Phase II study of intraoperative dosimetry for prostate brachytherapy using registered ultrasound and fluoroscopy

PurposeTo assess the performance of a system of intraoperative dosimetry and obtain estimates of dosimetry outcomes achieved when utilizing the system in a Phase II clinical trial.Methods and MaterialsForty-five patients undergoing permanent Pd-103 seed implantation for prostate cancer were prospectively enrolled. Seed implantation was performed and dose was tracked intraoperatively using intraoperative registered ultrasound and fluoroscopy (iRUF). Three-dimensional seed locations were computed from X-rays and registered to ultrasound for intraoperative dosimetry, followed by adaptive plan modification to achieve prostate V100 ≥95% and ≥95% D90. Time required for iRUF was recorded. Postoperative CT/MRI scans were performed 1 day after the implantation and used as reference for dosimetric analysis. Dosimetric parameters for the prostate and urethra were compared between standard ultrasound-based dosimetry (USD), iRUF, and postoperative CT/MRI.ResultsMean total time for iRUF was <30 min. A mean of four seeds (0–12) were added per implant to correct cold spots discovered by iRUF. Day 1 CT/MRI prostate V100 was ≥95% for 44/45 patients; 1 patient had Day 1 V100 93%. No patient had rectal V100 exceeding 1 cc. Compared to CT/MRI, iRUF dosimetry had significantly smaller mean differences and higher correlations for all prostate and urethral dosimetric parameters examined than USD. Both USD and iRUF tended to overestimate dose, but with less bias in iRUF than USD.ConclusionsIntraoperative dosimetry utilizing iRUF was associated with acceptable increase in procedure time and enabled very high rates of achieving excellent prostate dose coverage. iRUF intraoperative dosimetry approximated postoperative CT/MRI dosimetry to a greater degree than USD for the prostate and urethra.     

Intercomparison of the dosimetric system used in industrial irradiation plants in Italy

Industrial applications of ionizing radiation (such as the sterilization of medical supplies, long-lasting food preservation) is a well-established technique all over the world. The efficacy of the treatment depends on accurate dosimetry, which assures both the achievement of irradiation purposes at the lowest cost and the safety of the consumers. This is why in 1986 the Istituto Superiore di Sanità (ISS) started an intercomparison programme among the industrial irradiation plants operating in Italy, aimed at optimizing the applied dosimetric procedures. The electron spin resonance (ESR) alanine-based dosimetry, set up at the ISS, was chosen for reference dosimetry. Each plant received 30 dosimeters to be irradiated in pre-fixed conditions in order to (i) compare the ESR/alanine system and the routine dosimetry; (ii) evaluate the homogeneity factor in the irradiated product; (iii) check the reproducibility of the irradiation technique. Results support the need for standardized dosimetric procedures for an optimization of the radiation treatment.     

Dosimetric Effects of Setup Uncertainties on Breast Treatment Delivery

This study aimed to assess the dosimetric impact of setup errors during the delivery of radiotherapy to the breast, and use this information to make recommendations on intervention tolerances for portal imaging of breast treatments. Translational and rotational setup errors were simulated for 10 recent breast patients using an Oncentra MasterPlan treatment planning system. The effect of these errors on the breast and tumor bed target volumes receiving 95% and 107% of the prescribed dose were assessed. For the majority of patients, shifts of up to 10 mm or a 4° patient rotation about the cranio-caudal axis had no significant effect on the dose distribution. Changes in dosimetry were more likely if the reference plan contained large hot or cold spots. For a typical patient, it is estimated that a shift of 5 mm in any one direction, or a 2° patient rotation would not cause more than a 5% change in the target volume receiving between 95% and 107% of the prescribed dose. If combinations of errors occur, greater dosimetric changes would be expected. It is concluded that individual patient shifts of up to 5 mm or rotations about the cranio-caudal axis of 2° or less are unlikely to affect dose-volume histogram parameters by an amount judged as clinically significant. Setup errors exceeding these values may cause large dosimetric changes for some patients, particularly those with larger hot or cold regions in the dose distribution, and intervention is therefore recommended.     

