Neck Phantom Research Articles

SU-FF-T-74: Accuracy & Precision of An IGRT Solution

Purpose: To establish the overall accuracy and precision of an image‐guidedradiotherapy treatment strategy. Method and Materials: A perspex head & neck phantom developed in‐house for the verification of IMRT techniques was used to test an image guided radiotherapy treatment. The treatment involved the imaging of the phantom in the treatment position, followed by the delivery of an IMRT plan designed to spare salivary function. The complex IMRT plan involved five step‐and‐shoot fields with a total 65 segments delivered at 6MV. The phantom was not moved between the imaging and delivery phases of the experiment. The dose was measured at six points distributed in the phantom simultaneously via the use of micro‐mosfets. Development of the phantom used in the experiment will eventually allow up to 20 points of interest being independently measured. As the overall accuracy of the treatment was sought the dose from the whole treatment was considered. Doses from individual beams were not analyzed. Results: Planned dose to the 6 micro‐mosfets ranged from 220cGy to 350cGy with a dose prescription of 440cGy per fraction to the treatment isocentre. The measured doses were on average within 0.5% (±1.2%) when compared to planned doses. No differences greater than 2% were found in the investigation. Repeat experiments without moving the phantom demonstrated reliable delivery of the IMRT plan with a standard deviation of 0.5% from the average of the mean. Conclusion: A system has been developed to test the accuracy and precision of an image‐guidedradiotherapy treatment. It has been confirmed that the accuracy of a complex IMRTdose delivery using an image‐guided approach using an Elekta Synergy linac is ±2% in dose. The precision of the delivery system was demonstrated to be within 1% (2SD). Conflict of Interest: This research is partly funded by Elekta.

TH-C-T-6E-04: Evaluation of a New 3D Polyurethane Dosimeter for IMRT Verification

Purpose: To demonstrate the dosimetric accuracy of PRESAGE™, a new type of three‐dimensional dosimeter, when used in the Radiological Physics Center's (RPC) Head and Neck Phantom. Method and Materials: An IMRTtreatment plan was developed for the RPC Head and Neck Phantom, which contained simulated planning target volumes and an organ at risk. A conventional dosimetry insert, containing radiochromic film and TLD, was in place while the IMRT plan was delivered. The insert was removed and replaced with a PRESAGE™ dosimeter and delivery of the IMRT plan was repeated. An additional PRESAGE™ dosimeter was irradiated to doses between 0.5 and 7.5 Gy with stereotactic beams to develop a calibration curve. The PRESAGE™ dosimeter was imaged using an OCT‐OPUS™ laser CT scanner 24 hours after irradiation. Two‐dimensional comparisons were performed between the treatment plan, the distribution measured with film/TLD and the distribution measured with PRESAGE™. Results: The PRESAGE™ dosimeter showed a monotonic and easily characterized response with dose. Optical density maps obtained following IMRT delivery were converted to dose and compared with the treatment plan and with film/TLD data. Film and PRESAGE™ data showed good agreement, and both indicated higher doses in the steep gradient region between the PTV and the organ at risk. Conclusion: This work demonstrated the use of PRESAGE™ as a dosimetric tool for the verification of IMRTtreatments. The dosimeter provided a three‐dimensional distribution that was used for comparisons with treatment plan distributions. The investigation was supported by PHS grants CA 10953 and CA81647 awarded by the NCI, DHHS.

SU‐FF‐J‐99: Automation of Image Registration and Verification for Image‐Guided Stereotactic Body Radiotherapy

Purpose: To facilitate image‐guided stereotactic body radiotherapy (SBRT) with high precision and fully automated target localization and patient position verification. Method and Materials: A mutual information based image registration algorithm was developed to register the patient's daily 3D computed tomography (CT) images with those taken for treatment planning to assess and correct patient position shifts in all three translational and three rotational directions. A digitally reconstructed radiograph (DRR) algorithm generates and displays the corresponding DRRs in one frame in a split screen format along with their intensity differences to enhance verification. In the same way, the portal image verification algorithm processes the portal image and displays it with the corresponding planning DRR in the same frame as final verification of the patient's position. The algorithms were programmed using MatLab and were assembled seamlessly in one package and the operation was fully automated. Results: Target localization and patient position verification process in our image‐guided SBRT is significantly simplified and the time needed for this process is reduced from 30 minutes or more to 3–4 minutes. The image registration algorithm was found to be accurate to within 0.1 mm with a head and neck phantom. The DRR generation algorithm generates high quality DRRs with a spatial resolution of 0.289×0.289 mm2/pixel. It removes unrelated anatomical structures for a cleaner background in the DRR to help identify the target isocenter more easily and more accurately. The DRR comparison and portal image verification algorithms readily detect misalignments of less than 0.2 mm. Conclusion: Application of the algorithms in determining patient position shift in actual SBRT cases demonstrated that they were accurate, fast, and reliable. They serve as a useful tool for image‐guided radiotherapy.

