Stratifying Response to Stereotactic Radiosurgery for Spinal Metastases by Primary Site Genomic Mutations

PURPOSE/OBJECTIVE(S)

Spine stereotactic radiosurgery (SSRS) is effective in achieving durable local control (LC) and palliation of pain in patients with spinal metastases. Per institutional standards, SSRS prescription dose is tailored by primary site histology, with radioresistant disease receiving escalated doses. While the association between tumor histology and radioresistance is well studied, the association between specific genotypic mutations and radioresistance to SSRS is not well known. We sought to determine if a relationship exists between primary tumor mutations and clinical outcomes following SSRS.

MATERIALS/METHODS

We performed a retrospective analysis of 201 patients with available primary site mutation profiles who underwent SSRS to 327 spinal metastases from 2007-2022 at a single institution. Associations with overall survival (OS) and LC were identified using univariate and multivariable Cox proportional hazards modeling adjusted for clinicopathologic and treatment-related factors.

RESULTS

The median age was 59 years (range: 11-85) at the time of SSRS. One hundred and five patients were male (52%). The median SSRS dose to the gross tumor volume was 24 Gy (range: 12-50) given in a median of 1 fraction (range: 1-5), corresponding to a biologically effective dose (BED10) of 81.6 Gy. The most common primary tumor sites were head and neck (20%), lung (19%), and genitourinary (15%). The most common spine subsites were thoracic (57%) and lumbar (27%). Twenty-four mutated genes were identified in primary tumors with the most common being TP53 (28%), KIT (15%), PIK3CA (15%), and KRAS (10%). Prior to receipt of SSRS, 19% of patients underwent surgical resection and 23% received conventional radiation therapy (RT) to their treated sites. The median follow-up time and OS following SSRS was 97 months (95% confidence interval [CI] 86-128) and 41 months (95% CI 20-31), respectively. The median LC at 1 and 2 years following SSRS was 88% (95% CI 84-92) and 75% (95% CI 70-82), respectively. On multivariable analysis, receipt of prior RT (hazard ratio [HR] 1.86; 95% CI 1.29-2.67; P = 0.0008) and TP53 mutation (HR 1.68; 95% CI 1.20-2.35; P = 0.0024) were associated with shorter OS, whereas STK11 mutation (HR 2.14; 95% CI 0.47-4.70; P = 0.0589) trended towards shorter OS. LC was more durable with increasing BED10 (HR 0.98; 95% CI 0.96-1.00; P = 0.0166) and less durable with mutations in ATM (HR 2.40; 95% CI 1.06-5.44; P = 0.0362), STK11 (HR 4.89; 95% CI 1.40-15.01; P = 0.0119), and CTNNB1 (HR 4.49; 95% CI 1.90-10.62; P = 0.0006).

CONCLUSION

While favorable outcomes among all patients receiving SSRS were seen, increasing BED10 was associated with more durable LC. Primary site ATM, STK11, and CTNNB1 mutations may be associated with poorer LC following SSRS. Further investigation into the prognostic value of individual mutations and combinations thereof is warranted.

Spine Stereotactic Radiosurgery for Primary and Metastatic Osteosarcoma

PURPOSE/OBJECTIVE(S)

Osteosarcoma is difficult to control due to its high propensity for metastasis and resistance to local and systemic therapies. High doses of radiation therapy (RT) may confer local control (LC) in some settings but for lesions involving the vertebral bodies, proximity to the spinal cord may limit the ability to deliver an adequate dose. In this analysis, we investigate the role of spine stereotactic radiosurgery (SSRS) to overcome this barrier and enable efficacious treatment of primary or metastatic osteosarcoma of the spine.

MATERIALS/METHODS

We retrospectively reviewed all patients treated with SSRS for osteosarcoma of the vertebrae between 2006 and 2022 at a single large tertiary cancer center. We utilized the Kaplan-Meier method to estimate overall survival (OS) and LC.

