Abstract P4-19-01: Attitudes of medical oncologists towards research biopsies

Background:

There is increasing interest in studying tissue from patients (pts) with metastatic breast cancer (MBC). Historically, limited tissue has been available. Possible barriers to research biopsies (bx) include pt and provider opinions; the contribution of each factor is unknown.

Methods:

309 academic breast medical oncologists (MOs) identified from the websites of each of the National Cancer Institute - designated cancer centers were invited to complete either a self-administered paper or online survey. Eligible MOs (MOs who saw breast cancer pts and who saw pts 4 hours/week.) were asked to predict what proportion of their pts with MBC would consent to additional bx (ABs, additional bx performed with a clinically indicated bx) or research purposes only bx(RPOBs, research bx performed as a standalone procedure). They were also asked about their comfort levels in asking pts with MBC to consider participating in ABs or RPOBs for various organs. Median values are reported. Two-sided Fisher's exact test was used to compare categorical variables using a a level of .05.

Results:

191 (101F,85M, 5 unknown) eligible MOs completed the survey. 29 MOs were ineligible (response rate = 191/280,68%). Median age was 50 (Range 33-80). Median years of oncology experience was 15 (Range 1-45). MOs predicted that 90%, 75%, 70% and 50%, of their pts would definitely/probably consider ABs of blood, skin, breast, or liver respectively. MOs predicted that 90%, 60%, 33%, and 20% of their pts would definitely/probably consider RPOBs of blood, skin, breast, or liver. 98% (95% CI 96%-100%), 96% (95% CI 92%-98%), 93% (95% CI 88%-96%) and 70% (95% CI 63%-77%) of MOs were very/somewhat comfortable asking pts for an AB of blood, skin, breast and liver respectively. 98% (95% CI 95%-99%), 93% (95% CI 89%-96%), 78% (95% CI 72%-84%) and 50% (95% CI 43%-58%) of MOs were very/somewhat comfortable asking pts to participate in a RPOB of blood, skin, breast and liver respectively.

No demographic characteristics (eg. sex, age) were associated with MOs’ comfort levels of asking pts to have an AB of blood, skin and breast.

Factors associated with increased comfort discussing an AB of the liver were: age < 50 years (p = 0.01), in practice for < 15 years (p = 0.01), ≥ 1 pt enrolled on clinical trials per month (p = 0.02), or having pts who had undergone bx for research in prior 3 months (p<0.01).

MOs with ≥ 4 patients enrolled on clinical trials/month or whose pts had undergone research bx in the past 3 months were more likely to feel comfortable asking pts to have a RPOB of the breast (p<0.01; p<0.01) or liver (p = 0.03; p<0.01).

The 3 most common reasons why MOs were reluctant to refer pts for participation in an AB include risk of a bx procedure (n = 128, 67%), pain/discomfort of a bx (n = 125, 65%), and logistical barriers (n = 42, 22%). These reasons are similar for RPOB; risk of a bx procedure, (n = 137, 72%), pain/discomfort of a bx (n = 134, 70%), and inconvenience to pt (time involved, travel, etc) (n = 58, 30%).

Conclusions:

Many MOs predict that the majority of their MBC pts will consider ABs of various organs. However, this decreases with RPOBs, particularly as the procedure becomes more invasive. More research is needed to understand factors that may influence MOs’ comfort levels asking pts to participate in such studies.

Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P4-19-01.

Attitudes of patients with metastatic breast cancer toward research biopsies

BACKGROUND

Research studies involving human tissue are increasingly common. However, patients' attitudes toward research biopsies are not well characterized, particularly when the biopsies are carried out outside the context of therapeutic trials.

PATIENTS AND METHODS

One hundred sixty patients with metastatic breast cancer (MBC) from two academic (n = 80) and two community (n = 80) hospitals completed a 29-item self-administered survey to evaluate their willingness to consider providing research purposes only biopsies (RPOBs) (as a stand-alone procedure) and additional biopsies (ABs) (additional needle passes at the time of a clinically indicated biopsy).

RESULTS

Eighty-two (51%) of 160 patients would consider having RPOBs, of which 42 (53%) and 40 (50%) patients were from academic and community hospitals, respectively. Patients who had more prior biopsies were less likely to consider RPOBs (RR = 0.6, 95% CI: 0.4-1.0, P = 0.03). Of 160 patients, 115 (72%) patients would consider having ABs. Of these, 64 (80%) and 51 (64%) patients from academic and community hospitals, respectively, would consider ABs (RR = 1.2, 95% CI: 1.0-1.5, P = 0.03).

