Choice of optimum megavoltage for accelerators for photon beam treatment

Over three decades ago, the development of megavoltage accelerators revolutionized radiation oncology and provided the therapist with photons and electrons of any desired energy. The initial advantages cited for high energy photon therapy, listed below, have proved valid and accelerators have almost totally replaced orthovoltage units. Initially, it appeared that most of these cited advantages should continue to improve with increasing energy, and there has been an impetus for the production of ever higher megavoltage accelerators. Some of these advantages are reviewed in this paper. Also, recent investigations have indicated increasing diffuseness of the photon beam boundary with increasing energy because of lateral transport of electrons. The impact on treatment planning as a function of energy of the increase in volume dose due to the diffuseness of beam boundaries, "build-down" and "rebuild-up" effects in tissues at cavity and inhomogeneity interfaces, bone absorption, and photoneutron production are discussed. Consideration of the behavior of these parameters indicates that optimum photon energies have been achieved and that the impetus for higher megavoltages is unwarranted for most treatment. For many therapeutic applications, there are major advantages of 4 MV to 8 MV photon beams relative to 60Co gamma rays. For large lesions in the abdomen or pelvis there is an advantage to energies above those provided by 15 MV units. The various considerations above are discussed and summarized as a function of lesion site and megavoltage.

Off-center ratios for three-dimensional dose calculations

A new method is proposed for computing the off-center ratios (OCR's) in three-dimensional dose calculations. For an open field, the OCR at a point is computed as the product of the primary OCR (POCR) and the boundary factors (BF's). The POCR describes the beam profile for an infinite field, that is, without the effect of the collimators. It is defined as the ratio of the dose at a point off the central ray to the dose at the point on the central ray at the same depth for an infinite field. The POCR is a function of radial distance from the beam central ray and depth. The BF describes the shape of the beam in the neighborhood of the field boundary defined by the collimators. It is defined as the ratio of the OCR at a point for a finite field to the OCR at the same point for an infinite field. The BF is a function of distance from the field boundary, depth, and field size. For a wedged field, we assume that the boundary factors remain the same as for open fields but the POCR's are altered. The changes in beam profiles are described by a factor called the wedge profile factor (WPF), defined as the ratio of the dose at a point for the largest wedged field to the dose at the same point for an open field of the same field size. The WPF is a function of lateral distance from the beam central plane and depth. Calculated OCR's using this new method are in agreement with the measured data along both the transverse and the diagonal directions of the field.

Retinal revascularisation in diabetic retinopathy

The case history of a 33-year-old diabetic patient who has had diabetes for 24 years is presented. When first seen in 1975 he had bilateral proliferative retinopathy with new vessels in the retinal periphery. He had large areas of capillary non-perfusion lateral to the macula in the right eye associated with the new vessels. Nine years later, after extensive repeated photocoagulation, revascularisation of large areas previously not perfused were seen. The vessels are in the plane of the retina and do not have the appearance of new vessels.

Differential pencil beam dose computation model for photons

Differential pencil beam (DPB) is defined as the dose distribution relative to the position of the first collision, per unit collision density, for a monoenergetic pencil beam of photons in an infinite homogeneous medium of unit density. We have generated DPB dose distribution tables for a number of photon energies in water using the Monte Carlo method. The three-dimensional (3D) nature of the transport of photons and electrons is automatically incorporated in DPB dose distributions. Dose is computed by evaluating 3D integrals of DPB dose. The DPB dose computation model has been applied to calculate dose distributions for 60Co and accelerator beams. Calculations for the latter are performed using energy spectra generated with the Monte Carlo program. To predict dose distributions near the beam boundaries defined by the collimation system as well as blocks, we utilize the angular distribution of incident photons. Inhomogeneities are taken into account by attenuating the primary photon fluence exponentially utilizing the average total linear attenuation coefficient of intervening tissue, by multiplying photon fluence by the linear attenuation coefficient to yield the number of collisions in the scattering volume, and by scaling the path between the scattering volume element and the computation point by an effective density.