RESULTS

We identified 18 patients treated with SSRS for 25 lesions of spinal osteosarcoma. Median follow-up was 17.2 months. Two patients and three separate lesions were treated with SSRS for primary osteosarcoma of the vertebrae. The remaining 16 patients and 22 lesions received SSRS to the spine for metastatic disease. Lesions were treated to a dose of 24Gy in one fraction (n = 20) 27Gy in 3 fractions (n = 4) or 50Gy in 5 fractions (n = 1). Treatment sites included the cervical spine alone (n = 4), thoracic spine alone (n = 12), lumbar spine alone (n = 4), sacrum alone (n = 3), or both the thoracic and lumbar spine (n = 2). At latest follow up, local failure was observed in 9/25 (36%) treated lesions and median LC was 22.5 months (95% CI 6-43 months). Per-lesion LC at 1 year was 64% (95% CI 35-83%). Per-patient median OS was 14 months (95% CI 7-68 months) and OS estimates at 1 and 2 years were 60% (95% CI 32-80%) and 35% (11-60%), respectively. Among 15 patients who received 24 Gy in one fraction, at 1 year per-lesion LC was 72% (95% CI 41-88%) and per-patient OS was 60% (95% CI 28-81%). The most common acute treatment related toxicity was pain flare (12%). Four patients (16%) developed compression fractures in the treated vertebrae after radiation, with incidence between 57 and 578 days after radiation. Two of these fractures required intervention and two were incidental findings on imaging. No patients developed CTCAE Grade 3 or higher adverse events including neurological toxicities.

CONCLUSION

SSRS appears to be safe and effective in the treatment of metastatic or primary osteosarcoma involving spinal bone. Future work should include further investigation of this technique with pooled multi-institutional studies and randomized trials.

Dosimetric Analysis of Local Failure Outcomes and Vertebral Compression Fracture Risk in Single-Fraction Spine Stereotactic Radiosurgery for Metastatic Sarcoma

PURPOSE/OBJECTIVE(S)

Sarcoma spinal metastases (SSM) are particularly difficult to manage given their poor response rates to chemotherapy and their inherent radioresistance. We sought to analyze dosimetric parameters impacting local failure and vertebral compression fracture outcomes in a homogenously treated cohort of patients with SSM treated with single-fraction spine stereotactic radiosurgery (SSRS).

MATERIALS/METHODS

A retrospective review was conducted on a cohort of patients with SSM treated with definitive SSRS at a single tertiary institution. 16-24 Gy was delivered to the GTV and 16 Gy uniformly to the CTV. Kaplan-Meier analysis was conducted to assess time to local failure (LF). The log-rank test was utilized to examine group differences. Patients were censored at time of last follow-up or death. Cox proportionate hazards modeling was used to determine hazard ratios (HR) and their respective 95% confidence intervals (CI).

RESULTS

A total of 66 patients with 96 lesions underwent SSRS for SSM. Median follow-up was 17 months (IQR 8-28). Median age was 55 years (IQR 41-63). The most common histological subtype was leiomyosarcoma (41%) followed by liposarcoma (9%). 81 lesions received 24 Gy to the GTV, 12 received 18 Gy and 3 received 16 Gy. Median GTV and CTV volume was 13.6cc (IQR 5-27) and 51.6cc (IQR 30-80) respectively. 27% of patients had Bilsky 1b or greater disease. 16 of 96 lesions demonstrated progression representing a crude local failure rate of 17% with median time to failure of 8 months (IQR 5-18). The 1-year actuarial progression free survival (PFS) was 89% with a median PFS of 13 months (IQR 16-63). Median overall survival (OS) was 15 months (IQR 8-28) from SSRS. 8% of patients developed vertebral compression fractures at a median of 13 months post SSRS (IQR 7-25). Every 1 Gy increase in GTV minimum dose (DMin) across the range (5.8-25cc) was associated with a reduced risk of local failure (HR = 0.875 [95% CI 0.787-0.974], p = 0.01). Stratifying thresholds for GTV DMin, a local control benefit was seen as low as 12 Gy and higher (HR = 0.329 [95% CI 0.11-0.97, p = 0.044) with a significantly greater magnitude benefit seen at 14 Gy (HR = 0.267 [95% CI 0.09-0.77, p = 0.014) and above 15 Gy (HR = 0.091 [95% CI 0.03-0.41], p = 0.0018). There were no other queried variables besides GTV Dmin associated with local control including: GTV: volume, mean, Dmax, D90, CTV: volume, Dmin, Dmean, Dmax, or D90. There was an increased risk of VCF with increasing CTV DMean (HR = 2.4 [95% CI 1.4-4.1], p = 0.002) and CTV D90 (HR = 2.2 [95% CI 1.2-4.0], p = 0.01); however, no association with GTV parameters.