Improved total-body irradiation dosimetry

The total-body irradiation (TBI) technique at St Bartholomew's Hospital has been developed to improve dose homogeneity within the patient. Using a standard 6 MV linear accelerator in an orthodox-sized treatment room, the midpoint doses in head, neck, shoulders, mid-mediastinum, pelvis, knees and ankles are +/- 5% of that of the umbilicus in our current technique. This homogeneity has been achieved by a four-field technique, a reproducible patient set-up, careful use of a new bolus material and an additional beam-flattening filter mounted near the machine head. In addition, thermoluminescent dosimetric data collected at a test irradiation before TBI are used to influence field weightings and further improve dosimetry. This individualised and empirical TBI technique has dosimetric advantages over theoretical TBI dosimetric considerations in reducing dose gradients within the patient. These advantages are discussed.     

A clinical application of an automated phantom-film QA procedure for validation of IMRT treatment planning and delivery

To quantify the correlation between planned and delivered intensity-modulated radiation therapy (IMRT) dose distributions, IMRT plans for 37 prostate carcinoma patients were analyzed. IMRT treatment plans were converted into hybrid phantom plans using a commercially available treatment planning system and delivered to a specialized film phantom via a static-tomotherapy technique. The films were analyzed using a commercial film dosimetry system. Hybrid phantom axial dose maps and film images were normalized, registered to one another, and subtracted to calculate the overall relative dose difference throughout the entire film area on a pixel-by-pixel basis. The average percentage of pixels with dose-difference values greater than ± 3% among analyzed hybrid patient plans was 8.6% ± 3%. The average percentage of pixels with dose differences greater than ± 5% was 1.7% ± 1.0%. The number of pixels with more than ± 10% dose differences was negligible. An initial subset of hybrid plans was used to develop a quantitative criterion to verify for positional accuracy based on dosimetric verification of intensity map of IMRT plans for prostate patients in our institution. Plans with less than 5% of the pixels outside the ± 5% dose-difference range were accepted. This method could be implemented for other anatomical sites or treatment planning and delivery systems. © 2003 American Association of Medical Dosimetrists.     

Correlation of ESR with lyoluminescence dosimetry using some sugars

Most applications involving ESR dosimetry currently center on aminoacids because of their relative tissue equivalence. Sugars, however, in addition to possessing high sensitivity and stability in their ESR and LL responses, are widely available as chemical reagents and as commercial sugar. In the present study, dosimetric characteristics of mannose, trehalose, sucrose and commercial sugar obtained by means of ESR and LL techniques are reported. Doses measured by both methods showed agreement within 5%.     

Directionality of extruded lithium fluoride thermoluminescent dosemeters in a cobalt-60 beam

An experimental investigation of the directionality of commercially available extruded lithium fluoride (LiF:Mg,Ti) thermoluminescent dosemeters was carried out in a cobalt-60 beam at a water depth of 5 cm. One-half of a batch of 60 chips (3.1 x 3.1 x 0.9 mm3) was exposed face-on (faces perpendicular to the beam central axis), and the other half was exposed in an edge-on orientation (two edges perpendicular and faces parallel to the beam axis). Measurements show that an edge-on exposure results in a thermoluminescence reading approximately 0.9% lower than a face-on exposure. Although this is of minor importance in every day patient dosimetry, it is relevant in evaluating errors in in-phantom dosimetric measurements where greater accuracy is required.     

A clinical application of an automated phantom-film QA procedure for validation of IMRT treatment planning and delivery

To quantify the correlation between planned and delivered intensity-modulated radiation therapy (IMRT) dose distributions, IMRT plans for 37 prostate carcinoma patients were analyzed. IMRT treatment plans were converted into hybrid phantom plans using a commercially available treatment planning system and delivered to a specialized film phantom via a static-tomotherapy technique. The films were analyzed using a commercial film dosimetry system. Hybrid phantom axial dose maps and film images were normalized, registered to one another, and subtracted to calculate the overall relative dose difference throughout the entire film area on a pixel-by-pixel basis. The average percentage of pixels with dose-difference values greater than ± 3% among analyzed hybrid patient plans was 8.6% ± 3%. The average percentage of pixels with dose differences greater than ± 5% was 1.7% ± 1.0%. The number of pixels with more than ± 10% dose differences was negligible. An initial subset of hybrid plans was used to develop a quantitative criterion to verify for positional accuracy based on dosimetric verification of intensity map of IMRT plans for prostate patients in our institution. Plans with less than 5% of the pixels outside the ± 5% dose-difference range were accepted. This method could be implemented for other anatomical sites or treatment planning and delivery systems. © 2003 American Association of Medical Dosimetrists.     

