Thyroid hormone replacement for subclinical hypothyroidism.

BACKGROUND: Subclinical hypothyroidism is defined as an elevated serum thyroid-stimulating hormone (TSH) level with normal free thyroid hormones values. The prevalence of subclinical hypothyroidism is 4% to 8% in the general population, and up to 15% to 18% in women who are over 60 years of age. There is considerable controversy regarding the morbidity, the clinical significance of subclinical hypothyroidism and if these patients should be treated. OBJECTIVES: To assess the effects of thyroid hormone replacement for subclinical hypothyroidism. SEARCH STRATEGY: We searched The Cochrane Library, MEDLINE, EMBASE and LILACS. Ongoing trials databases, reference lists and abstracts of congresses were scrutinized as well. SELECTION CRITERIA: All studies had to be randomised controlled trials comparing thyroid hormone replacement with placebo or no treatment in adults with subclinical hypothyroidism. Minimum duration of follow-up was one month. DATA COLLECTION AND ANALYSIS: Two authors independently assessed trial quality and extracted data. We contacted study authors for missing or additional information. MAIN RESULTS: Twelve trials of six to 14 months duration involving 350 people were included. Eleven trials investigated levothyroxine replacement with placebo, one study compared levothyroxine replacement with no treatment. We did not identify any trial that assessed (cardiovascular) mortality or morbidity. Seven studies evaluated symptoms, mood and quality of life with no statistically significant improvement. One study showed a statistically significant improvement in cognitive function. Six studies assessed serum lipids, there was a trend for reduction in some parameters following levothyroxine replacement. Some echocardiographic parameters improved after levothyroxine replacement therapy, like myocardial relaxation, as indicated by a significant prolongation of the isovolumic relaxation time as well as diastolic dysfunction. Only four studies reported adverse events with no statistically significant differences between groups. AUTHORS' CONCLUSIONS: In current RCTs, levothyroxine replacement therapy for subclinical hypothyroidism did not result in improved survival or decreased cardiovascular morbidity. Data on health-related quality of life and symptoms did not demonstrate significant differences between intervention groups. Some evidence indicates that levothyroxine replacement improves some parameters of lipid profiles and left ventricular function.

Cytogenetic profile of 109 lipomas.

Cytogenetic analysis of short-term cultures was carried out on 109 lipomas from 92 patients. Clonal chromosomal abnormalities were present in 50% of the tumors analyzed. Based on the results, three main cytogenetic groups were identified and included: (a) tumors with normal karyotypes, (b) tumors with abnormalities involving region q13-15 on chromosome 12, and (c) tumors with other clonal aberrations. Within each of these groups, cytogenetic subgroups could be identified, each characterized by a specific anomaly. Tumors with abnormalities of 12q included specific subgroups with t/ins(1;12)(p32-33;q13-15), t(2;12)(p21-22;q13-14), t(3;12)(q28;q14), t(12;21)(q13;q21), complex, and nonrecurrent aberrations. The group containing heterogeneous clonal aberrations included subgroups with del(13)(q12q22), der(6)(p21-23), der(11)(q13), and nonspecific aberrations. Chromosome bands 1p36, 1p32-33, 2p21-22, 3q27-28, 6p21-23, 11q13, 12q13-15, 13q12, 13q22, 17p13, 17q21, and 21q21-22 were preferentially involved in structural rearrangements in lipomas. The identification of these sites of nonrandom rearrangements may serve to identify genes (at or near the junctions of chromosomal aberrations) involved in normal cellular growth control. Statistical analysis of the data revealed a correlation among karyotypic abnormalities and clinical data, such as age and sex of the patient, and tumor depth, site, and size.

A novel small molecule that directly sensitizes the insulin receptor in vitro and in vivo.

