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L T Tan

Publications and source records attributed to L T Tan.

33 records · Page 2Linked to original sources
PubMed1996

Case report: low dose rate and high dose rate intracavitary brachytherapy in a patient with carcinoma of the cervix.

The role of low dose rate (LDR) intracavitary radiotherapy in the management of carcinoma of the uterine cervix is well established. However, there are selected groups of patients in whom high dose rate (HDR) intracavitary radiotherapy may have particular advantages. A case report of a patient with carcinoma of the cervix who received both LDR and HDR intracavitary radiotherapy is presented. A comparison of the normal tissue doses produced by the two forms of brachytherapy is made. The choice of dose for the HDR treatment was calculated using standard linear quadratic isoeffect equations. The relative clinical merits of LDR and HDR treatments in gynaecological cancer are discussed.

Brachytherapy↗
PubMed1996

Treatment of carcinomas of the uterine cervix which remain bulky after initial external beam radiotherapy: a pilot study using integrated cytotoxic chemotherapy prior to brachytherapy.

The use of cytotoxic chemotherapy (CT) between external beam radiotherapy (EBRT) and intracavitary brachytherapy (BT) in patients with bulky carcinomas of the uterine cervix which regress poorly after initial EBRT has been evaluated in a pilot study. The aim of CT was to limit tumour clonogen repopulation while awaiting further tumour regression in order to improve the BT dose distribution. Between 1989 and 1992, 22 patients with FIGO Stage IIA, IIB and IIIB cervical carcinomas were given two to three cycles of cisplatin-based CT between EBRT and intracavitary BT. Patients were selected for CT if there was bulky residual tumour extending beyond the range of point "A" after completion of EBRT. The median survival of patients with Stage IIA/B and Stage IIIB disease was 24 months and 13 months, respectively. The 5 year actuarial survival rate for patients with Stage IIA/B disease was 42%. There were no long-term survivors among patients with Stage IIIB disease. Survival difference between Stage IIA/B patients and Stage IIIB patients was statistically significant (p < 0.04). 5 year actuarial pelvic control rates were 38% and 0% for Stage IIA/B and Stage IIIB patients, respectively. There were no serious late radiation complications in the entire study group. Bulky carcinomas of the cervix which respond poorly to initial EBRT have a particularly poor prognosis. For Stage IB-IIB patients with persistent bulky disease after EBRT, published reports suggest that a 5 year actuarial survival rate of around 40% can be obtained using higher doses of radiation alone, but the risk of serious late morbidity is considerable. The results of our study suggest that in Stage IIA/B carcinomas of the cervix which remain bulky after initial EBRT, the use of integrated cytotoxic chemotherapy prior to intracavitary BT can give similar 5 year survival rates but with minimal treatment related morbidity.

Adenocarcinoma↗
PubMed1996

The influence of air cavities on the optic chiasm dose during pituitary radiotherapy for acromegaly.

The treatment planning of acromegalic patients can be complicated by the presence of abnormally large frontal air sinuses which allow increased anterior beam transmission causing increased dose to the optic nerve and chiasm which, if excessive, can result in blindness. This potential problem has been investigated by computer treatment planning exercises which have allowed for a variable thickness (1-3 cm) of air cavity beneath the frontal field and for different weightings of the lateral opposed fields relative to the anterior field. The resultant overdosage can be greater than 5% of the intended dose. The increased biological effect from errors of this magnitude is sufficient to increase the risk of optic nerve damage since neural tissue is very sensitive to small increments in fraction size. Even when the reduced attenuation is allowed for in the computer plan, the thickness of the air sinuses varies across the field so that irradiation through the frontal enlarged frontal air sinuses causes inhomogeneous dose distributions in the target volume. Acromegalic patients should therefore be treated with a more superior beam which would avoid the air sinuses altogether.

Acromegaly↗
PubMed1996

Bladder dose estimation during intracavitary brachytherapy for carcinoma of the cervix using a single line source system.