Apocytochrome c induces pH-dependent vesicle fusion

The ability of apocytochrome c and the heme containing respiratory chain component, cytochrome c, to induce fusion of phosphatidylcholine (PC) small unilamellar vesicles containing 0-50 mol % negatively charged lipids was examined. Both molecules mediated fusion of phosphatidylserine (PS):PC 1:1 vesicles as measured by energy transfer changes between fluorescent lipid probes in a concentration- and pH-dependent manner, although cytochrome c was less potent and interacted over a more limited pH range than the apocytochrome c. Maximal fusion occurred at pH 3, far below the pKa of the 19 lysine groups contained in the protein (pI = 10.5). A similar pH dependence was observed for vesicles containing 50 mol % cardiolipin (CL), phosphatidylglycerol (PG), and phosphatidylinositol (PI) in PC but the apparent pKa values varied somewhat. In the absence of vesicles, the secondary structure of apocytochrome c was unchanged over this pH range, but in the presence of negatively charged vesicles, the polypeptide underwent a marked conformational change from random coil to alpha-helix. By comparing the pH dependencies of fusion induced by poly-L-lysine and apocytochrome c, we concluded that the pH dependence derived from changes in the net charge on both the vesicles and apocytochrome c. Aggregation could occur under conditions where fusion was imperceptible. Fusion increased with increasing mole ratio of PS. Apocytochrome c did induce some fusion of vesicles composed only of PC with a maximum effect at pH 4. Biosynthesis of cytochrome c involves translocation of apocytochrome c from the cytosol across the outer mitochondrial membrane to the outer mitochondrial space where the heme group is attached. The ability of apocytochrome c to induce fusion of both PS-containing and PC-only vesicles may reflect characteristics of protein/membrane interaction that pertain to its biological translocation.

Validity of the concept of separating primary and scatter dose

The technique of separating dose into primary and scatter components for calculating photon dose distributions is widely used. The primary and scatter dose models ignore ignore the fact that electrons have a finite range. This may be a good approximation for 60Co photons but not for higher energies. For the latter, the range of electrons may be several centimeters in soft tissue and even longer in lungs and will lead to errors in computed dose in regions where electronic equilibrium does not exist. Ignoring the finite range of electrons will affect dose at points such as those near the beam boundaries, near inhomogeneity boundaries, and at bone-soft-tissue interfaces. Other possible problems associated with the definition and use of "primary" and "scatter" dose in dose distribution calculations result from extrapolation of measured data to obtain data for zero and very large field sizes and from the use of these quantities, which are defined for central axis, for points at large distances from the central axis. This paper examines the limits of the validity of these assumptions.

Retinopathy in tropical pancreatic diabetes

In a study of the retinopathy profile of 40 patients with tropical pancreatic diabetes, a form of secondary diabetes seen in tropical countries, diabetic retinopathy was detected in 13 patients. Ten patients had background retinopathy and three patients had proliferative retinopathy requiring laser photocoagulation. In two patients, fibrous retinitis proliferans was present. Three patients had evidence of diabetic maculopathy. To our knowledge, this is the first report of severe and sight-threatening retinopathy occurring with secondary diabetes. Retinopathy in these forms of diabetes has hitherto been considered to be rare or, if present, only of a mild background type.

Energy and angular distributions of photons from medical linear accelerators

For accurate three-dimensional treatment planning, new models of dose calculations are being developed which require the knowledge of the energy spectra and angular distributions of the photons incident on the surface of the patient. Knowledge of the spectra is also useful in other applications, including the design of filters and beam modifying devices and determination of factors to convert ionization chamber measurements to dose. We have used Monte Carlo code (EGS) to compute photon spectra for a number of different linear accelerators. Both the target and the flattening filter have been accurately modeled. We find the mean photon energy to have a value lower than the generally perceived value of one-third the maximum energy. As expected, the spectra become softer as the distance from the central axis increases. Verification of the spectra is performed by computing dose distributions and half-value layers in water using the calculated spectra and comparing the results with measured data. We also examined the angular distributions of photons incident on the surface of the phantom. In currently used models of dose computations, it is assumed that the angular distribution of photons with respect to fan lines emanating from the source is negligible. Although the angular spread of photons with respect to the incident direction has been found to be small, its contribution to the diffuseness of the beam boundaries is significant.

