The Intersection of Antimicrobial Stewardship, the Pharmaceutical Industry, and the Federal Legislature

To mitigate the dangers of inappropriate antimicrobial use leading to increased multidrug-resistant organisms and mortality, antimicrobial stewardship programs have become a mainstay in many health systems. Unfortunately, some pharmaceutical manufacturers simultaneously have ended antimicrobial research and development efforts altogether due to suboptimal return on investments. An optimal and sustainable antimicrobial armamentarium requires a broad alliance between antimicrobial stewardship programs, the pharmaceutical industry, the legislature, and federal and state agencies. Public–private relationships such as the Combating Antibiotic-Resistant Bacteria Biopharmaceutical Accelerator (CARB-X) and legislative policies creating push and pull incentives, including the Generating Antibiotic Incentives Now (GAIN), Developing an Innovative Strategy for Antimicrobial-Resistant Microorganisms (DISARM), and Pioneering Antimicrobial Subscriptions to End Upsurging Resistance (PASTEUR) Acts, are each a step in the right direction, but more work remains. Understanding these legislative actions is imperative for all clinicians, as is teamwork from those involved in the antimicrobial field to develop and maintain the life cycle of each drug that harbors societal value.

Pharmacologic management of Mycobacterium chimaera Infections: A Primer for Clinicians

Mycobacterium chimaera, a member of the Mycobacterium avium complex, can cause infections in individuals after open heart surgery due to contaminated heater-cooler units. The diagnosis can be challenging, as the incubation period can be quite variable, and symptoms are nonspecific. In addition to aggressive surgical management, combination pharmacologic therapy is the cornerstone of therapy, which should consist of a macrolide, a rifamycin, ethambutol, and amikacin. Multiple second-line agents may be utilized in the setting of intolerances or toxicities. In vitro susceptibility of these agents is similar to activity against other species in the Mycobacterium avium complex. Drug–drug interactions are frequently encountered, as many individuals have chronic medical comorbidities and are prescribed medications that interact with the first-line agents used to treat M. chimaera. Recognition of these drug–drug interactions and appropriate management are essential for optimizing treatment outcomes.

66. Improving Antimicrobial Stewardship through Allergy Testing Referrals

Background

Penicillins and cephalosporins (PCN/CEPH) are considered first-line antibiotics for numerous infections for their efficacy, tolerability, and cost effectiveness. Unfortunately, their use may be precluded in approximately 10% of the general adult population who self-report ‘allergy’. As a result, suboptimal antimicrobials are substituted which may increase toxicities, length of hospitalizations, and antimicrobial resistance with subsequent expense and morbidity. Multiple organizations endorse beta-lactam allergy skin testing (BLAST) as an essential component of antimicrobial stewardship programs.^.In an attempt to better describe this patient population as well as to protocolize and improve rates of referral to allergy/immunology clinic, a quality initiative was undertaken at our institution.

Methods

Adult inpatients for whom an infectious disease consult was placed over a 6-month period were chart-reviewed for PCN/CEPH allergy. Inappropriately charted allergies were reconciled and patients were recommended referral to allergy/immunology for formal evaluation with BLAST when appropriate. Referrals were placed for agreeable patients who were then evaluated for appropriateness through history and then scheduled for BLAST. Patients who tolerated oral exposures without adverse effects had the allergy removed from their chart and were educated.

Results

322 patients met inclusion criteria for allergy referral. Of those, 103 agreed to further evaluation, and referrals were placed for 100%. Unfortunately, 7 patients died before referrals could be completed, and 88 referred patients did not complete BLAST for other reasons. In total 8 patients completed BLAST, and allergy was de-labeled in 75% (N= 6) of those cases.

Conclusion

Our data indicated similar prevalence of reported PCN/CEPH allergy between our institution and the general population. We achieved our aim of improving allergy referral rates among this population, however there was a high rate of attrition in the transitions of care. Qualitative review of selected patients highlights common thematic barriers including the COVID-19 pandemic, fiscal concerns, and acuity of condition. Future directions should include BLAST at the point of care or making referrals from the primary care setting.

