Dose to the contralateral breast: a comparison of two techniques using the enhanced dynamic wedge versus a standard wedge.

The dose to the contralateral breast has been associated with an increased risk of developing a second breast malignancy. Varying techniques have been devised and described in the literature to minimize this dose. Metal beam modifiers such as standard wedges are used to improve the dose distribution in the treated breast, but unfortunately introduce an increased scatter dose outside the treatment field, in particular to the contralateral breast. The enhanced dynamic wedge is a means of remote wedging created by independently moving one collimator jaw through the treatment field during dose delivery. This study is an analysis of differing doses to the contralateral breast using two common clinical set-up techniques with the enhanced dynamic wedge versus the standard metal wedge. A tissue equivalent block (solid water), modeled to represent a typical breast outline, was designed as an insert in a Rando phantom to simulate a standard patient being treated for breast conservation. Tissue equivalent material was then used to complete the natural contour of the breast and to reproduce appropriate build-up and internal scatter. Thermoluminescent dosimeter (TLD) rods were placed at predetermined distances from the geometric beam's edge to measure the dose to the contralateral breast. A total of 35 locations were used with five TLDs in each location to verify the accuracy of the measured dose. The radiation techniques used were an isocentric set-up with co-planar, non divergent posterior borders and an isocentric set-up with a half beam block technique utilizing the asymmetric collimator jaw. Each technique used compensating wedges to optimize the dose distribution. A comparison of the dose to the contralateral breast was then made with the enhanced dynamic wedge vs. the standard metal wedge. The measurements revealed a significant reduction in the contralateral breast dose with the enhanced dynamic wedge compared to the standard metal wedge in both set-up techniques. The dose was measured at varying distances from the geometric field edge, ranging from 2 to 8 cm. The average dose with the enhanced dynamic wedge was 2.7-2.8%. The average dose with the standard wedge was 4.0-4.7%. Thermoluminescent dosimeter measurements suggest an increase in both scattered electrons and photons with metal wedges. The enhanced dynamic wedge is a practical clinical advance which improves the dose distribution in patients undergoing breast conservation while at the same time minimizing dose to the contralateral breast, thereby reducing the potential carcinogenic effects.

Larger field sizes: an advantage of the dynamic wedge.

This case study describes a clinical situation in which dynamically wedged beams provide treatment options that are not available using conventional physical wedges. The process of planning and delivery are reviewed briefly.

A method for total skin electron treatment for infants.

Diseases such as mycosis fungoides require the treatment of a patient's total skin surface with superficial radiation. In a unique clinical situation, a 14-month-old child presented with a need for total skin treatment. A typical total skin technique requires overlapping electron beams, using 6 body positions, each with the gantry rotated for 2 angulations, or '6 positions-12 fields'. Adaptation of this technique for infants is complicated by the small diameter of some body parts, and by the necessity to treat while the patient is anesthetized. Even degraded, low energy electrons can easily penetrate fingers and toes. Therefore, dose from 6 positions becomes additive, and the total dose to small circumferences can be 3 to 4 times more than skin dose on the torso, raising concerns about uneven bone growth in the developing child. Special phantoms were designed for extensive dosimetry needed to determine both dose rate and dose summation from the overlapping beams. Computerized electron pencil beam calculations were compared to TLD measurements. Unique compensating techniques were used to deliver uniform dose. A modification of the 6 position-12 field technique will be described; and accessories used to reduce high dose regions will be illustrated.

Developmental changes in the contextual control of recognition.

In three experiments, we explored potential developmental differences in the contextual control of target recognition using elementary perceptual features that differed only in their spatial relations. Three- and 6-month-old infants were trained in the presence of black horizontal and vertical line segments arranged as Ls, Ts, or +s on a pink background and were tested in the presence of the same line segments in a different spatial arrangement. At both ages, target recognition required a match between the primitive perceptual features in the test context and those present during training. Delayed recognition by younger but not older infants also required a match between specific spatial arrangements of the horizontal and vertical segments (+s versus Ls or Ts), despite the fact that infants of both ages had initially encoded these relations. These data suggest that by 6 months, spatial information in the periphery is detected preattentively via a parallel-processing mechanism, as it is in children and adults.

An in vivo study of the effects of ischaemia on uterine contraction, intracellular pH and metabolites in the rat.

There are no data concerning the functional or metabolic effects of hypoxia in vivo in smooth muscle. We have therefore used 31P-NMR spectroscopy and intra-uterine pressure measurements to examine simultaneously, in vivo, the effect of ischaemia on uterine metabolites, intracellular pH (pHi) and force. A 1-2 cm portion of uterus from day 1 postpartum anaesthetized rats was exteriorized and an NMR surface coil placed on it. A balloon catheter in the uterine lumen recorded intra-uterine pressure changes from the same area. Reversible occluders were placed around the uterine artery. Occlusion produced a decrease and then abolition of contractions, within 10 min. In four of five animals contraction was abolished within 2 min. Upon reperfusion force was rapidly restored (1 min), in all preparations. The mean level of force was significantly above control (pre-occlusion) 20-30 min after reperfusion. The NMR data showed a significant fall in [ATP] (28%) and [phosphocreatine] (34%) during occlusion. Inorganic phosphate doubled in concentration during this period. Metabolites recovered slowly upon reperfusion, taking 20-30 min to return to pre-occlusion levels. The mean pHi fell from 7.32 to 7.00 upon occlusion and was rapidly reversed upon reperfusion. The changes in pHi closely correlated with the changes in uterine force. Decreases of pHi of a similar magnitude in vitro have previously been shown to abolish contractions; thus it is suggested that during ischaemia in vivo the depression of contraction is caused by the large fall in pHi.