Clinical experience with a computerized record and verify system

To improve the quality of patient care by detecting and preventing many types of treatment mistakes, we have implemented a computerized system for recording and verifying external beam radiation treatments on our therapy machines. It inhibits the radiation beam if treatment machine settings do not agree with prescribed values to within maximum permissible deviations (tolerances). The tolerances are determined from experience and adjusted when necessary to make the system more effective and less susceptible to "false alarms." The system uses a common data base for all treatment machines. As a result, it permits statistical analysis and generation of reports based on data encompassing the entire patient population as well as verification of treatments of patients transferred from one machine to another. Reports of verification failures reveal patterns of mistakes. Knowing these, attempts can be made to reduce the frequency of verification failures. "Significant" mistakes that were prevented are extracted by treatment planning personnel from these reports. Analysis of data indicates a rate of approximately 150 "significant" mistakes detected and prevented per machine per year, representing 1.0% of all fields treated. We present and discuss our experiences with the system and with the frequency, patterns, and significance of verification failures. We selected a few of the patients for whose treatments significant set-up mistakes were made, and were detected and prevented by the Record and Verify System. We include discussions of the overall effect these mistakes would have had on dose distribution had they not been prevented.

High prevalence of maturity-onset diabetes of the young (MODY) among Indians

This article describes the high prevalence of maturity-onset diabetes in the young (MODY) in an Indian clinic population of diabetic patients. MODY appears to be more common among Indians than among Caucasians. Only 27% of MODY patients had definite autosomal-dominant inheritance. In 73% the mode of inheritance was not definite. Microvascular complications were common and macrovascular complications rare. The high prevalence of MODY in this diabetes clinic might suggest an ethnic variation in diabetes.

Measurements of radiation dose distributions for shielded cervical applicators

Cervical applicators with shielded ovoids are employed to reduce dose to the rectum and bladder. Because of asymmetries introduced by the shields, dose distribution calculations for individual patients will require extensive computer reference data for the ovoid sources. Requisite 3-D dose distributions were measured for an unshielded and a shielded ovoid containing a Cs-137 source, using a computerized system employing a diode in a water phantom. The probe stops at each measurement point and accumulates dose for several seconds. The system automates horizontal positioning of the detector and angular motion of the ovoid to obtain dose in one plane. The detector is moved manually to other planes for a complete three dimensional set of measurements. In order to suppress the energy and directional dependence of the diode, final dose distributions are calculated from ratios of shielded to unshielded data in conjunction with independently measured TLD data for unshielded sources.

Computation of radiation dose distributions for shielded cervical applicators

While cervical applicators with shielded ovoids are used widely in brachytherapy, we know of no system for calculating dose distributions for them. For shielded sources, because of a lack of symmetry and because of a rapid variation of dose as a function of position relative to the source, extensive measured data in three dimensions are required. In the method we have developed, the dose at a given point from a source in a shielded ovoid is calculated by multiplying the dose from an unshielded source by the "effective attenuation factor" of the shields. The latter quantity is obtained by linear-interpolation in a three-dimensional table generated from measurements described in an earlier paper. The unshielded-source dose is calculated as the product of source strength, time of implant, distance-dependent geometry factor and a tabulated quantity called the "relative dose rate factor". Relative dose rate factor is obtained by dividing measured dose rate by the product of geometry factor and source strength. Division by the geometry factor reduces the amount of data required with respect to accuracy in linear-interpolation. Input localization data must include not only the position of the end points defining the source but also a third reference point to define the orientation of the shields.