Disclosures

Lisa A. Clough, MD, Merck (Scientific Research Study Investigator)Merck (Research Grant or Support)ViiV (Scientific Research Study Investigator)

Modifiable Risk Factors Associated With Disposal of Unused Prescription Drugs by Parents of Adolescents

The safe disposal of unused medications is one primary prevention strategy to reduce nonmedical prescription drug use among adolescents. We sought to identify modifiable risk factors associated with disposal of unused prescription drugs by parents of adolescents residing in ten south central Kentucky counties with disposal programs. In the fall of 2017, 4148 parents of adolescents participated in an anonymous, paper-based survey. We conducted generalized logit mixed models adjusted for within-school clustering to assess the relationship between disposal behaviors and modifiable risk factors while controlling for respondents' sociodemographic characteristics. The analytic sample consisted of parents in households in which someone had been prescribed an opioid medication within the past 12 months (N = 627). Our findings indicated that almost 42% of parents reported disposing of unused prescription medication within the past 12 months, and the majority disposed of medications at home rather than using a disposal program. Parents who perceived that any, compared to none, of their child's close friends engaged in nonmedical prescription opioid use had higher odds of reporting use of a disposal program. Parents who were aware of disposal programs, compared to those who were not aware, had greater odds of using them, rather than not disposing at all or disposing unused prescription medications at home. Compared to parents who perceived prescription drugs to be hard for adolescents to obtain for nonmedical use, parents who believed that prescription drugs were easily accessible to adolescents for nonmedical use had lower odds of using disposal programs than disposing of medications at home. Collectively, our findings suggest that enhancing awareness of disposal programs, while addressing parents' perceptions of their children's peers' use of nonmedical prescription opioids, should be considered to facilitate the disposal of unused medications and optimize current public health prevention efforts related to adolescent nonmedical use of these drugs.

COVID-19 and Religious Ethics

The editors of the JRE solicited short essays on the COVID-19 pandemic from a group of scholars of religious ethics that reflected on how the field might help them make sense of the complex religious, cultural, ethical, and political implications of the pandemic, and on how the pandemic might shape the future of religious ethics.

Power of the Peer and Parent: Gender Differences, Norms, and Nonmedical Prescription Opioid Use Among Adolescents in South Central Kentucky

This study examined risk factors of nonmedical prescription opioid use (NMPOU) among adolescents and how risk factors differ by gender. In the fall of 2017, adolescents attending 6th through 12th grades across 44 schools in 10 south central Kentucky counties were invited to participate in an anonymous, school-based survey. A total of 11,761 adolescents completed the survey. Logistic regression was conducted to examine the association between NMPOU and constructs of the Theory of Reasoned Action (i.e., attitudes and subjective norms), descriptive norms (i.e., peer use), and parental control of prescription medications in the home. There were 297 (2.7%) adolescents who reported NMPOU in the past 12 months. In the adjusted multivariate logistic regression model, for both males and females, the adolescents who perceived that more of their peers engaged in NMPOU were significantly more likely to endorse NMPOU, whereas male and female adolescents who perceived their peers disapproved of use were significantly less likely to report NMPOU. Parent disapproval was significantly associated with decreased NMPOU for females only. Moderated regression analyses revealed that gender moderated the relationship between parental disapproval and NMPOU. We found that during adolescence, NMPOU is influenced by peer norms for both genders and parental norms for females. These results indicate that prevention efforts should focus on changing adolescents' peer and parental norms related to NMPOU.

2087. Electronic Capture and Feedback of Standardized Antibiotic Clinical Indications Data Among Community Hospitals

Background

Antibiotic clinical indications allow stewardship programs to assess therapy appropriateness; however, many hospitals that require antibiotic indications upon order entry lack standardized mapping of indications leading to variability in entered values. Electronic capture and feedback of standardized antibiotic clinical indications data may allow hospitals to more effectively compare indication-specific prescribing trends among facilities.