SU‐FF‐T‐148: IMRT Head and Neck Phantom Irradiations: Correlation of Results with Institution Size

Purpose: To analyze the results from 136 IMRT H&N phantom irradiations. Method and Materials: A mailable anthropomorphic IMRT head and neck phantom was irradiated 136 times by 104 institutions. Some institutions irradiated multiple times. Institutions imaged the phantom, planned an IMRT treatment, performed their routine IMRT QA checks, and irradiated the phantom according to their plan. The phantom contained imageable structures representing a planning target volume (PTV) close to an organ at risk (OAR), simulating an oropharyngeal tumor and the spinal cord. The phantom also contained a secondary PTV that simulated peripheral nodes. TLDs were placed in each structure and a set of orthogonal radiochromic films intersected in the primary PTV. The following criteria were used to evaluate the measurements: TLD/institution dose − ± 7%; distance‐to‐agreement in the high dose gradient region near the OAR − ⩽4 mm. The failure rate of institutions that housed 3 or fewer megavoltage therapy machines was compared to that of larger institutions. Results: 41 irradiations failed to meet one or more of the criteria. 24 of the failures were dose discrepancies measured with TLD, 5 were dose distribution discrepancies measured with radiochromic film and 12 were disagreements in both TLD and film measurements. There was a 38% discrepancy rate in first‐time irradiations at the institutions with 3 or fewer machines and a 26% rate at the larger institutions. All of the institutions that failed multiple times were smaller institutions. Conclusion: Institutions of all sizes are capable of making mistakes in IMRT treatments. Sufficient physics coverage is an important aspect of IMRT quality assurance. Conflict of Interest: The investigation was supported by PHS grants CA10953 and CA81647 awarded by the NCI, DHHS.

Detection of emboli in vessels using electrical impedance measurements—phantom and electrodes

A phantom was constructed to simulate the electrical properties of the neck. A range of possible electrode configurations was then examined in order to improve the sensitivity of the impedance measurement method for the in vivo detection of air emboli. The neck phantom consisted of simulated skin, fat and muscle layers made of agar and a conductive rubber tube mimicking the common carotid artery. The ring-shaped electrodes with a guard electrode showed the highest sensitivity to emboli at short distances.

Assessment of the local SAR distortion by major anatomical structures in a cylindrical neck phantom

The objective of this work is to gain insight in the distortions on the local SAR distribution by various major anatomical structures in the neck. High resolution 3D FDTD calculations based on a variable grid are made for a semi-3D generic phantom based on average dimensions obtained from CT-derived human data and in which simplified structures representing trachea, cartilage, spine and spinal cord are inserted. In addition, phantoms with dimensions equal to maximum and minimum values within the CT-derived data are also studied. In all cases, the phantoms are exposed to a circular coherent array of eight dipoles within a water bolus and driven at 433 MHz. Comparisons of the SAR distributions due to individual structures or a combination of structures are made relative to a cylindrical phantom with muscle properties. The calculations predict a centrally located region of high SAR within all neck phantoms. This focal region, expressed as contours at either 50% or 75% of the peak SAR, changes from a circular cross-section in the case of the muscle phantom to a doughnut shaped region when the anatomical structures are present. The presence of the spine causes the greatest change in the SAR distribution, followed closely by the trachea. Global changes in the mean SAR relative to the uniform phantom are <11%, whilst local changes are as high as 2.7-fold. There is little difference in the focal dimensions between the average and smallest phantoms, but a decrease in the focal region is seen in the case of the largest phantom. This study presents a first step towards understanding of the complex influences of the various parameters on the SAR pattern which will facilitate the design of a site-specific head and neck hyperthermia applicator.

Thermometry studies of radio-frequency induced hyperthermia on hydrogel based neck phantoms.

A cylindrical phantom, resembling average human neck, was prepared by using hydrogel sheets containing vinyl and polysaccharide. The phantom was used to obtain temperature distributions for 6 values of input power of radio frequency (RF) at 8MHz,by invasive thermometry technique, using thermistor probes. The inclusion of cervical vertebrae and calcium carbonate pieces (human bone representative) with a hollow tube (windpipe equivalent) in the phantom simulates the change in thermal distributions. This is similar to the alterations in heat disposition obtained in the real human neck, during RF induced heating, without extensive distortion of the uniform temperature distribution provided by the RF heating instrument. This paper compares the hydrogel neck phantom with other phantoms, that have been developed for studying thermal distributions and optimization of novel non invasive thermometry techniques in hyperthermic oncology.

