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T Kron

Publications and source records attributed to T Kron.

At least 19 recordsLinked to original sources
PubMed2003

High dose-rate brachytherapy source localization: positional resolution using a diamond detector.

A potential real-time source position verification process for high dose-rate (HDR) brachytherapy treatment is described. This process is intended to provide immediate confirmation that a treatment is proceeding according to plan, so that corrective action can be taken if necessary. We show that three dosimeters are in principle sufficient and demonstrate the feasibility of the process using a diamond detector and an Ir-192 source. An error analysis including all identified sources of error shows that this detector is capable of locating the distance to the source to within 2 mm for distances up to 12 cm. This positional accuracy is less than the diameter of typical HDR catheters indicating that a diamond detector can be used to accurately determine the distance to the source. The uncertainty in the distance is found to increase with distance.

Algorithms↗
PubMed2003

Optimization of helical tomotherapy treatment plans for prostate cancer.

Helical tomotherapy (HT) is a novel treatment approach where the ring gantry irradiation geometry of a helical CT scanner is combined with an intensity-modulated megavoltage x-ray fan beam. An inverse treatment planning system (TomoTherapy Inc., Madison) was used to optimize the treatment plans for ten randomly selected prostate patients. Five different sets of margins (2, 5, 7.5 and 10 mm uniform 3D margins and a non-uniform margin of 5 to 10 mm) were employed for the prostate (GTV2) and seminal vesicles (GTV1). The dose distribution was evaluated in targets, rectum, bladder and femoral heads. HT plans are characterized by a rapid dose fall off around the target in all directions resulting in low doses (less than 30% of the dose at ICRU reference point) to the femurs in all cases. Up to a margin of 5 mm for target structures, it was always possible to satisfy the requirements for dose delivery set by RTOG protocol P-0126. Using a 'class solution', HT plans require minimal operator interaction and result in excellent sparing of normal structures in prostate radiotherapy.

Dose-Response Relationship, Radiation↗
PubMed2003

Investigation of dose reduction in neonatal radiography using specially designed phantoms and LiF:Mg,Cu,P TLDs.

Many departments still do not use recommended radiographic parameters to X-ray neonates. Direct, accurate dose measurements of individual examinations may assist a department in justifying technique modifications that provide a substantial dose reduction without a significant loss of image quality. The aim of this study was to investigate dose reduction techniques for neonates in the intensive care unit. Alterations in beam energy (kVp and filtration) and collimation were investigated using specially designed phantoms mimicking a 700 g and 2000 g neonate, and ultrasensitive LiF:Mg,Cu,P thermoluminescence dosimeters (TLDs). Differences in entrance surface dose (ESD) and dose at depth (3 cm or 5 cm) were compared for two, overlapping fields centred individually on the chest and abdomen (Technique 1) and one large chest-abdomen field (Technique 2 or babygram). The large phantom was irradiated at 54 kVp, 60 kVp and 70 kVp without additional filtration and at 66 kVp and 70 kVp with a rare-earth hafnium filter. Focus-film distance (FFD) and mAs were adjusted to maintain optical density (OD) on each radiograph. The baseline dose at 54 kVp and 100 cm FFD was (46+/-2) micro Gy. Increasing the tube potential from 54 kVp to 70 kVp without additional filtration reduced the ESD by 27%. However, the addition of a 0.05 mm hafnium filter at 66 kVp further reduced the radiation dose by 13%, to produce an ESD of (28+/-2) micro Gy. All contrast details were observable at 66 kVp with hafnium filtration. Technique 1 may lead to an increase in effective dose due to field overlap, which diverges at depth, and increased scatter at the periphery of the fields.

Critical Care↗
PubMed2002

LiF:Mg,Cu,P 'pin worms': miniature detectors for brachytherapy dosimetry.

Dose measurements in brachytherapy 192Ir implants are often difficult due to large dose gradients and complex photon spectra. Therefore, tissue-equivalent detectors with a high spatial resolution, such as the highly promising LiF:Mg,Cu,P thermoluminescent detectors (TLDs) are required. It was the aim of the present work to ascertain if miniature LiF:Mg,Cu,P TLDs can effectively measure the dose distribution around 192Ir implants. 'Pin worm' TLDs (type MCP, diameter 0.6 mm, length 2 mm) were compared with GR-200R (SSDL, Beijing) rods cut in half. The TLDs were tested for reproducibility and energy dependence using high dose rate (HDR) and low dose rate (LDR) brachytherapy units. 192Ir measurements were performed in a tissue equivalent phantom accommodating hollow needles and catheters routinely used in brachytherapy. Pin worms had an average reproducibility of less than +/-2% (1 SD) and a detection limit of less than 10 microGy. The small dimensions of the pin worms allowed their placement within brachytherapy needles and catheters. The measured relative dose distribution was in good agreement with the predictions of a computerised treatment planning system (ADAC Pinnacle); however, limitations in the TLD energy correction did not allow for absolute dose comparison.