Methods

We collected antibiotic indications from electronic medication orders for 6 DASON hospitals. These indications were mapped to a list of 15 standardized indication categories created by consensus of the DASON stewardship team. To demonstrate the feasibility and utility of standardized clinical indications mapping, we evaluated agents given for the indication C. difficile infection (CDI) in 2018. Differences between the hospitals were compared with highlight the added benefit of standardized indication data in evaluating antibiotic use and adoption of local guidelines.

Results

For 249,916 antibiotic days of therapy (DOT) with an indication available, a total of 125 unique indications were reported. Of note, 3 facilities allowed more than one indication to be entered at prescriber discretion. The distribution of antibiotic DOT mapped to the standardized indication list can be seen in Figure 1. The most common indication was the other category (19.5%). These were primarily other, no additional information (47%) or empiric therapy for an unknown source of infection (17%). Additional indications in the other category included chronic obstructive pulmonary disease exacerbations and sexually transmitted infections (< 5% each). Figure 2 depicts the agents used for CDI indication between facilities. Despite universal adoption of local guidelines where oral vancomycin is the drug of choice for treating CDI, there was variability seen in vancomycin CDI DOT (range: 60 – 80% of CDI DOT).

Conclusion

Stewardship programs can implement standardized antimicrobial indications to facilitate electronic capture, feedback, and comparison and efficiently identify stewardship targets. Additionally, hospitals may use these data to explore the appropriateness of antibiotic use.

Disclosures

All authors: No reported disclosures.

Outcome Following Concussion: Prospective Investigation into the Cumulative Effects of a Prior History of Brain Injury and a History of Repeated Sub-Concussive Injuries on Recovery

Objective

To investigate the impact of patient history of repeated head injury (RHI) and mild traumatic brain injury (mTBI) on recovery from an acute concussion.

Methods

There were 64 participants from 3 military installations treated for an acute concussion ( < 72 hours) and monitored at < 72 hours, 1 week, 1 month, 3 months, and 6 months. Participants had no concussion in the last year and no history of moderate/severe TBI. TBI history was based on the Ohio State University TBI Identification Method and limited to mTBI with loss of consciousness. Primary outcome was symptoms evaluated using the Neurobehavioral Symptom Inventory. A total of 33 participants completed the study.

Results

Participants were as follows: n = 12 TBI(-)/RHI(-); n = 6 TBI(+)/RHI(-); n = 9 TBI(-)/RHI(+); n = 6 TBI(+)/RHI(+). A multivariate ANOVA examining TBI/RHI group effect on symptoms was significant (F = 2.31, p = 0.010), whereby the TBI(+)/RHI(+) had higher symptom scores than other groups at < 72 hours, 1 week, and 6 months. At 1 month, TBI(+)/RHI(+) had more symptoms than the TBI(-)/RHI(+). At 3 months, the TBI(+)/RHI(+) had higher symptom scores than the TBI(-)/RHI(-) and the TBI(-)/RHI(+). The TBI(-)/RHI(+) did not differ from TBI(-)/RHI(-) except at 1 week where they had lower overall symptoms.

Conclusions

Participants with an acute mTBI had more severe symptoms if they had RHI and a prior remote mTBI. The combination of these two factors had a negative impact on recovery for up to 6 months. RHI in the absence of TBI did not impact recovery.

Treatment using the Progressive Return to Activity Clinical Recommendation may return individuals with elevated symptomatology to normal levels at an expedited rate

Objective

To determine whether the use of a clinical recommendation (CR) for activity progression to treat concussion will benefit symptom profile months post-injury beyond what is seen with usual care.

Methods

This study included 58 individuals who participated in the Defense and Veterans Brain Injury Center Progressive Return to Activity (PRA) Study. Eligibility included sustaining a concussion within 72 hours of study enrollment and no previous concussion within 12 months of injury. Poisson regression with robust error variance was used to evaluate the relationship between treatment group assignment (usual care versus PRA-CR) and having normal symptomatology (Neurobehavioral Symptom Inventory score below the upper 10th percentile) at the 1-, 3-, and 6-month follow-up. Participants with normal symptomatology within 72 hours post-injury were excluded from analyses.