CONCLUSION

This study represents one of the most homogenously treated and the largest cohorts of patients with sarcoma spinal metastases treated with single-fraction SSRS. Despite inherent radioresistance, SSRS confers durable and high rates of local control in SSM without unexpected long-term toxicity rates. Increasing GTV minimum dose is significantly associated with superior local control with no corresponding increased risk of VCF.

Validation of the Prognostic Index for Spine Metastasis (PRISM) Score for Stratifying Survival in Patients Treated with Spinal Stereotactic Radiosurgery

PURPOSE/OBJECTIVE(S)

Stereotactic spinal radiosurgery (SSRS) has been increasingly utilized as a first-line treatment for the management of spine metastases due to its ability to prolong survival and improve symptom control. Studies have shown that SSRS is helpful for select patients; however, there is no universal scoring system utilized to predict patient response to treatment. The Prognostic Index for Spinal Metastases (PRISM) score was shown to predict the likelihood of patients benefiting from SSRS. We sought to further demonstrate its generalizability by performing validation with a large dataset from a second high-volume institution.

MATERIALS/METHODS

We performed a retrospective review from 2017-2019 of 424 patients treated with SSRS at a single institution. Patients were stratified on the previously described PRISM criteria: Female sex (+2), solitary bone disease (+3), performance status (0 through +3.5), prior surgery at the SSRS site (+1), number of other metastatic sites (-N), prior radiation at the SSRS site (-1), and latency to treatment ≥ 5 months (+3). Patients were grouped based on PRISM scores: >7, Group 1; 4-7, Group 2; 1-3, Group 3; <1 Group 4. There were 89, 188, 88, and 59 patients in Groups 1, 2, 3, and 4, respectively. Most patients were male (70%) with a performance status of 0 (53%). The most common tumor histologies were prostate (34%), renal (18%), and lung (11%). The median biological effective dose (BED10) was 60 Gy (interquartile range [IQR], 60-82). We performed Cox proportional hazards analysis on overall survival (OS) based on PRISM score and patient and tumor characteristics. Concordance indices created from PRISM criteria and the multivariate Cox proportional analysis were compared.

RESULTS

The median follow-up time was 50.5 months (95% confidence interval [CI], 45.8-54.7) with a median overall survival of 30.3 months (95% CI, 27.3-38.4). The median overall survivals for PRISM Groups 1, 2, 3, and 4 were 57.1, 37, 23.7, and 8.8 months, respectively. There were significant differences in overall survival among PRISM groups with hazard ratios of 0.49 (95% CI, 0.35-0.69; P<0.001) for Group 1, 0.71 (95% CI, 0.55-0.91); P<0.007) for Group 2, 1,45 (95% CI, 1.08-1.94); P = 0.010) for Group 3, and 3.47 (95% CI, 2.56-4.70; P<0.001) for Group 4. Multivariable Cox analysis for patient and tumor characteristics revealed only the number of organs involved and performance status as significant clinicopathologic prognostic attributes. However, the C-index using the PRISM criteria was 0.76, which was superior to the C-index when using the significant clinicopathologic attributes by themselves (0.71).

CONCLUSION

These data demonstrate robust validation of the PRISM score to stratify OS in patients treated with SSRS and may help guide optimal treatment selection in prospective trials and clinical settings.

Osseous Pseudoprogression in Vertebral Bodies Treated with Stereotactic Radiosurgery: A Secondary Analysis of Prospective Phase I/II Clinical Trials

BACKGROUND AND PURPOSE

Osseous pseudoprogression on MR imaging can mimic true progression in lesions treated with spine stereotactic radiosurgery. Our aim was to describe the prevalence and time course of osseous pseudoprogression to assist radiologists in the assessment of patients after spine stereotactic radiosurgery.

MATERIALS AND METHODS

A secondary analysis of 2 prospective trials was performed. MRIs before and after spine stereotactic radiosurgery were assessed for response. "Osseous pseudoprogression" was defined as transient growth in signal abnormality centered at the lesion with a sustained decline on follow-up MR imaging that was not attributable to chemotherapy.