Insulin resistance, an important feature of type 2 diabetes, is manifested as attenuated insulin receptor (IR) signaling in response to insulin binding. A drug that promotes the initiation of IR signaling by enhancing IR autophosphorylation should, therefore, be useful for treating type 2 diabetes. This report describes the effect of a small molecule IR sensitizer, TLK16998, on IR signaling. This compound activated the tyrosine kinase domain of the IR beta-subunit at concentrations of 1 micromol/l or less but had no effect on insulin binding to the IR alpha-subunit even at much higher concentrations. TLK16998 alone had no effect on IR signaling in mouse 3T3-L1 adipocytes but, at concentrations as low as 3.2 micromol/l, enhanced the effects of insulin on the phosphorylation of the IR beta-subunit and IR substrate 1, and on the amount of phosphatidylinositol 3-kinase that coimmunoprecipitated with IRS-1. Phosphopeptide mapping revealed that the effect of TLK16998 on the IR was associated with increased tyrosine phosphorylation of the activation loop of the beta-subunit tyrosine kinase domain. TLK16998 also increased the potency of insulin in stimulating 2-deoxy-D-glucose uptake in 3T3-L1 adipocytes, with a detectable effect at 8 micromol/l and a 10-fold increase at 40 micromol/l. In contrast, only small effects were observed on IGF-1-stimulated 2-deoxy-D-glucose uptake. In diabetic mice, TLK16998, at a dose of 10 mg/kg, lowered blood glucose levels for up to 6 h. These results suggest, therefore, that small nonpeptide molecules that directly sensitize the IR may be useful for treating type 2 diabetes.

The structures of human glutathione transferase P1-1 in complex with glutathione and various inhibitors at high resolution.

The human pi-class glutathione S-transferase (hGST P1-1) is a target for structure-based inhibitor design with the aim of developing drugs that could be used as adjuvants in chemotherapeutic treatment. Here we present seven crystal structures of the enzyme in complex with substrate (glutathione) and two inhibitors (S-hexyl glutathione and gamma-glutamyl- (S-benzyl)cysteinyl-D-phenylglycine). The binding of the modified glutathione inhibitor, gamma-glutamyl-(S-benzyl)cysteinyl-D-phenylglycine, has been characterized with the phenyl group stacking against the benzyl moiety of the inhibitor and making interactions with the active-site residues Phe8 and Trp38. The structure provides an explanation as to why this compound inhibits the pi-class GST much better than the other GST classes. The structure of the enzyme in complex with glutathione has been determined to high resolution (1.9 to 2.2 A) in three different crystal forms and at two different temperatures (100 and 288 K). In one crystal form, the direct hydrogen-bonding interaction between the hydroxyl group of Tyr7, a residue involved in catalysis, and the thiol group of the substrate, glutathione, is broken and replaced by a water molecule that mediates the interaction. The hydrogen-bonding partner of the hydroxyl group of Tyr108, another residue implicated in the catalysis, is space-group dependent. A high-resolution (2.0 A) structure of the enzyme in complex with S-hexyl glutathione in a new crystal form is presented. The enzyme-inhibitor complexes show that the binding of ligand into the electrophilic binding site does not lead to any conformational changes of the protein.

Predicting ligand binding to proteins by affinity fingerprinting.

BACKGROUND: There are many ways to represent a molecule's properties, including atomic-connectivity drawings, NMR spectra, and molecular orbital models. Prior methods for predicting the biological activity of compounds have largely depended on these physical representations. Measuring a compound's binding potency against a small reference panel of diverse proteins defines a very different representation of the molecule, which we call an affinity fingerprint. Statistical analysis of such fingerprints provides new insights into aspects of binding interactions that are shared among a wide variety of proteins. These analyses facilitate prediction of the binding properties of these compounds assayed against new proteins. RESULTS: Affinity fingerprints are reported for 122 structurally-diverse compounds using a reference panel of eight proteins that collectively are able to generate unique fingerprints for about 75% of the small organic compounds tested. Application of multivariate regression techniques to this database enables the creation of computational surrogates to represent new proteins that are surprisingly effective at predicting binding potencies. We illustrate this for two enzymes with no previously recognizable similarity to each other or to any of the reference proteins. Fitting of analogous computational surrogates to four other proteins confirms the generality of the method; when applied to a fingerprinted library of 5000 compounds, several sub-micromolar hits were correctly predicted. CONCLUSIONS: An affinity fingerprint database, which provides a rich source of data defining operational similarities among proteins, can be used to test theories of cryptic homology unexpected from current understanding of protein structure. Practical applications to drug design include efficient pre-screening of large numbers of compounds against target proteins using fingerprint similarities, supplemented by a small number of empirical measurements, to select promising compounds for further study.

Glubodies: randomized libraries of glutathione transferase enzymes.