Results

Among participants with abnormal symptoms within 72 hours post-concussion, those receiving the PRA-CR to treat concussion had a 70% (CI = 1.06, 2.72) greater prevalence of normal symptomatology by 1 month post-injury, and a 39% (CI = 1.04, 1.87) greater prevalence of normal symptomatology by 3 months post-injury compared to those receiving usual care. By 6 months post-injury, there was no difference in the prevalence of having normal symptoms between groups.

Conclusion

The findings of this study show that participants with abnormal symptomatology decreased in symptom level to normal at a faster rate among those receiving the PRA-CR compared to those receiving usual care, providing evidence for the use of the PRA-CR as a means to enhance treatment of patients with concussion.

1804. Impact of Susceptibility Testing Method on Antibiotic Selection for Methicillin-Resistant Staphylococcus Aureus (MRSA) Bacteremia

Background

The selection of intravenous (IV) antibiotics for methicillin-resistant Staphylococcus aureus (MRSA) bacteremia can be influenced by the vancomycin minimum inhibitory concentration (MIC). This study explores the changes in antibiotic use and inpatient mortality for patients with MRSA bacteremia after switching the MIC testing methods.

Methods

At University of Kentucky Medical Center, Etest™ was implemented in November 2013 for all Staphylococcus aureus blood isolates. In April 2016, this was changed to Phoenix™ automated system. Data regarding antibiotic usage for patients with MRSA bacteremia were collected from July 2014 to December 2015 (Etest™) and September 2016 to March 2017 (Phoenix™). Only patients started on IV vancomycin were included. Daptomycin and ceftaroline use was monitored by the antimicrobial stewardship team with focus on guideline adherence.

Results

A total of 119 and 62 patients were identified before and after switching to Phoenix™. MICs of 2 μg/mL were significantly decreased (P < 0.001) after changing to Phoenix™ (Table 1). Daptomycin use (alone or in combination) decreased from 37% (44/119) to 21% (13/62) (P = 0.013). Ceftaroline use (alone or in combination) decreased from 32% (38/119) to 19% (12/62) (P = 0.036). The reason for escalation in 13 of 44 (30%) patients with daptomycin and 6 of 38 (16%) patients with ceftaroline was an MIC of 2 μg/mL. Overall, IV vancomycin use (alone or in combination) increased from 50% (60/119) to 69% (43/62) (P = 0.007). All-cause inpatient mortality was 16% (19/119) before and 10% (6/62) (P = 0.24) after switching to Phoenix.

Conclusion

A switch in vancomycin susceptibility testing from Etest™ to Phoenix™ automated system was associated with a significant decrease in daptomycin and ceftaroline use and an increase in IV vancomycin use without any change in all-cause inpatient mortality.

Disclosures

All authors: No reported disclosures.

From dispensed to disposed: evaluating the effectiveness of disposal programs through a comparison with prescription drug monitoring program data

BACKGROUND

Organized disposal of controlled medications, such as take-back events and permanent drug donation boxes, is a prevention strategy that has been widely used to reduce the availability of controlled medications for diversion or abuse. However, little is known as to whether this strategy actually reduces the overall availability of these medications for the purposes of diversion or abuse.

OBJECTIVES

The objective of this study was to compare the number and types of controlled medications that were disposed through organized efforts to the number dispensed in local communities.

METHODS

The quantity and type of controlled medication collected from three take-back events and permanent drug donation boxes over 4-week-long periods in five counties in south-central Kentucky was measured and compared to the number of controlled medications dispensed, as reported by Kentucky All Schedule Prescription Electronic Reporting system.

RESULTS

In 2013, 21,121,658 controlled medications units were dispensed in the participating counties. Of those, 46.9% were opioid analgesics, 13.1% tranquilizers, and 37.3% "other." During the assessment periods, a total of 21,503 controlled medication units were collected. Of those, 39.9% were opioid analgesics, 2.7% tranquilizers, and 57.4% "other." Annually, controlled medications disposed were estimated to account for 0.3% of those dispensed.