RESULTS

From the initial set of 223 patients, 37 lesions in 36 patients met the inclusion criteria and were selected for secondary analysis. Five of the 37 lesions (14%) demonstrated osseous pseudoprogression, and 9 demonstrated progressive disease. There was a significant association between single-fraction therapy and the development of osseous pseudoprogression (P = .01), and there was a significant difference in osseous pseudoprogression-free survival between single- and multifraction regimens (P = .005). In lesions demonstrating osseous pseudoprogression, time-to-peak size occurred between 9.7 and 24.4 weeks after spine stereotactic radiosurgery (mean, 13.9 weeks; 95% CI, 8.6-19.1 weeks). The peak lesion size was between 4 and 10 mm larger than baseline. Most lesions returned to baseline size between 23 and 52.4 weeks following spine stereotactic radiosurgery.

CONCLUSIONS

Progression on MR imaging performed between 3 and 6 months following spine stereotactic radiosurgery should be treated with caution because osseous pseudoprogression may be seen in more than one-third of these lesions. Single-fraction spine stereotactic radiosurgery may be associated with osseous pseudoprogression. The possibility of osseous pseudoprogression should be incorporated into the prospective criteria for assessment of local control following spine stereotactic radiosurgery.

Prevention and reversal of experimental posthemorrhagic vasospasm by the periadventitial administration of nitric oxide from a controlled-release polymer

OBJECTIVE

Despite improvements in the care of patients with aneurysmal subarachnoid hemorrhage, delayed cerebral vasospasm remains a major cause of morbidity and death. There is now evidence that a decrease in the local availability of nitric oxide (NO) plays a role in delayed cerebral vasospasm. We evaluated a controlled-release polymer containing the NO donor (Z)-1-[2-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate (DETA/NO) for the treatment of chronic posthemorrhagic vasospasm in the rat femoral artery model.

METHODS

The release kinetics of ethylene/vinyl acetate copolymers loaded with 20% (w/w) DETA/NO were determined in vitro. Chronic vasospasm was induced in the left femoral artery of adult male Fischer 344 rats (n = 35) by exposure to autologous blood. At 1, 3, or 7 days after blood exposure, either a 5-mg polymer loaded with 20% (w/w) DETA/NO or an empty 5-mg polymer was placed in the periadventitial space next to the left femoral artery. At the same time, an empty 5-mg polymer was placed next to the right femoral artery. On the 8th day after blood exposure (at the peak of vasospasm in this model), rats were transcardially perfused with 4% paraformaldehyde, and the left and right femoral arteries were removed for histological processing and morphometric analyses. Vasospasm was expressed as the percent lumen patency of the treated left artery, compared with the control right artery.

RESULTS

The in vitro release kinetics demonstrated that the 20% DETA/NO-loaded polymers released up to 15% of their total drug load during a 9-day period. DETA/NO treatments initiated at 1, 3, or 7 days after blood deposition all significantly inhibited vasospasm, compared with control values (94.6 +/- 7.2% versus 67.6 +/- 5.8%, 104.6 +/- 5.5% versus 64.9 +/- 1.7%, and 102.4 +/- 5.1% versus 73.6 +/- 1.4%, respectively; mean +/- standard error of the mean percent lumen patency; P < 0.001). No adverse effects of treatment were observed.

CONCLUSION

The diazeniumdiolate NO donor DETA/NO can be effectively released from ethylene/vinyl acetate polymers. Administration of DETA/NO into the periadventitial space can prevent the development of chronic posthemorrhagic vasospasm in the rat femoral artery and can reverse established vasospasm. No adverse effects of DETA/NO were observed in this model.

Total en bloc lumbar spondylectomy. Case report

The authors describe a technique for total en bloc spondylectomy that can be used for lesions involving the lumbar spine. The technique involves a combined anterior-posterior approach and takes into account the unique anatomy of the lumbar spine. This technique allows for the en bloc resection of lumbar vertebral tumors, thus optimizing outcome while minimizing the risk of neurological injury. The technique is described in detail with the aid of neuroimaging studies, photographs of gross pathological specimens, and illustrations, and a discussion of other authors' experiences is provided for comparison.

Simultaneous anterior-posterior approach to the thoracic and lumbar spine for the radical resection of tumors followed by reconstruction and stabilization

OBJECT

Thoracic or lumbar spine malignant tumors involving both the anterior and posterior columns represent a complex surgical problem. The authors review the results of treating patients with these lesions in whom surgery was performed via a simultaneous anterior-posterior approach.