Effects of hypoxia on force produced by agonists and depolarization and arising spontaneously in the rat uterus.

The effects of cyanide and nitrogen on contractile activity in rat uteri was investigated. Hypoxia significantly reduced contractile activity produced either spontaneously, or by application of carbachol (50 mumol l-1) or oxytocin (20 nmol l-1) in preparations from pregnant and nonpregnant rats. Hypoxia had, however, significantly smaller effects on agonist-evoked than on spontaneous contractions. Application of agonists under hypoxic conditions restored some degree of force to preparations in which spontaneous activity had been abolished. This result suggests that the loss of spontaneous contractions was, in part, due to decreased excitability of the uterus, rather than to an impairment of the contractile machinery. Hypoxia significantly decreased the force produced by depolarization of the uterus. The effects of hypoxia on contraction produced by agonists or depolarization were not significantly different, suggesting that a similar mechanism may maintain force under these conditions, and that this mechanism does not occur during spontaneous activity. Lowering the external Ca2+ concentration to 0.1 mmol l-1 resulted in production of significantly less force in the presence or absence of agonist. The ability of hypoxia to decrease agonist-induced force was found not to be due to the intracellular acidification it produces. It was concluded that uterine hypoxia may decrease uterine contractions in vivo and a possible role in dystocia during labour was discussed.

Alternative pathways for training the general internist and the medical subspecialist.

Considerable attention is being directed from within the discipline of internal medicine to a perceived need for changes in the curriculum for postgraduate training of the internist. Much of this attention has been focused on the training required to properly prepare the general internist for the evolving system of health care. A proposal is presented to change the pattern of training of both the general internist and the medical subspecialist. After 2 years of core training, the general internist would enter 2 additional years of training that would include new material and additional experience in areas such as ambulatory and continuity care. After the core experience, the subspecialist would begin training in the subspecialty. The general internist would be certified as such, and the subspecialist would be certified only in the subspecialty. The proposal is based on the assumptions that 1) more general physicians are needed to meet the nation's health care needs; 2) more time will be required to properly train the general internist; 3) all of the additional and expanded training experiences for the generalist are not essential for training the subspecialist; and 4) the generalists of the future will not come from the ranks of subspecialists. The training needs of the general internist and the subspecialist should be viewed as separate, and curriculum changes should be considered without the restrictions of current patterns of training.

Documentation of ovoid cap size.

The need for precise documentation in radiation oncology is paramount; assurance of the quality of therapy delivered is the responsibility of everyone engaged in the treatment of cancer patients. Although all aspects of quality assurance require meticulous attention to detail, the documentation of brachytherapy procedures, both written dose prescription and film verification, is particularly important as no single method of dose reporting is universally accepted and used. The size of ovoid caps used in brachytherapy applications for gynecologic malignancies cannot be verified on treatment planning films, as the caps are not radio-opaque. If the cap size is improperly reported by the physician, resulting surface dose calculations used to determine source loading and implant duration could cause over- or underdosing. In an effort to improve and refine gynecologic brachytherapy record-keeping, we have devised a method to verify and document cap size. Narrow stainless steel bands have been embedded into grooves cut circumferentially on the surface of the ovoid caps. These bands are readily visible on localization films, producing documentation of the cap diameter, reducing the risk of error in dose reporting, and providing a permanent record of ovoid size. We will review the nature of possible dose errors affecting clinical decisions.

Improved dose homogeneity in electron arc therapy achieved by a multiple-energy technique.

Improved dose homogeneity throughout the treatment volume defined for electron arc therapy is achieved through superposition of multiple arcs of different electron energy to the same treatment surface. The relative weights for each arc segment and energy are determined by computer optimization which minimizes the variation in radial depth dose across the treatment volume. In addition to the standard electron beam energies of 6 MeV, 9 MeV, 12 MeV, 16 MeV and 20 MeV, a new electron arc field is created by adding bolus to the treatment surface during an additional pass using 6 MeV electrons. This modified field, having maximum dose on the patient surface and a reduced range, supplements the dose delivered by the standard electron arc fields in the buildup region. Through use of this multiple-energy technique, depending on clinical indications, electron arc therapy can be planned and delivered to allow either skin sparing or a uniform dose from the patient's surface to the desired treatment depth.

Design and production of customized field shaping devices for electron arc therapy.