BACKGROUND: The immunoglobulin framework has been mutagenized to engineer recombinant libraries of proteins as potential diagnostics and novel catalysts, although the often shallow binding cleft may limit the utility of this framework for binding diverse small organic molecules. By contrast, the glutathione S-transferase (GST) family of enzymes contains a deep binding cleft, which has evolved to accommodate a broad range of hydrophobic xenobiotics. We set out to determine whether GST molecules with novel ligand-binding characteristics could be produced by random mutagenesis of segments of the binding cleft. RESULTS: We have identified two ligand-recognition segments (LRSs) in human GST P1, which are near the active site in the folded protein, but have characteristics indicating that the integrity of their sequence is not essential for the overall structure or activity of the protein. Libraries of GST P1-derived proteins were produced by substituting randomized sequences for an LRS or inserting random sequences into an LRS. The recombinant proteins in the libraries, collectively designated as 'glubodies,' generally retain enzymatic activity but differ markedly both from each other and from the parent enzyme in sensitivity to inhibition by diverse small organic compounds. In some instances, a glubody is inhibited by completely novel structures. CONCLUSIONS: We have shown that a non-antibody framework can be used to create large libraries of proteins with a wide range of binding specificities for small organic molecules. The glubodies provide a rich source of data for correlating the structural and functional features of proteins relevant to ligand binding. The criteria applied for identifying an LRS in GST P1 are generally applicable to other protein frameworks.

Gastrointestinal cytomegalovirus infection in heart and heart-lung transplant recipients.

Cytomegalovirus (CMV) causes major morbidity in organ transplant recipients. Gastrointestinal disease was the most prominent manifestation of CMV infection in a population of heart and heart-lung transplant patients, with an incidence of 9.9%, compared with pneumonitis (4.0%) and retinitis (0%), and occurred most frequently in CMV-seronegative recipients of organs from CMV-seropositive donors. Clinical manifestations included gastritis (nine patients), gastric ulceration (four patients), duodenitis (three patients), esophagitis (one patient), pyloric perforation (one patient), and colonic hemorrhage (one patient). Patients with gastrointestinal CMV infection were treated with intravenous ganciclovir sodium therapy, 5 mg/kg twice daily, for 2 to 8 weeks, with positive clinical, endoscopic, histologic, and virologic responses. Relapses occurred in four of nine patients who were followed up for a median period of 18 months. Retreatment resulted in healing of endoscopic lesions and in viral clearing. We conclude that early endoscopic evaluation for CMV is indicated in heart and heart-lung transplant patients with gastrointestinal symptoms. This study further suggests that intravenous ganciclovir therapy is effective for the treatment of gastrointestinal CMV in these patients.

Amino acid preferences at protein binding sites.

An analysis of the amino acid distribution at protein binding sites was carried out using 50 diverse macromolecules for which crystallographic data with a bound ligand are available. The purpose of this study is to determine whether differential trends in amino acid distributions exist at binding sites compared to other regions in the proteins. The results indicate that some residues, particularly Arg, His, Trp and Tyr are substantially more frequent at the binding sites, compared to the number of times these residues are present in proteins generally. These effects go beyond the differences seen comparing surface exposed residues to bulk protein. The resemblance in the residue utilization at the binding sites of unrelated proteins restricts the possible types of interactions with ligands, possibly accounting for the repetition of substructural motifs in chemicals with diverse pharmacological action. Further, the use of these diagnostic features may permit identification of ligand binding pockets in a protein structure deduced from sequence information or from data in the absence of a ligand. Some of these findings complement and extend previously described trends for antibody binding sites.

Molecular electrostatic potential of D1 and D2 dopamine agonists.

The molecular electrostatic potential (MEP) of four selective D1, four nonselective D1/D2, and three selective D2 agonists has been calculated in a three-dimensional grid surrounding the molecules. The local density functional program DMol was used to evaluate the MEP. A comparison of the MEPs of all compounds revealed that while the electrostatic effects may be important for the affinity in both D1- and D2-selective ligands, it only appears to be a subtle modulator of the selectivity. Slight differences were found in the negative regions in the vicinity of the catechol ring that can account for the D1 versus D2 selectivity in the compounds studied.

Toward the design of chemical libraries for mass screening biased against mutagenic compounds.