CONCLUSION

Controlled medications collected by take-back events and permanent drug donation boxes constituted a miniscule proportion of the numbers dispensed. Our findings suggest that organized drug disposal efforts may have a minimal impact on reducing the availability of unused controlled medications at a community level.

Widespread distribution of ability to oxidize manganese among freshwater bacteria

Manganese-oxidizing heterotrophic bacteria were found to comprise a significant proportion of the bacterial community of Lake Washington (Seattle, Wash.) and Lake Virginia (Winter Park, Fla.). Identification of these freshwater bacteria showed that members of a variety of genera are capable of oxidizing manganese. Isolates maintained in the laboratory spontaneously lost the ability to oxidize manganese. A direct correlation was found between the presence of plasmid DNA and the ability of the organism to oxidize manganese.

National school nurse certification. Part I: An ongoing process

The National Association of School Nurses proposed the concept of certification for school nurses in the 1970s. The development and evolution of the school nurse certification process, from concept to reality, is described in this article. It is the first of a 2-part series on national certification for school nurses. Part 2 will examine issues and answer questions about the certification, give information from past and current presidents and certified school nurses about their experiences, and present a forecast about the future of school nurse certification.

Epidemiological study of Campylobacter spp. in broilers: source, time of colonization, and prevalence

From October 1993 to August 1994, broiler chickens in four grow-out houses, two previously used (houses 1 and 2) and two newly constructed (houses 3 and 4), were used in a study to determine the source, time of infection, and prevalence of Campylobacter spp. Cecal droppings and cecal samples were obtained from the broilers. Samples were also obtained from water, feed, litter, soil, fans, and workers' boots. Samples were obtained from domestic animals and wildlife species (rectal swabs), including insects, on or near the premises. Broilers in houses 2, 3, and 4 became infected in the second or third week and were fully colonized by day 42. Campylobacter appeared in house 1 during week 2 in a low percentage of the birds, disappearing by the fourth week. Isolates were also recovered from domestic pigs and water on this farm. In house 3, the organism was isolated from workers' boots and a wild bird prior to isolation from the broilers. Following isolation from cecal droppings, the organism was isolated from water, feed, litter, feathers, flies, cattle, feces, and wild animals. In house 2, Campylobacter was isolated from cattle feces and wild birds prior to week 5, when the broilers first became infected, and thereafter from water, feed, insect, and wildlife, and cecal droppings. It was subsequently isolated from workers' boots, cattle feces, feathers, insects, and other wildlife. All ceca taken from 20 birds each from houses 2 and 3 were positive at time of slaughter (day 49). All ceca from house 1 were negative. No ceca were collected from birds originating in house 4. No specific source could be identified from the samples obtained, although apparently the organism permeates the environment and several potential sources are discussed in this paper.

Cancer mortality among electric utility workers exposed to polychlorinated biphenyls

OBJECTIVES

To assess whether excess mortality from cancer, malignant melanoma of the skin, and cancers of the brain and liver in particular, is associated with long term occupational exposure to polychlorinated biphenyls (PCBs).

METHODS

An epidemiological study of mortality was conducted among 138,905 men employed for at least six months between 1950 and 1986 at five electrical power companies in the United States. Exposures were assessed by panels composed of workers, hygienists, and managers at each company, who considered tasks performed by workers in 28 job categories and estimated weekly exposures in hours for each job. Poisson regression was used to examine mortality in relation to exposure to electrical insulating fluids containing PCBs, controlling for demographic and occupational factors.

