have received CT-based, 3D-planned conformal CSI in our Institute. Patient immobilization was made in prone position in a vacuum bed, using skull and pelvis masks. Organ-at-risk (OAR) contours were made by radiographers. PTV was contoured by radiation oncologists. The prescribed dose to the PTV was 36 Gy with 1.8 Gy dose per fraction. In the planning process the following aspects were taken under consideration: all points of the PTV had to receive at least 95% of the prescribed dose (according to ICRU 50, 62); at junction field edges the overlapping parts were eliminated using a multisegmental technique, where the adjacent segment ends of the neighbouring fields were shifted two times 2 cm, so that the three equally weighted segments used in one field had 2-2 cm distance from each other. In the CSI planning the shape of the patient and so the length of the PTV has made a big emphasis on determining the number of field matching. Thus in some cases instead of two, only one field matching was sufficient - this could be achieved by increasing the source-to-skin distance (SSD) of the fields. The verification made with a solid-water phantom justified the precision of the field matching. The offset used at junction field edges in between one treatment facilitates the verification of field matching - and so the patient positioning. Thus the possibility of having overdosed regions could be reduced, which was very important from a radiation biological point of view.

SU‐E‐E‐14: Fabrication of Divergent Wax Based Tissue Compensators for Head and Neck Radiotherapy

Purpose: Tissue obliquity is a common problem in conventional radiotherapy to head and neck region. This results in generation of hot spots and non‐uniformity of dose in the planned treatment volumes. Ellis compensators made of non divergent aluminum metal rods have been suggested. The purpose of this study is to develop a divergent wax based tissue compensator (DWTC) to address tissue obliquity and verify it dosimetrically before clinical implementation. Methods: A method is developed for a photon field of size 20 cm × 20 cm from a telecobalt machine having Source Skin Distance (SSD) of 80 cm and Source Diaphragm Distance (SDD) of 45 cm. A jig was made with 3 Perspex sheets kept at specified distances which were perforated to allow radial divergence of metal spokes (depicting pencil beams) to pass through. Dose compensator is fabricated with dental wax based on the measured tissue thickness variations in different planes. Patient anatomy was reproduced on a POP which was placed at the bottom of the jig and a divergent compensator was developed at the top of the jig by placing wax on the upper end of the metal spokes. The efficacy of DWTC in the treatment plane was confirmed with Imatrix device using a wedge shaped and two semi neck water phantoms with and without compensators. Results: The profiles with and without compensator verified by Imatrix device showed adequate tissue compensation for clinical purposes. The uniformity of the dose in the treatment region is well within 3%, 3mm criteria (dose evaluation parameter). Conclusions: The method developed for DWTC could be adopted for a linear accelerator with different SSD and SDD. The entire process of making a DWTC is simple, accurate and can be easily fabricated.

TH-E-BRB-10: A New Arc-Modulated Inverse-Planned Technique for Total Body Irradiation

Purpose: To develop a new inverse‐planned total body irradiation (TBI) technique for treating a patient as they lay on a stationary couch, near to the floor, that improves dose homogeneity over the conventional forward‐ planned treatment. Methods: The developed technique uses a collection of open‐field beams distributed along an arc to deliver dose to the patient. A 170 cm long, full‐body anthropomorphic water phantom was constructed for validating the technique. Treatment plans were optimized on a CT of the phantom with Pinnacle using the standard beam model. These plans explored the optimal field size and total number of beams. Additionally, these plans were compared to the conventional extended source‐to‐skin distance (SSD) treatment and also tested for the benefits of multi‐leaf collimator(MLC) motion. From each plan the fraction of the volume within 10% of the prescription dose, V(±10), is extracted. The optimal treatment plan was delivered to the phantom on a Siemens Artiste. Measurements of the delivered dose were performed with a combination of a linear diode array and an ion chamber at six different positions inside the phantom. These measurements were compared directly to the dose predictions. Results: The optimal setup for this phantom has treatment beams distributed in gantry angle at 5° steps from 305° to 55°, with a 40×40 cm field size. For the extended SSD, open field and MLC techniques, the values for V(±10) were 78%, 85% and 85%, respectively. The root of the mean square difference between dose predictions and measurements was 2.5%. Conclusions: The developed technique improves homogeneity over the conventional TBI method, allows treating a patient in a comfortable lying position and enables for dose delivery in a standard‐sized vault. A specific TBI beam model is being developed to further improve agreement between dose measurements and predictions. Work supported in part by Siemens OCS.