A key element in the implementation of electron arc therapy is the use of customized field shaping devices on or near the patient's surface to protect normal tissue surrounding the treatment surface. Techniques for design and production of field shaping devices have evolved to meet the requirements of improved efficiency, patient comfort and protection, and reproducibility of patient set-up. Techniques in current use at the University of Utah are described and illustrated.

Optimization of electron arc therapy doses by multi-vane collimator control.

Retrospective computer simulations, based on clinical treatment planning data available from over 50 patients treated by electron arc radiotherapy to the chestwall following mastectomy, show that a dramatic improvement in dose uniformity can, in many clinical situations, be achieved by dynamic shaping of the electron arc collimator, under computer control, as a function of gantry angle and distance superior or inferior to the central plane. The greatest improvement in dose uniformity is seen in calculational planes in which the patient contour has the greatest departure from a circular shape. Dosimetric studies demonstrate this improvement. Indicators for use of variable-width multi-vane electron arc collimators include the following: (1) Mechanical constraints of the therapy equipment may limit the placement of isocenter to an inadequate depth which causes large variation in the SSD around the arc; (2) Out of the central plane, the shape of the chest wall may change dramatically across the limits of the arc, creating large variations in the dose distribution; (3) Clinical definition of the treatment surface to include surgical scars or other at-risk volume may create an irregularly shaped treatment surface, thereby changing the fraction of the arc included in the treatment surface from one plane to the next. Electron arc collimator shape determines both the dose rate and the electron arc beam profile. Both the dose rate and the beam profile must be included in the integration of dose to a point within the arc. The dose to a point within the arc can be modified by as much as a factor of 1.5 to 2.0 by increasing the collimator width from 3 cm to 7 cm. A multi-vane collimator allows these changes to be made in each specific plane to compensate for changes in patient contour.

Prediction of cerebral death by cranial sector scan.

Real-time cranial ultrasonography (cranial sector [CS] scan) was prospectively evaluated for its usefulness in identifying irreversible cerebral injury in critically ill children admitted to the neonatal and pediatric ICUs at our institution. Absence of pulsatile movement of the anterior and middle cerebral arteries was determined to represent absent cerebral perfusion and thus profound irreversible injury. Of 800 consecutive patients representing 1900 CS scans, 12 met the criteria. Six were neonates and 6 were older infants. Mechanisms of injury included severe hypoxia, head trauma, and CNS infection. Absence of brain function by clinical examination and EEG confirmed CS findings in 11 who expired. One neonate survived with intact brainstem function but no identifiable cerebral function or growth. No false positives were demonstrated. We found the CS scan to be a reliable bedside adjunct in the determination of irreversible cerebral injury or cerebral death.

Volume reabsorption by the loop of Henle: a micropuncture study.

The observed extension of glomerulo-tubular beyond the proximal tube is thought to be due either to flow dependent reabsorption by non-accessible proximal segments and pars recta, or to osmotic volume flow out of the descending limb of the loop of Henle.

A study in vivo of peritubular oncotic pressure and proximal tubular reabsorption in the rat.

1. Peritubular capillary microperfusion was used to examine the effects of protein-free and hyperoncotic homologous plasma on fluid reabsorption by proximal convoluted tubules in the hydropenic rat. 3H-labelled p-aminohippurate was added to perfusates for the purpose of estimating the extent to which tubules under study were bathed by the perfusates. [14C]Mannitol was added to perfusates in order to detect contamination of collected tubular fluid by perfusates. 2. Hydrostatic pressures were monitored in the peritubular microvasculature and adjacent proximal tubules during perfusion. Evidence for secretion of p-aminohippurate from perfusate into tubules under study was determined by collecting tubular fluid from both early and late puncture site. Fractional and absolute reabsorption were not affected by either the protein-free or the hyperoncotic plasma. 3. When acetazolamide was added to the perfusate both fractional and absolute reabsorptive rates decreased by an average of 36%, indicating that the techniques were capable of detecting a decrease in proximal tubular reabsorption. 4. It is concluded that under the conditions of this study changes in peritubular capillary protein concentrations have no detectable effect on the rate of proximal convoluted tubule fluid reabsorption.

The relationship between sodium excretion and renin secretion by the perfused kidney.

The effect of altered tubular sodium reabsorption on renin secretion (RSR) was examined under conditions in which other factors influencing renin release could be controlled or excluded. To do this, isolated canine kidneys were perfused at constant pressure with blood circulating from donor animals. Volume expansion or hemorrhage of the donor dogs produced large changes in the animal's blood pressure, renal function, sodium excretion (UNaV), and RSR, but were without effect on renal hemodynamics, UNaV, or RSR in the perfused kidney. Hemodilution without volume expansion, resulted in hypotension, decreased UNaV and increased RSR in the donor dogs, and increased UNaV and suppressed RSR in the perfused kidney. These effects of hemodilution in the perfused kidney were partially reversed when plasma protein concentration was restored to control levels with hyperoncotic albumin, and, overall, there was a significant inverse relationship between electrolyte excretion and RSR. These results provide new evidence for the hypothesis that the rate at which sodium is delivered to the macula densa is an important determinant of the rate of renin secretion.