Limitations of conventional internal dosimetry at the cellular level

A theoretic examination of the validity at the cellular level of assumptions used in classic internal dosimetry has been undertaken. An alternate dosimetric model accounting for the consequences of selective uptake of a radiolabeled compound by specific cells in a multicellular cluster of hexagonal geometry has been developed. At the cellular level, derived dose estimates for electrons have been compared to dose estimates obtained employing the assumptions of conventional internal dosimetry. The study has been performed for all electron energies and then applied specifically to electrons emitted by 99mTc, 201Tl, 111In, and 123I. The dosimetric consequences of altering (a) the intracellular-to-extracellular radionuclide concentration, (b) the labeled cell density, and (c) the cell size have been examined for the labeled and nonlabeled cells in a cell cluster, and the conditions in which conventional dosimetry underestimates or overestimates the dose to individual cells have been indicated. It is shown that when selective intracellular uptake of a radiolabeled compound occurs in specific cells within a cell cluster, conventional dosimetry underestimates the radiation dose delivered to the labeled cells by twofold to more than 25-fold if the emitted electrons have ranges of a few micrometers or less, i.e., energies smaller than approximately 10 keV. Under the same conditions, conventional dosimetry overestimates slightly (20% to 50%) the electron radiation dose to the nonlabeled cells of the cell cluster. It is shown that inclusion of photons in the calculation of the total dose to individual cells does not alter significantly the conclusions of the present investigation.     

Dosimetry of Small Circular Fields for 6-MeV Electron Beams

Small field electron beams used in the clinic present complex dosimetry. This investigation reports dosimetric characteristics of small 6-MeV electron field measurements and Monte Carlo calculations, and examines their impact on patient dosimetry. Radiographic film and ionization chamber were used for dosimetric measurements for a 6-MeV electron beam from a Varian 21EX linac. A set of circular cerrobend cutouts with diameters ranging from 2 to 3.0 cm was made. A clinical Monte Carlo algorithm was used to calculate dosimetric parameters. As the cutout diameter decreased, the build-up portion of the percentage depth dose (PDD) curves shifted toward the phantom surface, the depth of maximum dose, d_max, decreased from 1.2 cm to 0.6 cm, but the practical range, R_p, remained constant at about 3.1 cm. The absorbed dose rate, ?_r, decreased linearly with decreasing diameter. Profile coverage ratios CR (ie, the ratio of a given isodose line to the cutout diameter) at d_max, for 90% and 80%, CR₉₀, CR₈₀, are approximately 55% and 73%, respectively. As the cutout diameter decreased, the 90% to 10% penumbra to diameter ratio, PR, increased from 0.49 to 0.56 for 3- to 2-cm cutouts. The total 90% to 10% penumbra was about the same size as the cutout diameter. The measured output factors were in good agreement with Monte Carlo calculation within 2.2%. Accurate small electron field measurements were performed using parallel plate ion chamber and film. The data were well supported by Monte Carlo calculations. These data facilitate routine clinical treatments for small cutouts as d_max, ?_r, and other data can be quickly obtained instead of performing labor-intensive individual patient measurements.     

Endocavitary in vivo Dosimetry for IMRT Treatments of Gynecologic Tumors

The accuracy and reproducibility of endometrial carcinoma treatment with intensity-modulated radiotherapy (IMRT) was assessed by means of in vivo dosimetry. Six patients who had previously undergone radical hysterectomy for endometrial carcinoma were treated with IMRT using a vaginal applicator with radio-opaque fiducial markers. An ion-chamber inserted into the applicator supplied an endocavitary in vivo dosimetry for quality assurance purposes. The ratio R = D/D_TPS between the in vivo measured dose D and the predicted dose by the treatment planning system D_TPS was determined for every fraction of the treatment. Results showed that 90% and 100% of the ratios resulted equal to 1 within 5% and 10%, respectively. The mean value of the ratios distribution for the 6 patients was R = 0.995 and the SD = 0.034. The ratio R* between the measured and predicted total doses for each patient was near to 1, within 2%. The dosimetric results suggest that the use of a vaginal applicator in an image-guided approach could make the interfractions target position stable and reproducible, allowing a safe use of the IMRT technique in the treatment of postoperative vaginal vault. In vivo dosimetry may supply useful information about the discrimination of random vs. systematic errors. The workload is minimum and this in vivo dosimetry can be applied also in the clinical routine.     