Spectrometric assessment of thyroid depth within the radioiodine test

Aim of this study is the validation of a simple method for evaluating the depth of the target volume within the radioiodine test by analyzing the emitted iodine-131 energy spectrum. In a total of 250 patients (102 with a solitary autonomous nodule, 66 with multifocal autonomy, 29 with disseminated autonomy, 46 with Graves' disease, 6 for reducing goiter volume and 1 with only partly resectable papillary thyroid carcinoma), simultaneous uptake measurements in the Compton scatter (210 +/- 110 keV) and photopeak (364-45/+55 keV) windows were performed over one minute 24 hours after application of the 3 MBq test dose, with subsequent calculation of the respective count ratios. Measurements with a water-filled plastic neck phantom were carried out to perceive the relationship between these quotients and the average source depth and to get a calibration curve for calculating the depth of the target volume in the 250 patients for comparison with the sonographic reference data. Another calibration curve was obtained by evaluating the results of 125 randomly selected patient measurements to calculate the source depth in the other half of the group. The phantom measurements revealed a highly significant correlation (r = 0,99) between the count ratios and the source depth. Using these calibration data, a good relationship (r = 0,81, average deviation 6 mm corresponding to 22%) between the spectrometric and the sonographic depths was obtained. When using the calibration curve resulting from the 125 patient measurements, the overage deviation in the other half of the group was only 3 mm (12%). There was no difference between the disease groups. The described method allows on easy to use depth correction of the uptake measurements providing good results.

Optimization of parathyroid imaging by simultaneous dual energy planar and single photon emission tomography

Single photon emission tomography (SPET) gives valuable 3-dimensional information, but prolongs the time of imaging and increases the possibility of patient movement. We therefore investigated a method for the optimization of SPET. Using an in-house fabricated thyroid/parathyroid neck phantom simultaneous dual energy (DE) 1223I/99mTc imaging to localize parathyroid glands was assessed both in planar and SPET modes. Experiments demonstrated improved spatial resolution and contrast for planar pinhole imaging compared to parallel collimation. For DE-SPET compared to planar pinhole imaging more glands in the phantom were visualized by the improved contrast. We conclude that DE-SPET for parathyroid imaging is feasible to aid the accurate localization of parathyroid adenomas. For planar imaging pinhole collimation is superior to parallel hole collimation.

Characterization of an in vivo thyroid 131I monitoring system using an imaging plate

The effects of neck diameter, thyroid volume, and prethyroid tissue thickness on a count-activity conversion coefficient and the detection limit of a thyroid 131I monitoring system with an imaging plate (IP) were estimated by using an anthropomorphic thyroid–neck phantom. The conversion coefficient and detection limit of the IP system was approximately constant for normal Japanese adults regardless of their neck diameters, thyroid volumes, and prethyroid tissue thicknesses. The IP system is a new option for thyroid 131I monitoring.

IMRT-application with an add-on MMLC.

In order to provide automatic IMRT dose delivery with an add‐on MMLC a technical integration of a MMLC system with a linear accelerator was realized. The principle of this integration and the changes and enhancements of the existing hard‐and software are briefly described. The system was tested by the automatic delivery of an IMRT plan designed for a head and neck phantom. A verification of dose delivery was performed with film dosimetry. The plan consisting of 78 “step and shoot” segments could be delivered within 17 minutes. A high spatial accuracy of the fluence pattern at the isocentre was reached by a resolution of 2.75×2.75 mm2. The measured dose profiles were within 3% of the maximum dose of the calculated profiles.PACS number(s): 87.53.–j, 87.90.+y

In vivo thyroid 131I monitoring system using an imaging plate.

The counting efficiency and detection limit of an in vivo thyroid 131I monitoring system using an imaging plate (IP) were estimated using an anthropomorphic thyroid neck phantom. The counting efficiency of the IP system was approximately constant for thyroid volumes between 11.7 and 20 ml and neck diameters between 10 and 14 cm. The detection limits were distributed from 288 to 451 Bq depending on the combination of neck diameter, thyroid volume and tissue thickness. The IP monitoring system gives a reliable counting efficiency notwithstanding the variation of thyroid volume and neck diameter. The IP system is a new option for thyroid 131I monitoring.