The estimation of maximum bladder doses from orthogonal radiographs is unreliable when triple source systems are used for intracavitary brachytherapy (BT) for gynaecological cancers. For single line source systems, the estimation of maximum bladder doses from radiographs should be more reliable due to the radial symmetry of the isodose distribution. A pilot study has been carried out to compare the estimated maximum bladder doses from standard radiographs with data obtained from CT for single line source BT treatments. 12 patients undergoing treatment for carcinoma of the cervix were selected for CT assessment of their bladder doses. For each patient, the dose rates at the International Commission of Radiation Units and Measurements (ICRU) bladder reference point B1 and a second reference point B2 2.5 cm cranially were computed from orthogonal radiographs and were compared with the maximum bladder dose rate as determined by CT scanning. Dose rates were computed for two different source loading patterns: (1) a 6 cm line source with uniformly distributed linear activity along its length; (2) a 6 cm line source with increased activity in the central 2 cm segment. The mean ratio of the maximum CT bladder dose rate to the dose rate at the ICRU reference point B1 on orthogonal radiographs for the line source with uniform linear activity was 1.32 (range 0.62-2.43, SD = 0.54). When the dose rates at both reference points B1 and B2 were considered, the mean ratio of the maximum CT dose rate to the maximum dose rate from radiographs was only 1.05 (range 0.72-1.72, SD = 0.24). For the line source with increased activity in the central segment, the mean ratio of the maximum CT bladder dose rate to the dose rate at B1 was 1.38 (range 0.60-2.63, SD = 0.64). When both B1 and B2 were considered, the variation in the ratio of the maximum CT dose rate to the maximum dose rate from radiographs was considerably smaller (mean ratio = 1.07, range 0.69-1.76, SD = 0.26). For single line source systems, single point dose estimation using the ICRU reference point on orthogonal radiographs will underestimate the maximum bladder dose although the discrepancy is less than for triple source systems. If the ICRU reference point is used in conjunction with a second reference point 2.5 cm cranially, then underestimation of the maximum bladder dose is unlikely to occur as at least one of the points is likely to be a reasonable estimate of the maximum bladder dose.

Brachytherapy↗
PubMed1996

Early experience with surgically inaccessible wide-necked intracranial aneurysm embolised with Guglielmi Electrically Detachable coils and electrothrombosis.

We describe our early successful experience with Guglielmi Electrically Detachable Coils (GDC) and electrothrombosis in treating a surgically inaccessible wide-necked intracranial aneurysm in a Chinese patient. The precautions taken to prevent unnecessary complications while performing this technique was also described. It provides a serious alternative to other less ideal types of embolising agents and direct surgery in treating patients with these difficult-to-treat aneurysms.

Cerebral Angiography↗
PubMed1995

The achievement of isoeffective bronchial mucosal dose during endobronchial brachytherapy.

PURPOSE: The use of endobronchial brachytherapy in the treatment of lung cancer is increasing due to the more widespread availability of high dose rate afterloading equipment. The complications include small airway (segmental and small lobar bronchi) fibrosis, stenosis, and obstructive complications in addition to hemorrhage. A progressive reduction in the diameter of the bronchial lumen occurs at each division of the bronchial tree. If uniform dwell times along a bronchial catheter treatment length are used, this will result in higher doses being given to the bronchial mucosa in the distal part of the treatment volume where the brachytherapy source mucosa distances are smaller, and underdosage proximally, where the source mucosa distances are larger. METHODS AND MATERIALS: The known mathematical relationships of the sequential reductions in the diameter of the bronchial lumen have been incorporated into two methods of optimization, which have been compared to uniform dwell times along a treatment length from trachea to segmental bronchus. RESULTS: The resulting isodose plots are presented, and demonstrate the extent of the overdosage distally, and the underdosage proximally when using uniform dwell times, and the achievement of isoeffective mucosal doses when using differential dwell times. CONCLUSION: This refinement in brachytherapy technique offers the potential for reduced normal tissue complications and possibly improved tumor control by reducing overdosage and underdosage, respectively.

Brachytherapy↗
PubMed1995

National Patient Master Index in Singapore.

It has been the practice of hospitals and medical institutions to keep their patients' medical records. Access to such valuable information will be confined within the respective health institutions if there is no proper infrastructure to facilitate sharing of the information. On the other hand, it has been the wish of medical professionals, as well as patients who frequent hospitals and medical institutions that patients' medical records, especially essential medical conditions, are readily available, whenever required, regardless of the health institution by which the patient is being attended to. In response, the National Patient Master Index (NPMI) system was conceived and implemented by the Singapore Ministry of Health in 1994. The NPMI system aims at providing a patient database at national level that allows authorised users faster access to a patient's essential medical data thereby facilitating patient management. This paper describes the implementation of the NPMI system, its data elements, system functionality and access control to address data confidentiality and privacy.