CONCLUSIONS

Many patients with MBC in both academic and community settings report willingness to consider undergoing biopsies for research. Further research is needed to understand ethical, logistical and provider-based barriers to broader participation in such studies.

A phase II study of ixabepilone and trastuzumab for metastatic HER2-positive breast cancer

BACKGROUND

A multicenter NCI-sponsored phase II study was conducted to analyze the safety and efficacy of the combination of ixabepilone with trastuzumab in patients with metastatic HER2-positive breast cancer.

PATIENTS AND METHODS

Two cohorts were enrolled: cohort 1 had received no prior chemotherapy or trastuzumab for metastatic disease and cohort 2 had received 1-2 prior trastuzumab-containing regimens for metastatic disease. Patients in both cohorts received ixabepilone 40 mg/m(2) as a 3-h infusion and trastuzumab on day 1 of a 21-day cycle. Tumor biomarkers that may predict response to trastuzumab were explored.

RESULTS

Thirty-nine women entered the study with 15 patients in cohort 1 and 24 patients in cohort 2. Across both cohorts, the overall RR was 44%, with a clinical benefit rate (CR + PR + SD for at least 24 weeks) of 56%. Treatment-related toxic effects included neuropathy (grade ≥2, 56%), leukopenia (grade ≥2, 26%), myalgias (grade ≥2, 21%), neutropenia (grade ≥2, 23%), and anemia (grade ≥2, 18%).

CONCLUSIONS

This represents the first study of the combination of ixabepilone with trastuzumab for the treatment of metastatic HER2-positive breast cancer. These results suggest that the combination has encouraging activity as first and subsequent line therapy for metastatic breast cancer.

Abstract P2-16-04: Attitudes of metastatic breast cancer patients towards research biopsies

Background: In the era of molecularly targeted therapy, developing an understanding of the molecular basis of cancer is a principal or secondary goal of many research studies. For this reason, studies collecting tissue for research purposes are increasingly common. Understanding patients' attitudes towards research biopsies may lead to improvement in accrual to research biopsy studies.

Methods: Patients with metastatic breast cancer from two academic and two community hospitals completed a self-administered paper survey consisting of 29 questions in clinic to evaluate their willingness to consider providing additional biopsies (additional biopsy performed with a clinically indicated biopsy) and research purposes only biopsies (RPOB) (research biopsy performed as a stand alone procedure).

Results: 160 patients (n = 80 academic, n=80 community) completed the survey, with a response rate of 98%. As expected, demographic variables differed between sites, with patients from academic sites likely to be younger (p = 0.01), more educated (p = 0.002), employed (p = 0.01), have prior trial participation (P <0.001) and have a longer travel time (P <0.0001). 64 (80%) academic patients and 51 (64%) community patients would definitely or probably consider additional biopsies. 42 (53%) academic patients and 40 (50%) community patients would consider RPOB.

In univariate analyses of patients' willingness to have additional biopsies, patients in academic sites were more likely to agree to additional biopsies than those at community sites (RR = 1.2, 95% CI 1.0–1.5, p = 0.03). Statistically significant differences based on demographic characteristics such as age, education, marital status, prior trial participation, number of prior biopsies, and travel time were not observed.

For RPOB, patients having had more prior biopsies were less likely to consider research biopsies (RR = 0.6, 95% CI 0.4–1.0, p = 0.03). The following variables did not reach statistical significance: type of practice, age, education, marital status, prior trial participation, and travel time.

Patients' willingness in both academic and community sites to consider RPOB declined with more invasive biopsies. Although differences were observed, none were statistically significant between academic and community; skin (56%, 65%), bone marrow (30%, 27%), breast (43%, 49%) or liver (24%, 19%).

Of the 13/160 (8%) patients who would not consider additional biopsies, the most common reasons cited included pain or discomfort (n = 8/13, 62%), risk of biopsy (n = 8/13, 62%) and anxiety related to the biopsy (n = 6/13, 46%). Of the 37/160 (23%) patients who would not consider RPOB, the most common reasons cited included pain or discomfort (n = 23/37, 62%), risk of biopsy (n = 15/37, 41%) and inconvenience of the procedure to the patient (n = 13/37, 35%).