Accommodating practical constraints for intensity modulated radiation therapy by means of compensators

The thesis deals with the practical implementation of intensity modulated radiation therapy (IMRT) generated by means of patient specific metal compensators. An elaborate comparison between several compensator-machining techniques, with respect to their suitability for production within a hospital workshop, is presented. The limitations associated with the selected compensator manufacturing technique are identified and implemented as constraints in an existing inverse treatment-planning algorithm. In order to obtain the profile of a compensator, which produces a desired intensity distribution, inverse modeling of the radiation attenuation within the compensator is required. Two novel and independent approaches, based on deconvolution and system identification, are proposed to accomplish this. To compare the approach with the “rival” state of the art beam modulation technique, a theoretical and experimental examination of the modulated fields generated by manufactured compensators and multileaf collimators is presented. This comparison focused on the achievable resolution of the intensity modulated beams in lateral and longitudinal directions. To take into account the characteristics of a clinical environment the suitability of the most common commercially available treatment couch systems for IMRT treatments is studied. An original rule based advisory system is developed to alert the operator of any potential collision of the beam with the movable supporting structures of the treatment couch. The system is capable of finding alternative positions for the supporting frames and, if necessary, can suggest alternative beam directions. Finally, a head and neck phantom is designed for gel dosimetry to assess IMRT treatment delivery techniques. The phantom is based on a simplistic but realistic design and contains the main anatomical features.

Paediatric absorbed doses from rotational panoramic radiography.

To determine the paediatric doses in rotational panoramic radiography with film/screen and photostimulable phosphor receptors. A paediatric anthropomorphic head and neck phantom was used. Absorbed doses were measured for two panoramic systems, the Orthophos (Sirona Dental Systems, Bensheim, Germany) and the PM 2002 CC (Planmeca Oy, Helsinki, Finland), with and without programmable child settings, using both screen/film and photostimulable phosphor receptors. Absorbed doses to the eye ranged from 5 to 24 micro Gy. Doses to the dental arches with the Orthophos unit ranged from 50 to 555 micro Gy with the adult and from 27 to 436 micro Gy with the child program; using the PM 2002 CC unit, doses ranged from 56 to 1040 micro Gy using the adult settings, and from 60 to 890 micro Gy with the paediatric settings. The paediatric exposure settings reduced doses at most locations for both panoramic systems. The highest doses were measured near the rotational axes of the X-ray beam. Paediatric settings with the Orthophos P10 resulted in the dose reduction more than 50% to the thyroid but not with the PM 2002 CC. When lower kVcp or mA settings were used, absorbed doses were effectively reduced for all combinations of machines, programs and detectors. Specific program settings for children reduced the absorbed doses from panoramic radiography irrespective of the machine or receptor used.

Comparative dose measurements by spiral tomography for preimplant diagnosis: The Scanora machine versus the Cranex Tome radiography unit

Objective. The purpose of this study was to determine the dose profile of the Cranex Tome radiography unit and compare it with that of the Scanora machine. Study design. The radiation dose delivered by the Cranex Tome radiography unit during the cross-sectional mode was determined. Single tooth gaps in regions 3 (16) and 30 (46) were simulated. Dosimetry was carried out with 2 phantoms, a head and neck phantom and a full-body phantom loaded with 142 thermoluminescent dosimeters (TLD) and 280 TLD, respectively; all locations corresponded to radiosensitive organs or tissues. The recorded local mean organ doses were compared with those measured in another study evaluating the Scanora machine. Results. Generally, dose values from the Cranex Tome radiography unit reached only 50% to 60% of the values measured for the Scanora machine. The effective dose was calculated as 0.061 mSv and 0.04 mSv for tooth regions 3 (16) and 30 (46), respectively. Corresponding values for the Scanora machine were 0.117 mSv and 0.084 mSv. Conclusion. Cross-sectional imaging in the molar region of the upper and the lower jaw can be performed with the Cranex Tome unit, which delivers only approximately half of the dose that the Scanora machine delivers. (Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2001;91:735-42)

In Vivo Thyroid 125I Monitoring with Radioluminography

The counting features of an in vivo thyroid monitoring system with an imaging plate (IP) were investigated by using an anthropomorphic thyroid-neck phantom. The realistic thyroid phantoms loaded with 125 I solution were embedded in a neck phantom. The IP was fixed on the neck phantom and exposed to 125 I photons emitted from the thyroid phantom by changing the pre-thyroid tissue thickness at the front of the thyroid. A clear thyroid image was obtained at short tissue thicknesses. A region of interest (ROI) was set so that the ROI contained the thyroid image. The count within the ROI was regarded as the number of 125 I photons detected by the IP. The IP counting efficiency was about 0.6% at maximum. Monitoring with IP has the advantage of allowing the worker to move freely during the monitoring period by wearing the IP on his neck. Radioluminography with IP has shown itself to be useful when monitoring thyroid 125 I.