Tropical pancreatic diabetes in South India: heterogeneity in clinical and biochemical profile

Clinical and biochemical studies were carried out in 33 patients with diabetes secondary to chronic calcific, non-alcoholic pancreatitis (tropical pancreatic diabetes) and in 35 Type 2 (non-insulin-dependent) diabetic patients and 35 non-diabetic subjects. Despite lower body mass indices, only 25% of patients with tropical pancreatic diabetes had clinical evidence of malnutrition. There was no history of cassava ingestion. Mean serum cholesterol concentration was significantly lower in the tropical pancreatic diabetic patients (p less than 0.01) in comparison with the Type 2 diabetic patients or non-diabetic subjects, due to a significantly decreased concentration of LDL cholesterol (p less than 0.01) and VLDL cholesterol (p less than 0.05). Basal and post-glucose stimulated concentrations of serum C-peptide were highest in those pancreatic diabetic patients (n = 11) who responded to oral hypoglycaemic drugs, intermediate in the majority (n = 17), who were insulin dependent and ketosis resistant and negligible in a small sub-group (n = 5) who were ketosis prone. The occurrence of microangiopathy in pancreatic diabetic patients was common and similar to that in Type 2 diabetic patients. Thus, tropical pancreatic diabetes in South India appears to be heterogeneous with respect to level of nutrition, severity of glucose intolerance, B-cell function, response to therapy and the occurrence of microvascular complications.

A computerized record and verify system for radiation treatments

We have developed a general purpose, comprehensive, and highly reliable computerized Record and Verify System to detect and prevent mistakes in the delivery of external beam radiation therapy. This system helps prevent accidental delivery of dangerous dose, improves quality control, and provides invaluable record keeping and report generating capabilities. Currently, treatment machine and couch parameter settings of four different machines are monitored by the system and compared with prescribed values. The system inhibits a machine from being turned on if the settings do not agree with the prescribed values to within specified maximum permissible deviations. The system is user-friendly and provides useful, complete, and easily accessible data. We describe many aspects of the system including hardware, software, data, and operation, and we conclude with a brief discussion of clinical experience and preliminary data.

Epidemiologic and laboratory observations of Chlamydia psittaci infection in pet birds

Psittacosis was diagnosed in 13 (24%) of 55 pet bird submissions in the first 3 months of 1983, compared with 18 (17%) of 103 in 1982, 4 (4.6%) of 87 in 1981, and 3 (8%) of 36 in 1980. The numbers and types of birds in which psittacosis was diagnosed were 2 of 47 budgerigars, 8 of 45 cockatiels , 3 of 11 small parrots ( lovebirds , conures ), 20 of 116 medium-sized parrots (African Grays, Amazons ), 2 of 21 macaws , and 3 of 21 cockatoos . Thirty-nine percent of psittacosis-positive birds were received from pet bird owners, compared with 21% from pet shops, 18% from pet bird breeders and fanciers, and 13% from pet bird jobbers and retailers . Most frequently observed clinical signs were anorexia, weight loss, and diarrhea or yellowish droppings. Major gross lesions were splenohepatomegaly (77%), followed by enteritis (53%), sinusitis (47%), airsacculitis (37%), pneumonitis (23%), and pericarditis (10%). Regarding the 38 cases of confirmed psittacosis in pet birds since 1980, transmission of the disease to human beings was confirmed in 2 cases and suspected in 4 cases.

Increased LDL cholesterol in non-insulin-dependent diabetics with maculopathy

Serum cholesterol and its various lipoprotein fractions and triglycerides were measured in 50 non-insulin-dependent diabetic patients, 25 without retinopathy (group A) and 25 with diabetic maculopathy (group B) initially before control of diabetes, and again, after achieving good metabolic control. The lipid parameters were also estimated in 25 healthy non-diabetic subjects. The groups were well matched for sex, age, duration of diabetes and body weight. Mean serum total cholesterol and LDL cholesterol levels were significantly higher in group B when compared to group A, before and after achieving good metabolic control of diabetes (p less than 0.001). Mean total/HDL cholesterol and mean LDL/HDL cholesterol ratios were also significantly increased in group B as compared to group A (p less than 0.01) both before and after good diabetic control in these patients. Mean serum HDL and VLDL cholesterol and triglyceride concentrations were similar in groups A and B. The mean serum lipid concentrations in the diabetic patients without retinopathy (group A) were comparable to those of non-diabetic subjects.