METHODS

The hospital records of 26 patients who underwent surgery via simultaneous combined approach for thoracic and lumbar spinal tumors at our institution from July 1994 to March 2000 were reviewed. Surgery was performed with the patients in the lateral decubitus position for the procedure. The technical details are reported. The mean survival determined by Kaplan-Meier analysis was 43.4 months for the 15 patients with primary malignant tumors and 22.5 months for the 11 patients with metastatic spinal disease. At 1 month after surgery, 23 (96%) of 24 patients who complained of pain preoperatively reported improvements (p < 0.001, Wilcoxon signed-rank test), and eight (62%) of 13 patients with preoperative neurological deficits were functionally improved (p = 0.01). There were nine major complications, five minor complications, and no deaths within 30 days of surgery. Two patients (8%) later underwent surgery for recurrent tumor.

CONCLUSIONS

The simultaneous anterior-posterior approach is a safe and feasible alternative for the exposure tumors of the thoracic and lumbar spine that involve both the anterior and posterior columns. Advantages of the approach include direct visualization of adjacent neurovascular structures, the ability to achieve complete resection of lesions involving all three columns simultaneously (optimizing hemostasis), and the ability to perform excellent dorsal and ventral stabilization in one operative session.

O6-benzylguanine potentiates the antitumor effect of locally delivered carmustine against an intracranial rat glioma

Local delivery of carmustine (BCNU) via biodegradable polymers prolongs survival against experimental brain tumors and in human clinical trials. O6-benzylguanine (O6-BG), a potent inhibitor of the DNA repair protein, O6-alkylguanine-DNA alkyltransferase (AGT), has been shown to reduce nitrosourea resistance and, thus, enhance the efficacy of systemic BCNU therapy in a variety of tumor models. In this report, we demonstrate that O6-BG can potentiate the activity of BCNU delivered intracranially via polymers in rats challenged with a lethal brain tumor. Fischer 344 rats received a lethal intracranial challenge of 100,000 F98 glioma cells (F98 cells have significant AGT activity, 328 fmol/mg protein). Five days later, animals receiving an i.p. injection of O6-BG (50 mg/kg) 2 h prior to BCNU polymer (3.8% BCNU by weight) implantation had significantly improved survival (n = 7; median survival, 34 days) over animals receiving either O6-BG alone (n = 7; median survival, 22 days; P = 0.0002) or BCNU polymer alone (n = 8; median survival, 25 days; P = 0.0001). Median survival for the control group (n = 8) was 23.5 days. Moreover, there was no physical, behavioral, or pathological evidence of treatment-related toxicity. These findings suggest that O6-BG can potentiate the effects of interstitially delivered BCNU and, for tumors expressing significant AGT, may be necessary for the BCNU to provide a meaningful therapeutic benefit. Given the clinical use of BCNU polymers against malignant gliomas, concurrent treatment with O6-BG may provide an important addition to our therapeutic armamentarium.

Paracrine delivery of IL-12 against intracranial 9L gliosarcoma in rats

OBJECT

Interleukin-12 (IL- 12) has potential for the treatment of tumors because it can stimulate an antitumor immune response and possesses antiangiogenic properties. In the study reported here, the authors investigated the therapeutic role of locally delivered IL-12 in a malignant brain tumor model.

METHODS

After genetically engineering 9L gliosarcoma cells to express IL-12 (9L-IL12 cells), the authors used these cells as a source of locally delivered cytokine. First, they investigated the behavior of these cells, which were implanted with the aid of stereotactic guidance into the rat brain, by using serial magnetic resonance imaging and histopathological examination. Second, they assessed the antitumor efficacy of proliferating, as well as nonproliferating (irradiated), 9L-IL12 cells by implanting these cells in animals challenged by wild-type 9L gliosarcoma (9Lwt) cells. The IL-12 expression in brain regions injected with 9L-IL12 was confirmed by reverse transcription-polymerase chain reaction. Last, the authors explored whether animals treated with 9L-IL12 cells developed an antitumor immunological memory by rechallenging the survivors with a second injection of 9Lwt cells. The authors demonstrated that local delivery of IL-12 into the rat brain by genetically engineered cells significantly prolongs survival time in animals challenged intracranially with a malignant glioma.

CONCLUSIONS

These findings support continued efforts to refine local delivery systems of IL-12 in an attempt to bring this therapy to clinical trials.