The extension of the range of NIM alanine/ESR dosimetric system to therapy level

The NIM alanine/ESR dosimetric system, which was designed for industrial radiation processing, has been improved to be suited for applications in therapy dose range. Two different procedures of dose intercomparisons between IAEA and NIM were carried out with the improved system in the range of 2.5 to 100 Gy. In the first procedure, a set of NIM alanine dosimeters were irradiated at IAEA dosimetric laboratory and part of dosimeters marked “for evaluated” were evaluated using the rest of those with “known dose” given by IAEA. Most of evaluated doses agreed with IAEA doses within 1%. In the second procedure, all above dosimeters were evaluated on the base of NIM dosimetry. The results indicated that the doses determined by NIM agreed with those given by IAEA within 3% on the average.     

Feasibility study of radiophotoluminescent glass rod dosimeter postal dose intercomparison for high energy photon beam

A radiophotoluminescent glass rod dosimeter (GRD) system has recently become commercially available. In this study we evaluated whether the GRD would be suitable for external dosimetric audit program in radiotherapy. For this purpose, we introduced a methodology of the absorbed dose determination with the GRD by establishing calibration coefficient and various correction factors (non-linearity dose response, fading, energy dependence and angular dependence). A feasibility test of the GRD postal dose intercomparison was also performed for eight high photon beams by considering four radiotherapy centers in Korea. In the accuracy evaluation of the GRD dosimetry established in this study, we obtained within 1.5% agreements with the ionization chamber dosimetry for the ⁶⁰Co beam. It was also observed that, in the feasibility study, all the relative deviations were smaller than 3%. Based on these results, we believe that the new GRD system has considerable potential to be used for a postal dose audit program.     

Five pediatric head and brain mathematical models for use in internal dosimetry.

Mathematical models of the head and brain currently used in pediatric neuroimaging dosimetry lack the anatomic detail needed to provide the necessary physical data for suborgan brain dosimetry. To overcome this limitation, the Medical Internal Radiation Dose (MIRD) Committee of the Society of Nuclear Medicine recently adopted a detailed dosimetric model of the head and brain for the adult. METHODS: New head and brain models have been developed for a newborn, 1, 5, 10 and 15 y old for use in internal dosimetry. These models are based on the MIRD adult head and brain model and on published head and brain dimensions. They contain the same eight brain subregions and the same head regions as the adult model. These new models were coupled with the Monte Carlo transport code EGS4, and absorbed fractions of energy were calculated for 14 sources of monoenergetic photons and electrons in the energy range of 10 keV-4 MeV. These absorbed fractions were then used along with radionuclide decay data to generate S values for all ages for 99mTc, considering 12 source and 15 target regions. RESULTS: Explicit transport of positrons was also considered with separation of the annihilation photons component to the absorbed fraction of energy in the calculation of S values for positron-emitting radionuclides. No statistically significant differences were found when S values were calculated for positron-emitting radionuclides under explicit consideration of the annihilation event compared with the traditional assumption of a uniform distribution of 0.511-MeV photons. CONCLUSION: The need for electron transport within the suborgan brain regions of these pediatric phantoms was reflected by the relatively fast decrease of the self-absorbed fraction within many of the brain subregions, with increasing particle energy. This series of five dosimetric head and brain models will allow more precise dosimetry of radiopharmaceuticals in pediatric nuclear medicine brain procedures.     

Comparison of sample preparation and signal evaluation methods for EPR analysis of tooth enamel.

In dose reconstruction by EPR dosimetry with teeth various methods are applied to prepare tooth enamel samples and to evaluate the dosimetric signal. A comparison of seven frequently used methods in EPR dosimetry with tooth enamel was performed. The participating Institutes have applied their own procedure to prepare tooth enamel samples and to evaluate the dosimetric signal. The precision of the EPR measurement and the dependence of the estimated dosimetric signal with irradiation up to 1000 mGy were compared. The obtained results are consistent among the different methods. The reproducibility of the dosimetric signal and its estimated relation with the absorbed dose was found to be very close for the applied methods with one possible exception.     