The midline dose distribution for a three-field radiotherapy technique

At the University of Florida, head and neck cancer often is irradiated using parallel opposed lateral fields (with inferior borders slanted superiorly) and an anterior low neck field. A common criticism is that overlap may occur at the match-line junction of the three fields, resulting in an increased risk of radiation myelitis. One setup for treatment of the oropharynx and two for the larynx were irradiated in an anthropomorphic head and neck phantom made of tissue-equivalent polyacrylamide gel with a two-dimensional thermoluminescent dosimeter array in its sagittal midplane. The results showed that no excess radiation dose was measured at the junction of the three fields. The “spinal cord dose,” as percentage of dose to the central axis of the primary field, was as follows: oropharynx setup, 15% to 100%; larynx setup with midline tracheal block, 10% to 90%; larynx setup without tracheal block, 10% to 90%. In conclusion, the University of Florida three-field technique for head and neck cancer produces no measured increase in dose at field junctions.

Hypothetical mortality risk associated with spiral computed tomography of the maxilla and mandible.

In the present study, dose measurements have been conducted following examination of the maxilla and mandible with spiral computed tomography (CT). The measurements were carried out with 2 phantoms, a head and neck phantom and a full body phantom. The analysis of applied thermoluminescent dosimeters yielded radiation doses for organs and tissues in the head and neck region between 0.6 and 16.7 mGy when 40 axial slices and 120 kV/165 mAs were used as exposure parameters. The effective dose was calculated as 0.58 and 0.48 mSv in the maxilla and mandible, respectively. Tested methods for dose reduction showed a significant decrease of radiation dose from 40 to 65%. Based on these results, the mortality risk was estimated according to calculation models recommended by the Committee on the Biological Effects of Ionizing Radiations and by the International Commission on Radiological Protection. Both models resulted in similar values. The mortality risk ranges from 46.2 x 10.6 for 20-year-old men to 11.2 x 10(-6) for 65-year-old women. Using 2 methods of dose reduction, the mortality risk decreased by approximately 50 to 60% to 19.1 x 10(-6) for 20-year-old men and 5.5 x 10(-6) for 65-year-old women. It can be concluded that a CT scan of the maxillofacial complex causes a considerable radiation dose when compared with conventional radiographic examinations. Therefore, a careful indication for this imaging technique and dose reduction methods should be considered in daily practice.

原子炉事故にともなう緊急時医療活動におけるスペクトロサーベイメータを用いた甲状腺｀１３１´Ｉの測定

Feasibility for the use of the collimated NaI (Tl) scintillation spectrosurvey meter SS-γ C3475 (Hamamatsu Photonics) in the estimation of thyroidal 131I burden in patients that received emergency exposure to radioiodine (131I) is demonstrated. When the NaI (Tl) crystal probe collimated well to a neck phantom simulating the human thyroid gland, the detection limit of 131I deposited in that organ was estimated at 0.58kBq, which was well below the generally adopted screening level of 30kBq. The detection system for thyroidal 131I presented in this report is remarkably inexpensive compared to the conventional whole-body counters, and is considered to provide a satisfactorily accurate evaluation of thyroidal 131I burden.

Dosimetry of high energy electron therapy to the parotid region

Well-known inadequacies in currently available electron planning systems, and two cases of temporal lobe necrosis following electron therapy of the parotid stimulated a comprehensive head and neck phantom dosimetric study of the use of high energy electrons for parotid treatments. A typical electron field employed for the treatment of parotid malignancy was examined in an anthropomorphic head phantom from which air cavities had been excavated. Thermoluminescent dosimeter measurements were compared with predicted point doses obtained from a Theraplan Treatment planning system (V05). Data was examined for three different electron energies: 12, 16 and 20 MeV and with the addition of contoured bolus for 20 MeV. A number of significant discrepancies between the measured and predicted dose were observed. Measured doses were seen to exceed predicted doses by up to 23% in the temporal lobe. Further under-predictions of dose were found behind the mandible and in the nasal cavity. Over-predictions of dose by the planning algorithm of up to 22% were observed beside the oropharynx. Some of these discrepancies were found to relate to Theraplan under-estimation of the dose in the fall-off region. Other errors are attributable to the difficulties in predicting dose at density interfaces. Localised over- and under-predictions of this magnitude must be accounted for by the clinician prescribing treatment in terms of possible late effects on the temporal lobe and, in particular, the nominated dose specification point.