Sources of electron contamination for the Clinac-35 25-MV photon beam

A detailed Monte Carlo approach has been employed to investigate the sources of electron contamination for the 25-MV photon beam generated by Varian's Clinac-35. Three sources of contamination were examined: (a) the flattening filter and beam monitor chamber, (b) the fixed primary collimators downstream from the monitor chamber and the adjustable photon jaws, and (c) the air volume separating the treatment head from the observation point. Five source-to-surface distances (SSDs) were considered for a single field size, 28 cm in diameter at 80 cm SSD. It was found that for small SSDs (80-100 cm), the dominant sources of electron contamination were the flattening filter and the beam monitor chamber which accounted for 70% of the unwanted electrons. Thirteen percent of the remaining electrons originated in the downstream primary collimators and the photon jaws, and 17% were produced in air. At larger SSDs, the fraction of unwanted electrons originating in air increased. At 400 cm SSD, 61% of the contaminating electrons present in the beam were produced in air, 34% originated in the flattening filter and beam monitor chamber, and 5% were due to interactions in the fixed collimators downstream from the monitor chamber and the adjustable photon jaws. These calculated results are substantiated by recent experiments.

Response of citrus to vesicular–arbuscular mycorrhizal inoculation in unsterile soils

Response of citrus to inoculation with the endomycorrhizal fungus Glomus fasciculatum in four soil types of India was studied under greenhouse conditions. Inoculation increased the dry weight of shoot and root and nutrient content of plants. Plants grown in sandy soils responded to a greater extent compared with plants grown in clayey or lateritic soils. The results brought out that mycorrhizal inoculation could be beneficial in certain unsterile soils in spite of the presence of native endophytes.

Standardization of therapy machine interface for treatment monitoring

Computerized monitoring of radiation treatments is an effective tool in detecting and preventing errors in treatment delivery. Most manufacturers now supply optional treatment monitoring (Record and Verify) systems. These differ significantly in their features and are, in general, incompatible with each other. Based on our examination of four different commercial systems, we believe that these commercial and individualized systems are not adequate and are difficult to use. In a radiation therapy facility with more than one treatment machine, it is highly desirable, for a variety of reasons, that such a system be implemented on a central computer which monitors all machines. A centralized record and verify system which allows the storage of all data on a long term basis in an on-line common data base permits much easier access to individual patient data as well as useful data analysis and reports. Every patient's demographic, prescription and treatment data are then available on all treatment machines. This is particularly useful for cases in which a patient may be transferred from one machine to the other. Such a system is also easier to operate. The development of such a treatment monitoring system requires that the protocol of communication between the therapy machine and the computer, and the data describing therapy machine settings transmitted to the computer be standardized. We propose that every manufacturer supply an optional interface meeting this standard. A description of the proposed standard evolved in four years of our work in this field is given.

Investigation of buildup dose from electron contamination of clinical photon beams

The contribution made by contaminating electrons present in a clinical photon beam to the buildup dose in a polystyrene phantom has been calculated and compared to measurements. A Monte Carlo technique was employed. The calculation was divided into two parts. First, the accelerator treatment head was simulated in detail using the EGS-PEGS electromagnetic shower code. Then, information obtained from these calculations was used to compute dose curves in a polystyrene phantom. Two cases were considered, one in which both electrons and photons were incident upon the phantom, and another in which electrons were eliminated from the incident beam. Results of these calculations agree with recent experimental findings. A decrease in buildup dose as well as a shift in dmax was observed when electrons were eliminated from the beam.