RESULTS

Neither all cause nor total cancer mortality was related to cumulative exposure to PCB insulating fluids. Mortality from malignant melanoma increased with exposure; rate ratios (RRs) relative to unexposed men for melanoma were 1.23 (95% confidence interval (95% CI) 0.56 to 2.52), 1.71 (0.68 to 4.28) and 1.93 (0.52 to 7.14) for men with < 2000, > 2000-10,000, and > 10,000 hours of cumulative exposure to PCB insulating fluids, respectively, without consideration of latency. Lagging exposure by 20 years yielded RRs of 1.29 (0.76 to 2.18), 2.56 (1.09 to 5.97), and 4.81 (1.49 to 15.50) for the same exposure levels. Mortality from brain cancer was modestly increased among men with < 2000 hours (RR 1.61, 95% CI 0.86 to 3.01) and > 2000-10,000 hours exposure (RR 1.79, 95% CI 0.81 to 3.95), but there were no deaths from brain cancer among the most highly exposed men. A lag of five years yielded slightly increased RRs. Mortality from liver cancer was not associated with exposure to PCB insulating fluids.

CONCLUSIONS

This study was larger and provided more detailed information on exposure than past investigations of workers exposed to PCBs. The results suggest that PCBs cause cancer, with malignant melanoma being of particular concern in this industry.

Fatal agricultural injuries in North Carolina by race and occupation, 1977-1991

Medical examiner reports and death certificates were reviewed for all fatal agricultural injuries (n = 228) that occurred on-the-job in North Carolina between 1977 and 1991. Data were collected on the decedents' age, gender, race, data and time of injury, means of injury, and occupation. Annual workforce estimates were derived from the 1980 and 1990 US Census of the Population. Overall, 54% of the fatal injuries were due to tractors. Farmers who suffered fatal injuries tended to be older (median age = 56 years) and Caucasian (87%), while farm workers who died on the job were younger (median age = 35 years) and more often African-American (60%). The crude mortality rate for farmers was 38 per 100,000 worker-years; the crude rate for farm laborers was 16 per 100,000 worker-years. Age-adjusted fatal injury rates were 2.5 times higher among African-American farmers than among Caucasian farmers; furthermore, between 1977 and 1991 the rate of fatal injury among African-American farmers increased an estimated 14.7% per year. African-American farmers in North Carolina have experienced rising rates of fatal injuries at a time when employment in the industry is declining due to consolidation of farm ownership and foreclosures of African-American owned farms. In order to address the growing racial disparity in farm fatalities, efforts need to be made to improve the conditions under which African-American farmers are working.

Genetic studies in Alzheimer's disease with an NACP/alpha-synuclein polymorphism

The non-Abeta component of Alzheimer's disease amyloid (NAC) is copurified with amyloid from the brain tissue of Alzheimer's disease victims and is immunohistochemically localized to amyloid fibrils. NAC is a hydrophobic peptide fragment from the NAC precursor protein (NACP/alpha-synuclein) that is localized to presynaptic terminals. We used a polymorphic dinucleotide repeat sequence in a genomic clone of NACP for genetic association and linkage studies. Screening of Alzheimer's disease families failed to establish linkage between NACP and Alzheimer's disease. Nevertheless, one of the NACP polymorphisms (NACP allele 2) was shown to have significant association with healthy elderly control individuals with apolipoprotein E risk. This may indicate a possible protective function of the allele.

Pathway from the zona incerta to the superior colliculus in the rat

In order to test the proposal that the zona incerta contributes to the generation of orienting movements, we examined the synaptic relationships between the incertotectal pathway and the cells of origin of the predorsal bundle. The predorsal bundle cells give rise to the major premotor pathway from the superior colliculus to the brainstem gaze centers. First, cytochrome oxidase histochemistry, gamma-aminobutyric acid (GABA), and glutamic acid decarboxylase (GAD) immunocytochemistry, and the axonal transport of markers were used to define the borders of a ventral subdivision of the zona incerta. This subdivision projects topographically to the same sublamina of the intermediate grey layer of the superior colliculus that contains the vast majority of the predorsal bundle cells. Experiments in which incertotectal cells were labeled by both retrograde transport and immunocytochemistry showed that this pathway is GABAergic. Retrograde and anterograde experiments also showed that this pathway is reciprocated by a pathway from the intermediate grey layer of the superior colliculus to the same ventral subdivision of the zona incerta. Finally, experiments combining axonal transport and electron microscopic methods showed that the incertotectal pathway is the source of a monosynaptic GABAergic input to the cells of origin of the predorsal bundle. The ventral subdivision of the zona incerta is contrasted with a second source of GABAergic input to the predorsal bundle cells, the substantia nigra pars reticulata.