Computer Security↗
PubMed1995

Derivation of the optimum dose per fraction from the linear quadratic model.

The linear quadratic equation for fractionated radiotherapy has already been adapted to include a time factor for tumour repopulation: loge cell kill (E) is given as a function of dose per fraction (d), number of fractions (n), overall treatment time (T) and the clonogen doubling time (Tp). By incorporating a normal tissue isoeffect and replacing the relationship between T and n by a function f, the equation for E can be rewritten as a more complex function of d. In this form, E and d are continuous variables so that the dose per fraction (d') required to produce maximum values of E for isoeffective late normal tissue effects can be found by differential calculus. The derived equation takes the form (beta kTp-alpha Tp)d2 + 1.386fd + 0.693fk = 0 and when solved for d provides a direct estimation of the optimum dose per fraction. Where normal tissue sparing is possible and the tumour dose z is related to the normal tissue dose d, the optimum dose per fraction z' can be found by solving the equation (beta kTp-alpha gTp)z2 + 1.386fgz + 0.693fk = 0 The results show that a critical minimum dose per fraction is required to counteract rapid tumour clonogen repopulation in both conventional and accelerated radiotherapy. The calculus method is reasonably accurate for larger fraction numbers, when clonogen doubling times are 3.5 days or longer and for conventional radiotherapy given 5 days per week. The model is even more accurate for accelerated hyperfractionated radiotherapy providing that there is complete repair between successive fractions. Where greater normal tissue sparing is possible, as with focal teletherapy methods and brachytherapy, higher tumour doses per fraction can be used to increase further the tumour cell kill without exceeding normal tissue tolerance. These predicted doses per fraction are consistent with clinical experience when the given constraints in terms of frequency of treatment are considered. The model described can be used for tumours in which repopulation occurs at a constant rate throughout treatment. For tumours in which accelerated repopulation occurs, the optimum dose per fraction can be separately calculated for the initial phase of slow repopulation (for which very small doses per fraction are optimal) and also for the second phase of rapid repopulation (for which either accelerated hyperfractionated treatments or hypofractionated focal methods of treatment would be appropriate). The limitations of the model are fully discussed including the need for accurate radiobiological predictive assays. In the future such assays of pre-treatment doubling times and tumour cell radiosensitivities could be used to determine reasonable ranges for the optimum dose per fraction in experimental tumours and subsequently in clinical trails. This approach could produce major improvements in the therapeutic potential of radiotherapy.

Cell Division↗
PubMed1994

A mathematical model of intraluminal and intracavitary brachytherapy.

The adaptation of the linear-quadratic model to allow for the effect of tumour regression and clonogen repopulation between initial teletherapy and subsequent brachytherapy has been extended to include the geometrical conditions encountered in intraluminal and intracavitary brachytherapy. For a radiation line source placed at the centre of a lumen or cavity, regression of any endoluminal tumour towards its mural origin will not result in any change in the minimum brachytherapy-tumour dose with time. In contrast, regression of transmural tumour will cause a potentially advantageous increase in the minimum brachytherapy-tumour dose with time. The latter effect will be opposed by tumour clonogen repopulation. The log(e) cell kill due to brachytherapy has been calculated for tumours of diameters 2, 4 and 6 cm at completion of teletherapy. The centres of the tumours were assumed to be at distances of 0, 1 and 2 cm from the radiation source. Tumour linear regression rates (lambda) ranging from 0.025 to 0.25 per week and tumour clonogen doubling times (Tp) of 2.5, 5 and 15 days were used in the calculations. The results demonstrate the critical importance of the distance of the tumour centre from the line source as well as the influence of tumour diameter, lambda and Tp. In some instances, both maximum and minimum values of log(e) cell kill occur. Calculations of tumour cure probabilities reveal that these variations in log(e) cell kill predicted by the model can produce highly significant differences in tumour control rates. Where the relevant parameters can be assessed directly or estimated from previous experience, the model provides a basis for the design of future intraluminal or intracavitary brachytherapy protocols.