Dosimetric properties of the Theraview fluoroscopic electronic portal imaging device

Electronic portal imaging devices (EPIDs) can be used for non-imaging applications in radiotherapy such as patient dosimetry. Of the systems available, the fluoroscopic camera-based EPID Theraview (InfiMed Inc.) has not been studied to date, and a review of the dosimetric properties of the system is presented here. In the "single set-up" mode of image acquisition, pixel intensity increases sublinearly with applied dose. The response was dependent on the system's video signal gain and showed a threshold dose to the detector in the range 0.05-0.35 cGy, and pixel saturation at detector doses in the range 1.2-1.6 cGy. Repeated exposures of the EPID were observed to be extremely reproducible (standard deviation 0.5%). The sensitivity of the system showed a linear decline of 0.04% day-1 over a 68-day period, during which time the relative off-axis response within 10 x 10 cm2 field was constant to within a standard deviation of 0.56%. The system shows spatial non-uniformity, which requires correction for application to dose measurements in two-dimensions. Warm-up of the camera control unit required a period of at least 40 min and was associated with an enhancement in pixel intensity of up to 12%. A radiation dose history effect was observed at doses as low as 0.2 Gy. Camera dark current was shown to be negligible at normal accelerator operation. No discernible image distortion was found. Mechanical stability on gantry rotation was also assessed and image displacement of up to 5 mm at the isocentre was observed. It was concluded that the device could be used for dosimetry provided necessary precautions were observed and corrections made.     

Influence of Pro-Qura–generated Plans on Postimplant Dosimetric Quality: A Review of a Multi-Institutional Database

The influence of Pro-Qura–generated plans vs. community-generated plans on postprostate brachytherapy dosimetric quality was compared. In the Pro-Qura database, 2933 postplans were evaluated from 57 institutions. A total of 1803 plans were generated by Pro-Qura and 1130 by community institutions. Iodine-125 (¹²⁵I) plans outnumbered Palladium 103 (¹⁰³Pd) plans by a ratio of 3:1. Postimplant dosimetry was performed in a standardized fashion by overlapping the preimplant ultrasound and the postimplant computed tomography (CT). In this analysis, adequacy was defined as a V₁₀₀ > 80% and a D₉₀ of 90% to 140% for both isotopes along with a V₁₅₀ < 60% for ¹²⁵I and < 75% for ¹⁰³Pd. The mean postimplant V₁₀₀ and D₉₀ were 88.6% and 101.6% vs. 89.3% and 102.3% for Pro-Qura and community plans, respectively. When analyzed in terms of the first 8 sequence groups (10 patients/sequence group) for each institution, Pro-Qura planning resulted in less postimplant variability for V₁₀₀ (86.2–89.5%) and for D₉₀ (97.4–103.2%) while community-generated plans had greater V₁₀₀ (85.3–91.2%) and D₉₀ (95.9–105.2%) ranges. In terms of sequence groups, postimplant dosimetry was deemed “too cool” in 11% to 30% of cases and “too hot” in 12% to 27%. On average, no clinically significant postimplant dosimetric differences were discerned between Pro-Qura and community-based planning. However, substantially greater variability was identified in the community-based plan cohort. It is possible that the Pro-Qura plan and/or the routine postimplant dosimetric evaluation may have influenced dosimetric outcomes at community-based centers.     

A phantom for dosimetric characterization of small radiation fields: design and use

An acrylic phantom was designed and constructed for the acquisition and verification of basic dosimetric data of narrow fields in stereotactic radiotherapy/radiosurgery (SRT/SRS) using thermoluminescent (TL) dosimetry. An array of holes to accommodate up to 426 dosimeters was used to allow the assessment of dose distribution in circular fields with a 1-mm spatial dose resolution with minimal field perturbation. It was found experimentally that there must be a minimum gap of 1 mm between neighboring dosimeters in 6-MV photon fields. Most of the dosimetric characteristics of a 6-MV x-ray SRS/SRT unit assessed using TL dosimetry and ion chamber dosimetry were in good agreement when the longitudinal axis of the chamber was parallel to the central beam axis. TL dosimetry showed that the penumbra width increased with increasing collimator aperture. The low cost of the phantom and the widespread use and familiarity of TL dosimetry in radiotherapy departments offer a significant advantage in the use of the proposed methodology.