Care and sterilization of endourologic instruments

All endourologic instruments must be handled properly if they are to continue to function properly. Care must be taken in washing and sterilizing of this equipment, as not all endoscopic equipment can endure all methods, and people who are working with these instruments must be taught the proper care and sterilization methods of each. For example, fiberoptic telescopes and light cables must never be autoclaved; ethylene oxide is the method of choice. Disinfectant is an alternative. Loops, sheaths, high-frequency cables, resectoscopes, and working elements should be sterilized by ethylene oxide. Loops and high-frequency cables should not be soaked in a disinfectant, but other instruments may be soaked. All instruments should be dried before sterilization. Too much emphasis cannot be placed on the importance of proper storage of these very fragile fiberoptic instruments. With proper care and sterilization, these instruments will need fewer repairs and function properly for a longer time.

Variation in the lung inhomogeneity correction factor with beam energy. Clinical implications

In order to determine the magnitude of the dosimetry error introduced by failing to correct for increased transmission through lung tissue in treating thoracic malignancies, measurements in a phantom were taken using different field sizes, inhomogeneity thicknesses and photon qualities. The results indicate that the error introduced by neglecting the inhomogeneity correction is greatest at lower photon energies, smaller field sizes and greater thickness of inhomogeneity. Correction factors to account for the lung inhomogeneity were obtained from phantom measurements and were compared with those calculated using the tissue-air ratio and Batho-Young algorithms; correlation coefficients describing the relationship between measured and calculated values exceeded 0.995. The calculated values tended to overestimate the correction factor and differed most from the measured correction factors at lower energies, smaller field sizes, and greater inhomogeneity thicknesses. The importance of these results in clinical radiation therapy is discussed.

Postnatal maturation of cerebellar mossy and climbing fibers: transient expression of dual features on single axons

We have studied the form and fine structure of developing afferent axons in postnatal mouse cerebellum, before and during the formation of synaptic connections. In slices of fresh brain, bundles of axons were injected with horseradish peroxidase (HRP), and individual axons were examined in the light and electron microscopes. At birth, before formation of cortical layers, axons with growing tips are rare in the peduncular tracts but instead ramify throughout the cerebellar anlage. All axons have similar structures; they branch infrequently and terminate in bud-like tips and/or small growth cones. Growth cones contain small and large vesicles in the flank and small vesicles in filopodia. Typical mossy and climbing fiber branching patterns and bouton shapes are recognizable after postnatal day (P) 5, even though fibers are still intermingled in a plexus beneath the newly formed Purkinje cell layer. Climbing fiber-like axon arbors are highly branched and covered with small foliate growing tips that contact Purkinje cells. Mossy fiber-like branches have large irregular expansions that give rise to long filopodia and resemble growth cones seen in vitro. The flanks of these growth cones contact granule cell dendrites and form glomeruli typical of mossy fibers, whereas the filopodia make primitive contacts or are associated with coated vesicles in adjacent profiles. A novel finding is the occurrence during the second postnatal week of many single axons that simultaneously have the morphology and synaptic connections of both climbing and mossy fibers. These "combination" axons have some branches that extend into the granule cell layer and others that enter the Purkinje cell layer, with the shape and synaptic connections of terminals on each branch type corresponding to the respective layer. Climbing fiber-like branches, including those on combination fibers, extend over several adjacent Purkinje cells. Combination fibers are rare in late postnatal or adult stages. These results suggest that long after arrival in the cerebellum, afferent axons have similar elementary forms and overlap in their projections. Mature axonal forms are not exhibited until cellular layers develop. During a limited period of postnatal maturation, some axons have dual morphologies and synaptic relations with appropriate and inappropriate partners. These aspects of cerebellar axonal development, particularly the transient exuberant branching onto two types of target cells, offer a valuable opportunity to examine, in developing cerebellum, the sorting out of afferents and the formation of specific synaptic connections.