The ability to develop a chemical into a drug depends on multiple factors. Beyond potency and selectivity, ADME/PK and the toxicological profile of the compound play a significant role in its evaluation as a candidate for development. Those factors are being brought into bear earlier in the discovery process and even into the design of libraries for screening. The purpose of our study is the comparative analysis of simple physical characteristics of compounds that have been reported to be mutagens and nonmutagenic ones. The analysis of differences can lead to the development of knowledge-based biases in the libraries designed for massive screening. For each of four Salmonella strains, TA-98, TA-100, TA-1535, and TA-1537, an analysis of the statistical significance of the deviance of the averages for a number of global properties was carried out. The properties studied included parameters, such as topological indices, and bit strings representing the presence or absence of certain chemical moieties. The results suggest that mutagens display a larger number of hydrogen bond acceptor centers for most strains. Moreover, the use of bit strings points to the importance of certain molecular fragments, such a nitro groups, for the outcome of a mutagenicity study. Development of multivariate models based on global molecular properties or bit strings point to a small advantage of the latter for the prediction of mutagenicity. The benefits of the bit strings are in accord with the use of fragment-based approaches for the prediction of carcinogenicity and mutagenicity in methods described in the literature.

LASSOO: a generalized directed diversity approach to the design and enrichment of chemical libraries.

Pharmaceutical discovery relies on the screening of chemical libraries that are as diverse as possible yet constrained in favor of compounds possessing attributes that are normally associated with successful drug candidates. We describe a new algorithm for simultaneously addressing both objectives, providing an effective means to increase structural diversity in a chemical library while maintaining a bias toward compounds that retain the desirable properties of drugs. The LASSOO algorithm exploits differences in descriptor distributions to identify novel compounds that are most dissimilar to the members of an existing screening library and most similar to members of a target library with desirable characteristics. We illustrate the LASSOO technique using publicly available compound databases and bit string descriptors. The architecture of the algorithm is general enough to allow any set of descriptors or similarity measures to be employed, and it is easily adaptable to other means of directing diversity, such as the avoidance of toxicity and/or poor pharmacokinetic properties.

Design, synthesis, and evaluation of latent alkylating agents activated by glutathione S-transferase.

In search of compounds with improved specificity for targeting the important cancer-associated P1-1 glutathione S-transferase (GST) isozyme, new analogs 4 and 5 of the previously reported glutathione S-transferase (GST)-activated latent alkylating agent gamma-glutamyl-alpha-amino-beta-[[[2-[[bis[bis(2-chloroethyl)amino]ph osp horyl]oxy]ethyl]sulfonyl]propionyl]-(R)-(-)-phenylglycine (3) have been designed, synthesized, and evaluated. One of the diastereomers of 4 exhibited good selectivity for GST P1-1. The tetrabromo analog 5 of the tetrachloro compound 3 maintained its specificity and was found to be more readily activated by GSTs than 3. The GST activation concept was further broadened through design, synthesis, and evaluation of a novel latent urethane mustard 8 and its diethyl ester 9. Interestingly, 8 showed very good specificity for P1-1 GST. Cell culture studies were carried out on 4, 5, 8, and 9 using cell lines engineered to have varying levels of GST P1-1 isozyme. New analogs 4 and 5 exhibited increased toxicity to cell lines with overexpressed GST P1-1 isozyme. The urethane mustard 8 and its diethyl ester 9 were found to be not as toxic. However, they too exhibited more toxicity to a cell line engineered to have elevated P1-1 levels, which was in agreement with the observed in vitro specificity of 8 for P1-1 GST isozyme. Mechanistic studies on alkaline as well as enzyme-catalyzed decomposition of latent mustard 3 provided experimental proof for the hypothesis that 3 breaks down into an active phosphoramidate mustard and a reactive vinyl sulfone. The alkylating nature of the decomposition products was further demonstrated by trapping those transient species as relatively stable diethyldithiocarbamic acid adducts. These results substantially extend previous efforts to develop drugs targeting GST and provide a paradigm for development of other latent drugs.

Comparison of MB-check and Löwenstein-Jensen media for recovery of mycobacteria.