Brachytherapy↗
PubMed2002

Measurements in radiotherapy beams using on-line MOSFET detectors.

When acquiring data to characterise radiation beams for radiotherapy treatment planning measurements in steep dose gradients such as beam penumbra or dose build-up are often required. A metal oxide semiconductor field effect transistor (MOSFET) with its inherent high spatial resolution was used for penumbra measurements in a 120 kVp X ray beam. The customised MOSFET system features a pulsed readout that allows the acquisition of data points in user defined time intervals of less than 1 s. Using a modified scanning beam data acquisition system the penumbra was acquired on-line in 0.1 mm steps. Measurements were made at different distances behind the beam collimator. From the extrapolation of the penumbra width to a location directly under the block the spatial resolution of the MOSFET system can be estimated to be better than 0.1 mm. This excellent spatial resolution has many potential applications in radiotherapy dosimetry, including the characterisation of multileaf collimator systems.

Equipment Design↗
PubMed2001

Extinction of the weakest.

PURPOSE: To examine whether changes in the effective doubling time of tumor cells during irradiation of head-and-neck cancer are linked to accumulating dose. METHODS AND MATERIALS: Optimal fitting of the results of four apparently iso-effective regimens in three recently reported randomized controlled trials (continuous hyperfractionated accelerated radiation therapy [CHART], Radiation Therapy Oncology Group [RTOG] 90-03, and Trans-Tasman Radiation Oncology Group [TROG] 91.01) was attempted using two different types of model of the change in effective doubling time that may occur during treatment. The first involved the traditional approach where doubling time changes at specific times after the start of treatment regardless of fractionation used (''fixed response time'' models). The second is where changes in doubling time are linked to accumulating biologic dose (''cellular depletion'' models). RESULTS: Changes in effective doubling time occur during radiotherapy for head-and-neck cancer. Data from the three trials can be fitted successfully by functions that imply a continuous reduction in effective doubling time. Models linking the reductions in effective doubling time to the cellular depletion that occurs in the tumor during radiotherapy fit the data satisfactorily. Effective doubling time ultimately reduces to 2 days, or slightly less, during high-dose radiotherapy regimens designed to cure squamous head-and-neck cancer. CONCLUSIONS: If the assumption of iso-effectivity is justified then this study indicates that the ''repopulation phenomenon'' may be best described by a function that depicts a reduction in effective tumor cell doubling time that decreases continuously during treatment down to a certain minimum value. Furthermore, this reduction may be linked to cellular depletion.

Cell Division↗
PubMed2001

A clinical comparison of different film systems for radiotherapy portal imaging.

Portal films are an important tool for verification of the shaping and positioning of external radiation fields to the target volume in radiotherapy. One limitation of port films is their inherent lack of contrast, which is due to the low attenuation of the exposing megavoltage radiation by the tissues being imaged. Recently, Kodak introduced a new portal film-cassette system, Kodak EC-L, with much improved contrast compared with conventional film. The aim of this study was to determine if the enhanced contrast of the Kodak EC-L system actually provides better clinical results. To simulate clinical use, port films were taken using an anthropomorphic phantom that was artificially shifted and/or rotated by a predetermined distance. Identical images were taken using a conventional port film system (AGFA-Gevaert Curix MR4 in lead-lined cassette) and the Kodak EC-L system. Twelve different operators (6 physicists and 6 radiation therapists) were asked to diagnose the problem from a total of 20 port films (10 per treatment site), allowing for direct comparison of the 2 types of films. While the diagnosis of the field displacement improved using the Kodak film, it did not speed-up the decision-making process. It was also found that experienced operators were more accurate at evaluating the films. The results indicate that, for the situations studied (head and neck, pelvis), the Kodak system exhibits better contrast and leads to improved decision making.

Humans↗
PubMed2001

Dose resolution in radiotherapy polymer gel dosimetry: effect of echo spacing in MRI pulse sequence.

In polymer gel dosimetry using magnetic resonance imaging, the uncertainty in absorbed dose is dependent on the experimental determination of T2. The concept of dose resolution (Dpdelta) of polymer gel dosimeters is developed and applied to the uncertainty in dose related to the uncertainty in T2 from a range of T4 encountered in polymer gel dosimetry. Dpdelta is defined as the minimal separation between two absorbed doses such that they may be distinguished with a given level of confidence, p. The minimum detectable dose (MDD) is Dpdelta as the dose approaches zero. Dpdelta and the minimum detectable dose both give a quantifiable indication of the likely practical limitations and usefulness of the dosimeter. Dpdelta of a polyacrylamide polymer gel dosimeter is presented for customized 32-echo and standard multiple-spin-echo sequences on a clinical MRI scanner. In evaluating uncertainties in T2, a parameter of particular significance in the pulse sequence is the echo spacing (ES). For optimal results, ES should be selected to minimize Dpdelta over a range of doses of interest in polymer gel dosimetry.