Late-onset facial nerve degeneration after vestibular schwannoma surgery: incidence, putative mechanisms, and prevention

Delayed facial nerve dysfunction after vestibular schwannoma surgery is a poorly understood phenomenon that has been reported to occur in 15 to 29% of patients undergoing microsurgery. It is a condition characterized by spontaneous deterioration of facial nerve function in a patient who has otherwise normal or near-normal facial function in the immediate postoperative period. This delayed paralysis is generally reported to occur in the first few days postsurgery, with the majority of patients eventually recovering their immediate postoperative facial function. However, infrequently, it can also occur more than 1 week after surgery (so-called late-onset facial nerve palsy). The authors reviewed facial nerve outcome in 611 patients who underwent microsurgery between 1973 and 1994. The facial nerve was anatomically preserved in 596 patients (97.5%), and 90% of patients had House-Brackmann[6] Grade 1 or 2 function 1 year after surgery. Late-onset facial dysfunction was seen in 13 patients (2.1%). All of these had significant deterioration in facial nerve function between 1 and 4 weeks postoperatively, and all showed improvement by 1 year. In this study, the focus on these patients who developed late-onset facial palsy. The incidence, treatment strategies, and outcomes will be discussed with emphasis on possible pathophysiological mechanisms that contribute to this relatively rare condition.

Cultured olfactory interneurons from Limax maximus: optical and electrophysiological studies of transmitter-evoked responses

1. The olfactory processing network in the procerebral (PC) lobe of the terrestrial mollusk Limax maximus exhibits a coherent oscillation of local field potential that is modulated by odor input. To understand the cellular basis of this oscillation, we developed a cell culture preparation of isolated PC neurons and studied the responses of isolated cells to stimulation with neurotransmitters known to be present in the PC lobe. 2. The distribution of PC soma diameters suggests at least two different populations of neurons. Approximately 95% of isolated cells had soma diameters of 7-8 microns, with the remaining cells having larger diameters (10-15 microns). 3. Extracellular measurements of action potentials and optical measurements of intracellular calcium concentrations in fura-2-loaded cells were made. Serotonin and dopamine excited PC neurons and promoted transitions from steady to bursty activity. Both amines elicited increases in intracellular calcium, presumably concomitant with the increase in action-potential frequency. 4. Glutamate suppressed action-potential firing and reduced intracellular calcium. This effect was seen most clearly when glutamate was applied to cells excited by high potassium medium. Quisqualate is an effective glutamate agonist in this system, whereas kainate is not. 5. Combined with anatomic and biochemical data and with studies of the effects of these neurotransmitters on the oscillating local field potential of the intact PC network, the data from isolated PC neurons are consistent with the hypothesis that dopamine and serotonin modulate network dynamics, whereas glutamate is involved in generating the basic oscillation of local field potential in the PC. 6. The optical studies of fura-2-loaded cells showed that several treatments that increase the rate of action-potential production lead to elevations in intracellular calcium. Optical studies of intracellular calcium may be useful for multisite measurements of activity in the intact, oscillating PC lobe network.

Coherent network oscillations by olfactory interneurons: modulation by endogenous amines

1. The procerebral (PC) lobe of the terrestrial mollusk Limax maximus contains a highly interconnected network of local olfactory interneurons that receives direct axonal projections from the two pairs of noses. This olfactory processing network generates a 0.7-Hz oscillation in its local field potential (LFP) that is coherent throughout the network. The oscillating LFP is modulated by natural odorants applied to the neuroepithelium of the superior nose. 2. Two amines known to be present in the PC lobe, dopamine and serotonin, increase the frequency of the PC lobe oscillation and alter its waveform. 3. Glutamate, another putative neurotransmitter known to be present in the lobe, suppresses the PC lobe oscillation by a quisqualate-type receptor and appears to be used by one of the two classes of neurons in the PC lobe to generate the basic LFP oscillation. 4. The known activation of second messengers in Limax PC lobe by dopamine and serotonin together with their effects on the oscillatory rhythm suggest the hypothesis that these amines augment mechanisms mediating synaptic plasticity in the olfactory network, similar to hypothesized effects of amines in vertebrate olfactory systems. 5. The use of a distributed network of interneurons showing coherent oscillations may relate to the highly developed odor recognition and odor learning ability of Limax.