We have compared the rate of recovery of mycobacteria with the MB-Check culture system (liquid phase) and the Löwenstein-Jensen (LJ) medium in 2,907 clinical specimens obtained from 830 patients submitted for mycobacterial culture during 1-year period. Direct smear examination was carried out by auramine-rhodamine staining. All primary isolates from the culture media were confirmed by Ziehl-Neelsen staining and identified by acridinium-ester-labeled DNA probes specific for Mycobaterium tuberculosis complex. A total of 214 isolates were of the M. tuberculosis complex (88 patients) and 54 of "potentially pathogenic environmental mycobacteria" (45 patients). A total of 117 (54.7%) samples were smear-positive and the remaining 97 (45.3%) were smear-negative. There was a significant difference in the percentage of positive cultures obtained by the MB-Check method (99.1%) as compared with the LJ medium (73.8%) (P < 0.05). This difference, however, occurred almost exclusively at the expense of the 97 smear-negative samples (positive cultures 97.95% by the MB-Check method vs. 42.3% by the LJ culture, P < 0.05). The number of patients diagnosed of tuberculosis by the MB-Check was significantly higher as compared with LJ medium (88 [100%] vs. 77 [87.5%], P < 0.05). In 11 (12.5%) patients, the diagnosis was only established by the MB-Check system. In smear-positive samples, the mean (+/-SD) detection time for M. tuberculosis complex was 14.8 +/- 8 days with MB-Check and 19.9 +/- 7 days with LJ medium. The corresponding figures in smear-negative samples were 22.8 +/- 3 days and 27.8 +/- 6 days, respectively. DNA probes directly applied to MB-Check liquid medium showed a sensitivity of 98.8% and specificity of 100%. These results indicate that the MB-Check system is more efficient for the recovery of mycobacteria than LJ medium.

Rate of disappearance of circulating endogenous gastrin in dogs.

In five awake mongrel dogs, endogenous gastrin was released by continuous irrigation of the antrum with acetylcholine. After 60 minutes of antral perfusion, the entire vascular supply of the antrum was suddenly and totally occluded, and serial samples of peripheral blood were taken for measurement of gastrin. The rate of disappearance of endogenous gastrin was caluclated by standard linear regression analysis; the calculated half-life of endogenous gastrin was 8.62 minutes. Analysis of the data suggests that the disappearance rate of endogenous gastrin could be explained by two distinct half-lives: one of 2.8 minutes (which is similar to the half-time in dogs of both 14- and 17 -amino acid gastrin), and another of 15.4 minutes (which is similar to the half-time of 34 -amino acid gastrin). Physiologically released gastrin is a mixture of three or more molecular forms of gastrin and the half-life of 8.62 minutes probably represents the disappearance half-time of this mixture.

Mechanisms of satiety and gastric emptying after gastric partitioning and bypass.

The patterns of fundic and antral motility as well as changes in fundic pressure in response to balloon distention were evaluated in 26 patients before and after gastric partitioning (18 patients) or gastric bypass (8 patients). In addition, the rate of gastric emptying, as measured by the ingestion of 99mTc-tagged chicken liver, was determined after gastric partitioning (12 patients) and bypass (7 patients). A striking alteration in the motility of the fundus occurs after both gastric partitioning and bypass. Normal pressure waves of the fundus disappear with both procedures. Fullness after both operations is secondary to distension of the pouch rather than to an increase in pressure. Changes in antral motility, inhibition, and recovery seem to be neurally mediated. The gastric emptying rate was 6.9% +/0 2.9% per minute after gastric bypass and 1.8% +/- 0.4% per minute after partitioning. Delayed emptying could be an additional benefit of gastric partitioning.

Posttreatment laparotomy for Hodgkin's disease.

We examined and surgically restaged (using posttreatment laparotomy) 26 patients with stage III and IV Hodgkin's disease treated with combination chemotherapy and in apparent remission to determine the status of their clinical remission. Eleven patients had normal clinical restaging and surgical restaging. Fifteen patients had abnormal clinical restaging, mainly consisting of abnormal lymphangiograms or abdominal CT scans. Ten lymphangiograms were abnormal and could not exclude persistent lymphoma. Two of the 15 patients proved to have Hodgkin's disease involving the para-aortic nodes and the spleen. With a median follow-up of 24 months, two patients had relapses in supradiaphragmatic sites and no patient with a negative laparotomy had a recurrence abdominal sites. Restaging laparotomy in selected patients with Hodgkin's disease with abnormal lymphangiograms or CT scans may identify additional patients with residual lymphoma who require further therapy and, more importantly, may identify those patients who have no residual disease and, therefore, may be spared additional therapy. Mortality and morbidity were nil.

Evidence for more than two central benzodiazepine receptors in rat spinal cord.

Competitive binding assays were performed in rat spinal cord membranes at 0 degrees C against [3H]Ro15-1788. The displacement curves of Ro15-1788 and alpidem produced pseudo-Hill slopes of 1.0 and 0.5, respectively. In addition, 5-10% of the specifically bound [3H]Ro15-1788 could not be displaced by greater than 100 microM of alpidem. These two pieces of evidence strongly suggest the presence of at least three central benzodiazepine binding sites in the spinal cord.