Biophysical Phenomena↗
PubMed2001

Participant supervision in co-therapy.

The present paper describes a model for supervision through co-therapy, developed in a training framework for interns of clinical psychology. The format presented includes a senior therapist and an intern as co-therapists. The model is conceptualized as one of participant supervision in psychodynamic psychotherapy. As such, the model is comprised of two elements. 1. Two therapists work together with one or more patient during a session. 2. The definition of the process as a supervisory situation, beyond the co-work in therapy of the two therapists. We offer the model as a significant and unique supplementary experience for both supervisor and supervise. The supervisory process is based on the principle of "reflection in action," and the actual participation of both partners in the clinical work, facilitating their mutual growth.

Adolescent↗
PubMed2000

Treatment-time-dependence models of early and delayed radiation injury in rat small intestine.

BACKGROUND: The present study modeled data from a large series of experiments originally designed to investigate the influence of time, dose, and fractionation on early and late pathologic endpoints in rat small intestine after localized irradiation. The objective was to obtain satisfactory descriptions of the regenerative response to injury together with the possible relationships between early and late endpoints. METHODS: Two- and 26-week pathologic radiation injury data in groups of Sprague-Dawley rats irradiated with 27 different fractionation schedules were modeled using the incomplete repair (IR) version of the linear-quadratic model with or without various time correction models. The following time correction models were tested: (1) No time correction; (2) A simple exponential (SE) regenerative response beginning at an arbitrary time after starting treatment; and (3) A bi-exponential response with its commencement linked to accumulated cellular depletion and fraction size (the 'intelligent response model' [INTR]). Goodness of fit of the various models was assessed by correlating the predicted biological effective dose for each dose group with the observed radiation injury score. RESULTS: (1) The incomplete repair model without time correction did not provide a satisfactory description of either the 2- or 26-week data. (2) The models using SE time correction performed better, providing modest descriptions of the data. (3) The INTR model provided reasonable descriptions of both the 2- and 26-week data, confirming a treatment time dependence of both early and late pathological endpoints. (4) The most satisfactory descriptions of the data by the INTR model were obtained when the regenerative response was assumed to cease 2 weeks after irradiation rather than at the end of irradiation. A fraction-size-dependent delay of the regenerative response was also suggested in the best fitting models. (5) Late endpoints were associated with low-fractionation sensitivity and treatment-time dependence even in animal groups that exhibited minimal early mucosal reactions. CONCLUSION: Radiation injury scores in this rat small intestinal experimental model cannot be adequately described without time correction. 'Consequential' mechanisms contribute to the development of late effects, even in animals that do not develop severe early mucosal injuries. The initiation of the regenerative response is subject to a fraction-size-dependent mitotic delay and is linked to the level of accumulated cellular depletion. The response does not cease at the end of therapy but probably continues until maximal healing has taken place.

Animals↗
PubMed2000

Where is the light field edge: perception of different operators on different surfaces.

On most radiotherapy treatment units, a light field indicates on the patient's skin where the treatment field will irradiate the patient. It was the aim of the present study to investigate the perception of the light field edge by different operators on different surfaces under different lighting conditions. Ten radiation therapists and physicists were asked to mark the light field edge of an 8 x 10-cm2 radiation field from a linear accelerator on prepacked radiographic film. Each operator marked the field 4 times each with the room light turned on and dimmed as usual for patient setup. Two operators marked the field on 5 different surfaces (film envelope, brown solid water, clear plastic used for the manufacturing of immobilization shells, black rubber, and Orfit patient immobilization material). The interoperator reproducibility (+/- 0.39 mm, ISD) was larger than the intraoperator reproducibility (+/- 0.27 mm). The light field was judged consistently to be 0.6 mm smaller in the light room than under dimmed light conditions and the physicists judged the field to be approximately 0.4 mm smaller compared to the radiation therapists' judgement. Compared to the yellow film wrapping, the light field on solid water, black rubber, and the clear plastic were judged to be 0.6 mm smaller by both operators. The same observation was made using a slotted block tray, which also gave the worst reproducibility of perceived field edges. While these systematic errors are relatively small and difficult to correct for, it appears to be important to be at least aware of them, in particular if the light field is used to junction radiation fields with steep penumbras, as commonly done in megavoltage treatments of head & neck and breast cancer.

Humans↗
PubMed2000

Decision-making models in the analysis of portal films: a clinical pilot study.