Brachytherapy↗
PubMed1994

Results of a questionnaire regarding the practice of radiotherapy for carcinoma of the cervix in the UK.

A questionnaire was sent to 50 departments of clinical oncology in the UK in September 1991. The aim was to determine the range of external beam and brachytherapy techniques employed at that time in the radical treatment of carcinoma of the cervix. Replies were received from 35 centres. This paper summarizes the preliminary findings of the study. Low dose rate (LDR) brachytherapy techniques predominated (34/35 = 97%) but 41% of departments (13/32) had future plans for the use of high dose rate (HDR) equipment. For low bulk (Stage I-II) carcinoma of the cervix, there was no detectable association between the total brachytherapy dose prescribed and the brachytherapy dose rate. In bulky (Stage I-II) carcinoma of the cervix treated by initial open teletherapy (without shielding), there was a statistically significant reduction in the prescribed brachytherapy dose with increasing dose rate. There was considerable variation between centres in the measurement or estimation of normal tissue doses during brachytherapy. The range of techniques used and the variation in expected complication rates should be closely monitored via medical audit and a further follow-up questionnaire may reveal important changes.

Brachytherapy↗
PubMed1994

Non-uniform dwell times in line source high dose rate brachytherapy: physical and radiobiological considerations.

The ability to vary source dwell times in high dose rate (HDR) brachytherapy allows for the use of non-uniform dwell times along a line source. This may have advantages in the radical treatment of tumours depending on individual tumour geometry. This study investigates the potential improvements in local tumour control relative to adjacent normal tissue isoeffects when intratumour source dwell times are increased along the central portion of a line source (technique A) in radiotherapy schedules which include a relatively small component of HDR brachytherapy. Such a technique is predicted to increase the local control for tumours of diameters ranging between 2 cm and 4 cm by up to 11% compared with a technique in which there are uniform dwell times along the line source (technique B). There is no difference in the local control rates for the two techniques when used to treat smaller tumours. Normal tissue doses are also modified by the technique used. Technique A produces higher normal tissue doses at points perpendicular to the centre of the line source and lower doses at points nearer the ends of the line source if the prescription point is not in the central plane of the line source. Alternatively, if the dose is prescribed at a point in the central plane of the line source, the dose at all the normal tissue points are lower when technique A is used.

Brachytherapy↗
PubMed1990

Ministry of Health computerisation programme.

The purpose of the paper is to give the readers an idea of the state of computerisation in the Ministry of Health, Singapore and to highlight some of the benefits of computerisation. The Ministry employs a wide range of computer systems from portable microcomputers, point-of-sale microcomputers, supermicros, minicomputers to mainframe computer. What are the roles of these computers? Why and how are they interconnected? Carry on reading if these questions are appealing to you.

Computer Communication Networks↗
PubMed1981

Between-technician variation in the measurement of spirometry with air and helium.

A study using a two-period cross-over design was conducted to determine the between-technician variation in the measurement of variables derived from spirograms with subjects breathing room air and also breathing a mixture of 80% helium and 20% oxygen (He-O2). Twenty-four subjects were allocated systematically into three 2-period cross-over studies (blocks) of 8 subjects each (4 in each sequence in each block). There were 3 technicians in the study, and the subjects in each block were tested by a pair of technicians. The following variables derived from the tracings were analyzed statistically: forced expiratory volume in one second (FEV1), forced vital capacity (FVC) while breathing room air and also while breathing He-O2, maximal mid-expiratory flow rate (MMRF), FEV1/FVC %, and the per cent decrease in time between 60% VC and 40% VC for the spirograms comparing He-O2 with air (delta t). Analyses were performed to assess the presence of residual effects in the cross-over design, the between-technician difference in the administration of the tests, and in the interpretation of tracings. The results indicated that: (1) significant differences were observed between technicians in the administration of the tests to obtain FVC while the subjects breathed room air and while they breathed He-O2, (2) there were significant differences between the 3 technicians in the interpretation of MMFR, and (3) the between-technician variation in the administration of the tests with respect to delta t and with He-O2 in general were not significantly different from that of the other spirometric variables studied.

Adult↗