Thermodynamic analysis of binding to the cerebellar type I benzodiazepine receptor.

The temperature dependence of binding to the type I benzodiazepine receptor in rat cerebellum was determined using [3H]Ro15-1788 and regression analysis techniques. The ligands chosen were from diverse chemical families and display different pharmacological properties. Included were the agonists flunitrazepam, CL 218,872, zolpidem and alpidem; the antagonists Ro15-1788 and propyl-beta-carboline-3-carboxylate (beta-CCP); the inverse agonists ethyl-beta-carboline-3-carboxylate (beta-CCE) and methyl-6,7-dimethoxy-beta-carboline-3-carboxylate (DMCM); and the selective muscle relaxant AHR-11797. Assays were performed at 0 degrees C, 20 degrees C and 37 degrees C and the K(i) at each temperature was used to construct a van't Hoff plot for each compound. The binding of all ligands, with the exception of DMCM, was enthalpy-driven. However, enthalpy alone does not determine the rank order of affinity. There was no relationship between the thermodynamic behavior of binding and the observation of agonism, antagonism or inverse agonism, indicating that activation and recognition are distinct steps in this receptor system.

Influence of local treatment on the recurrence rate of ductal carcinoma in situ.

BACKGROUND: Screening mammography has resulted in a significant increase in the diagnosis of ductal carcinoma in situ (DCIS). The role of breast conservation therapy and the long-term recurrence rate are still controversial. This article compares mastectomy, wide excision alone, and wide excision with radiation as treatments for DCIS. STUDY DESIGN: One hundred twenty-four cases of DCIS were retrospectively reviewed and were found to be pure DCIS by a senior pathologist. The mean age at diagnosis was 60 years (range, 33 to 81). Originally, 101 patients (81 percent) presented with calcification on mammogram, and 23 (19 percent) presented with a palpable mass. Histologic data showed that 54 (44 percent) had noncomedo type lesions, 46 (37 percent) had comedo type, and 24 (19 percent) had unknown type DCIS. RESULTS: Four of the 124 patients had a recurrence during a mean follow-up period of 43 months. Recurrence is defined as any development of DCIS or invasive carcinoma in the ipsilateral breast. There was one (1.3 percent) recurrence in the 75 patients treated with mastectomy (an adenocarcinoma of the chest wall), which occurred at 59 months. Treatment was 5,750 cGy to the chest wall and the patient is free of disease 37 months postradiation. There were three (11 percent) recurrences at 14, 21, and 29 months, respectively, in the 28 patients treated with wide excision alone. All three recurrences were found by calcifications on mammogram and all patients had comedo type original lesions. Two recurrences were pure DCIS of the breast. Both patients were treated with mastectomy and are free of disease at 33 and five months, respectively. The third recurrence was an invasive colloid carcinoma of the breast. Treatment was a modified radical mastectomy; the patient is free of disease after 62 months. There were no recurrences in the 21 patients who were treated with wide excision and radiation. Average total dose of radiation was 5,835 cGy (range, 4,500 to 6,480). CONCLUSIONS: The results of this study indicate that both mastectomy and wide excision with radiation are associated with very low recurrence rates. Wide excision alone is associated with a higher recurrence rate. However, all recurrences were detected mammographically and all lesions were salvaged by mastectomy. Therefore, the ultimate local control and survival rates were similar for all three modalities.

Influence of field size on a PTW type 23342 plane-parallel ionization chamber's response.

The response of a PTW type 23342 plane-parallel ionization chamber, both in air and in phantom, was evaluated for x-ray tube potentials between 30 and 100 kV and radiation field diameters ranging from 30 to 70 mm. The experiments were performed with a calibrated Pantak x-ray machine and made use of the same set of x-ray qualities adopted by the PTB primary laboratory for the calibration of such chambers. A Plexiglas phantom (1.18 g cm(-3)) 110 mm long, 110 mm wide, and 80 mm deep was used for phantom measurements. X-ray qualities were characterized by using 99.99% pure aluminum filters. On the basis of the IAEA's TRS 398, the article discusses the dependence of the plane-parallel ionization chamber readings with field size in air and in phantom, its implication with regard to clinical dosimetry, cross-calibration, and dissemination of calibration factors.