Portal films continue to play an important role in the verification of radiotherapy treatment. There is still some discussion, however, as to what action should be taken after a port film has shown a radiation field deviation from the prescribed volume. It was the aim of the present pilot study to investigate the performance of three decision-making models ('Amsterdam', 'Quebec' and 'Newcastle') and an expert panel basing their decision on intuition rather than formal rules after portal film acquisition in a clinical setting. Portal films were acquired on every day during the first week of treatment for five head and neck and five prostate cancer patients (diagnostic phase). If required, the field position was modified according to our normal practice following the recommendation of the expert panel. In order to analyse the results of the models, however, additional port films were taken in the following 3 treatment weeks with the patient moved as required by the different models (intervention phase). The portal films were taken over 4 consecutive days, positioning the patient according to each of the different models on one day each. None of the models diagnosed a field misplacement in the head and neck patients, while the 'Amsterdam' and 'Quebec' models predicted a move in one prostate patient. The 'Newcastle' model, which is based on Hotelling's T2 statistic, proved to be more sensitive and diagnosed a systematic displacement for three prostate patients. The intervention phase confirmed the diagnosis of the model, even if the three portal films taken with the patient position adjusted as required by the model proved to be insufficient to demonstrate an improvement. The 'Newcastle' model does not rely on assumptions about the random movement of patients and requires five portal films before a decision can be reached. This approach lends itself well to incorporation into electronic portal imaging 'packages', where repeated image acquisitions present no logistical difficulty.

Decision Support Techniques↗
PubMed2000

Verification of the dose distribution for 192Ir mould treatments using radiochromic film and LiF:Mg,Cu,P TLDs.

Radioactive moulds are an effective way of treating skin lesions. The aim of the study was to verify the dose distribution in a low dose rate 192Ir hand mould treatment using radiochromic film and LiF:Mg,Cu,P TLDs. Measurements were compared with two computerised treatment planning systems--Theraplan VO5B and ADAC Pinnacle3 V4.0b. Radiochromic film measured doses that were typically 25-30% lower than the TLD and planning computer doses (which agreed within +/- 5%). However, radiochromic film provided a two-dimensional map, which is particularly useful for dose distributions that are difficult to predict. This was demonstrated in the effects of adding thumb shielding to the hand mould. TLD results provided only point dose verification. Dose rates to the inner surface of the thumb were reduced from 15-20 cGy/hr to 2-4 cGy/hr by using 7.5 mm lead shielding. This is consistent with three half value layer reductions. For unshielded treatments, TLD results agreed with the treatment planning computers all within +/- 13%, including an uncertainty of +/- 10% on the source strength certificate for the activity. Due to the detector's sensitivity, miniaturisation would be the next approach for further more accurate verification with LiF:Mg,Cu,P TLDs.

Brachytherapy↗
PubMed2000

The potential for systematic field placement errors when using asymmetric collimation for photon field junctions.

During radiotherapy of breast or head and neck cancer it is often necessary to junction photon radiation fields to cover primary disease as well as the draining lymphatics. One increasingly popular technique to achieve this is a mono-isocentric technique for all abutting fields employing independent jaws. While this technique has many advantages it was found in a study of quality assurance requirements that the mono-isocentric technique was prone to some systematic errors in field placement due to incorrect calibration of the collimator read-outs and the isocentre definition of linear accelerator. It is not only important that operators are aware of these potential problems but also that quality assurance activities specific to the use of asymmetric collimation are implemented.

Biophysical Phenomena↗
PubMed2000

Evaluation of a second set of lateral lasers for easier patient positioning in radiotherapy.

On modern linear accelerators patient set-up is often awkward due to the height of the isocentre. A second set of lateral lasers was mounted approximately 20 cm below the isocentre allowing patient positioning at an ergonomical height. A small randomised study involving 16 heavy patients treated for pelvic malignancies demonstrated that using the lower lasers for patient set-up is not only likely to reduce strain on staff but also speed up the set-up process.

Aged↗
PubMed1999

Selective in vivo dosimetry in radiotherapy using P-type semiconductor diodes: a reliable quality assurance procedure.

Since 1994, our center has conducted entrance dose measurements on selected patients receiving 6MV x-ray therapy by utilizing a commercial set of p-type semiconductor diodes. We report on three years results representing 386 patients having 1005 measurements and the usefulness of such a system in a radiotherapy department. The 386 patients represent approximately 20% of our total radical treatments. Minimal disruption to patient treatment was achieved. Measurements showed an average variation from expected dose of 0.5% +/- 2.2%. Specific treatment site groups were investigated. Our results show that in vivo dosimetry on a selected group of patients is an effective method of providing an independent verification of dose delivery accuracy.

Humans↗