Geometric Performance and Efficiency of an Optical Tracking System for Daily Pre-treatment Positioning in Pelvic Radiotherapy Patients

The purpose of this study was to characterize the accuracy of a novel in-house optical tracking system (OTS), and to determine its efficiency for daily pre-treatment positioning of pelvic radiotherapy patients compared to conventional optical distance indicator (ODI) methodology. The OTS is comprised of a passive infrared stereoscopic camera, and custom control software for use in assisting radiotherapy patient setup. Initially, the system was calibrated and tested for stability inside a radiation therapy treatment room. Subsequently, under an ethics approved protocol, the clinical efficiency of the OTS was compared to conventional ODI setup methodology through 17 pelvic radiotherapy patients. Differences between orthogonal source-to-skin distance (SSD) readings and overall set-up time resultant from both systems were compared. The precision of the OTS was 0.01 ± 0.01 mm, 0.02 ± 0.02 mm, and -0.01 ± 0.06 mm in the left/right (L/R), anterior/posterior (A/P), and cranial/caudal (C/C) directions, respectively. Discrepancies measured between the linac radiographic center in the treatment room and the calibrated origin of the camera (OTS) by two independent observers was submillimeter. Analysis of 146 fractions from 17 patients showed a high correlation between the SSD readings of the OTS and ODI setup methodologies (r = 0.99). The average time for pre-treatment positioning using the OTS couch shift calculation was 2.60 ± 0.69 minutes, and for conventional ODI setup, 3.62 ± 0.82 minutes; the difference of 1.02 minutes was statistically significant (p < 0.001). In conclusion, the OTS is a precise and robust tool for use as an independent check of treatment room patient positioning. The system is indicated as geometrically equivalent to current methods of daily pre-treatment patient positioning with potential for gains in efficiency by decreasing setup times in the treatment room.

565 ASSESMENT OF PEDIATRIC RADIATION EXPOSURE DURING URETEROSCOPY

You have accessJournal of UrologyPediatrics: Imaging/Infections & Vesicoureteral Reflux1 Apr 2011565 ASSESMENT OF PEDIATRIC RADIATION EXPOSURE DURING URETEROSCOPY Paul Kokorowski, Jeanne Chow, Keith Strauss, Melanie Pennison, and Caleb Nelson Paul KokorowskiPaul Kokorowski Boston, MA More articles by this author , Jeanne ChowJeanne Chow Boston, MA More articles by this author , Keith StraussKeith Strauss Boston, MA More articles by this author , Melanie PennisonMelanie Pennison Boston, MA More articles by this author , and Caleb NelsonCaleb Nelson Boston, MA More articles by this author View All Author Informationhttps://doi.org/10.1016/j.juro.2011.02.1375AboutPDF ToolsAdd to favoritesDownload CitationsTrack CitationsPermissionsReprints ShareFacebookTwitterLinked InEmail INTRODUCTION AND OBJECTIVES Pediatric patients are more sensitive to ionizing radiation than adults. The ′as low as reasonably achievable′ (ALARA) principle requires efforts to minimize exposure to the patient, but little is known about radiation exposure during urologic procedures in children, including ureteroscopy (URS) for urolithiasis. The aim of this study was to measure radiation exposure during pediatric URS and identify opportunities for quality improvement. METHODS We performed a prospective direct observation of URS procedures as part of a quality improvement initiative. Information regarding pre-operative patient characteristics, operative factors, fluoroscopy unit settings and radiation exposure were recorded. Primary outcomes were total fluoroscopy time (min) and patient skin entrance dose (SED, in mGy). Specific modifiable factors were identified from observations as targets for potential quality improvement. Univariate tests of association were used to identify specific patient and operative factors associated with primary outcomes. RESULTS Direct observation was performed on 48 consecutive ureteroscopic procedures at a single institution. 34 procedures were included in our analysis after excluding two procedures for arterio-venous malformations, one terminated procedure, and 11 non-pediatric patients (≥21 years of age). Median patient age was 16.5 years old (range 7.4 to 19.2). 21% (7/34) procedures resulted in failure to treat a stone due to spontaneous passage or difficult access. Median total fluoroscopy time was 2.85 min (range 0.4–6.7) and median SED was 42.7mGy (range 2.23–223.4).For reference purposes, SED for chest x-ray in a 15 year old is ∼0.16 mGy. There was considerable variability across multiple examined factors and thus no single factor was significantly associated with fluoroscopy time and only source to skin distance (SSD) was associated with skin entrance dose (p=0.02). However, several other modifiable factors including fluoroscopic image rate and field of view demonstrated opportunities for dose reduction. CONCLUSIONS Pediatric patients receive considerable radiation exposure during ureteroscopic procedures. No specific modifiable factors were consistently predictive of total fluoroscopy time and only SSD was associated with skin entry dose. Large variations limited the power to detect differences. Multiple potential methods for potential radiation reduction were identified and future efforts will focus on implementing dose reduction strategies consistent with the ALARA principle. © 2011 by American Urological Association Education and Research, Inc.FiguresReferencesRelatedDetails Volume 185Issue 4SApril 2011Page: e228 Advertisement Copyright & Permissions© 2011 by American Urological Association Education and Research, Inc.MetricsAuthor Information Paul Kokorowski Boston, MA More articles by this author Jeanne Chow Boston, MA More articles by this author Keith Strauss Boston, MA More articles by this author Melanie Pennison Boston, MA More articles by this author Caleb Nelson Boston, MA More articles by this author Expand All Advertisement Advertisement PDF downloadLoading ...