Conclusions: The majority of patients in this study indicated they would consider research biopsies, with a larger proportion willing to consider additional biopsies; patients seen at academic hospitals were more likely to consider additional biopsies compared to those seen at community hospitals. Breast cancer patients' willingness to undergo research biopsies may be higher than generally expected by clinicians and may not be the primary barrier to obtaining research biopsies.

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P2-16-04.

Responses to subsequent anti-HER2 therapy after treatment with trastuzumab-DM1 in women with HER2-positive metastatic breast cancer

BACKGROUND

Women with human epidermal growth factor receptor 2 (HER2)-positive metastatic breast cancer (MBC) can respond to multiple lines of anti-HER2 therapy. It is unknown whether these patients will derive further clinical benefit following treatment with trastuzumab-MCC-DM1 (T-DM1).

PATIENTS AND METHODS

We retrospectively identified HER2-positive MBC patients treated with T-DM1 and characterized outcomes during subsequent lines of anti-HER2 therapy. Response was determined by a blinded radiology review. Time-dependent analyses were carried out using Kaplan-Meier estimates.

RESULTS

We identified 23 patients treated with single-agent T-DM1 and report on the 20 patients who discontinued protocol therapy. All patients received trastuzumab-based metastatic therapy before initiation of T-DM1 [median 7 regimens (range 3-14)]. Of these 20 patients, 75% (15 of 20) received further therapy with or without anti-HER2 agents after discontinuing T-DM1. Partial response to either first- or second-subsequent line(s) of therapy was seen in 5 of 15 (33%) treated patients, including 33% (4 of 12) who received a regimen containing trastuzumab and/or lapatinib. Median durations of therapy to first- and second-subsequent regimens after T-DM1 were 5.5 and 6.4 months, respectively.

CONCLUSIONS

In heavily pretreated HER2-positive MBC patients, prior exposure to T-DM1 does not exhaust the potential benefit of ongoing anti-HER2 therapy with trastuzumab- and/or lapatinib-based regimens.

PD05-07: Prospective Validation and Characterization of HER2 Positive Circulating Tumor Cells in Patients with HER2 Negative Metastatic Breast Cancer

Background: Circulating tumor cells (CTCs) with evidence of HER2 amplification can occur in patients (pts) with clinically HER2 negative metastatic breast cancer. While these findings potentially have profound implications for CTCs as a biomarker for treatment, prospective validation and characterization of this subgroup is necessary.

Methods: We created a prospective cohort of pts with metastatic breast cancer that were HER2 negative by IHC and/or FISH on all available primary and metastatic biopsies. Blood samples were collected at study entry and then again at ≥ 3 weeks if available. CTCs were enumerated by a modification of the Veridex CellSearch Profile kit. FISH was performed on each CTC sample and reported as positive if the HER2/CEP17 ratio was ≥ 2.0. Analyses are descriptive.

Results: 66 pts were consented for study and this report includes the 65 pts with detectable CTCs. Median number of CTCs was 226 (range 112 to > 3000). At initial testing, 23 pts (35%) had HER2 positive CTCs, median HER2:CEP17 ratio of 3.4. 50% (11 of 22) of the pts with lobular or ductal/lobular histology had HER2 amplified CTCs, compared to only 27% (10 of 36) of patients with ductal histology. Women with ER positive disease had HER2 positive CTCs in 40% of cases (20 of 49) compared to 19% of ER negative pts (3 of 16). To assess concordance of HER2 amplification of CTCs over time, 34 pts consented to be retested at a median 5.9 weeks after initial screening (range 3.3 - 17 weeks) and all but 1 had detectable CTCs. Baseline characteristics of these 34 pts were similar to the original population, with HER2 amplified CTCs detected in 35% (12 of 34) pts at initial screening. HER2 positive CTCs were concordant at time of retesting in 83% (10 of 12) pts; the 2 women with discordant CTCs were receiving HER2 directed therapy. Of the pts with HER2 negative CTCs at initial screening, 81% (17 of 21) continued to have HER2 negative CTCs at time of retesting.

Conclusion: We observed a higher prevalence of HER2 positive CTCs among pts with ER positive disease and evidence of lobular histology. The presence of HER2 positive CTCs is concordant over time in the majority of pts. The functional significance of HER2 positive CTCs in patients with clinically HER2 negative breast cancer will be tested in a prospective study with HER2−directed therapy.

Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr PD05-07.

P2-09-07: Metabolic Response by FDG-PET in Patients (pts) Receiving Trastuzumab (T) and Lapatinib (L) for HER2+ Metastatic Breast Cancer (MBC): Correlative Analysis of TBCRC 003

Background

We evaluated the safety and efficacy of L+T in pts with 0–2 prior lines of chemotherapy (CT) for HER2+ MBC. In the context of this phase II trial, we evaluated metabolic response by FDG-PET and explored the relationship between metabolic response and clinical outcomes.

Methods: Pts with measurable, HER2+ MBC were eligible. Cohort 1: No prior T, L, or CT +T for MBC, and >1 yr from adjuvant T, if received. Cohort 2: 1–2 prior lines of CT for MBC, including T, or relapse within 1 yr of adjuvant T. Pts received L 1,000 mg QD + T (2 mg/kg weekly or 6 mg/kg Q3W). Staging studies were done with CT or MRI at baseline (BL) and every 2 cycles (1 cycle=4 weeks [wks]). Objective response was assessed by local investigator according to RECIST 1.0. FDG-PET/CT was performed at BL, Wk 1, and Wk 8 per NCI guidelines. Central quality assurance, review, and analysis were performed on FDG-PET studies. Up to 5 target lesions were identified on BL FDG-PET images based on hypermetabolic uptake. Percent change in the summed maximum standardized uptake value (SUVmax) of target lesions was calculated at Wk 1 or Wk 8, compared to BL. Metabolic response was assessed according to EORTC criteria for % change in SUVmax (progressive disease [PD]: ≥25% increase; partial response [PR]: ≥25% decrease; stable disease [SD]: <25% change). Metabolic response at Wk 1 was compared to Wk 8 as well as to clinical outcome, including objective response, clinical benefit, and progression-free survival (PFS).

Results: 87 pts were registered to the study. Of these, one pt did not begin protocol therapy and one pt did not have MBC on further testing, and are not included. 81/85 pts had FDG-PET data at Wk 1; 75/85 had data at Wk 8. Metabolic PR at Wk 1 was observed in 28/39 (72%) pts in Cohort 1 and 20/42 (48%) pts in Cohort 2. Metabolic PR at Wk 8 was observed in 27/34 (79%) pts in Cohort 1 and 18/41 (44%) pts in Cohort 2. Wk 1 and Wk 8 metabolic responses were similar. In cohort 1, 18/28 (64%) pts who achieved Wk 1 metabolic PR had clinical benefit by RECIST. Of pts with Wk 1 metabolic SD, 2/9 (22%) had clinical benefit. In cohort 2, 9/20 (45%) pts who achieved Wk 1 metabolic PR had clinical benefit; 5/22 (23%) who achieved Week 1 metabolic SD had clinical benefit. Exploratory analysis of progression-free survival (PFS) showed that pts in Cohort 1 who achieved Wk 1 metabolic PR experienced a median PFS of 9.3 months ([mos]; 95% CI 5.6−22.3); for pts with metabolic SD, median PFS was 1.9 mos (95% CI 0.8−5.5). For pts in Cohort 2, Wk 1 metabolic PR was associated with median PFS of 5.6 mos (95% CI 3.7−7.8), whereas for pts with metabolic SD, median PFS was 3.7 mos (95% CI 1.8−5.5).

Conclusions: L+T is associated with a high rate of early and sustained metabolic response by FDG-PET. Exploratory analyses suggest that metabolic PR may be associated with clinical benefit and longer PFS.

Abstract P3-14-08: Responses to Subsequent Anti-HER2 Therapy after Treatment with Trastuzumab-DM1 in Women with HER2-Positive Metastatic Breast Cancer

Background: Women with HER2-positive metastatic breast cancer (MBC) can have clinical responses to multiple lines of anti-HER2 therapy. However, it is unknown whether these patients will derive further clinical benefit following treatment with the novel antibody-drug conjugate trastuzumab-MCC-DM1 (T-DM1), which has significant activity in trastuzumab-refractory breast cancer.

Methods: We retrospectively identified patients treated with T-DM1 monotherapy on clinical trials for HER2-positive MBC at Dana-Farber Cancer Institute and characterized clinical outcomes during subsequent lines of anti-HER2 therapy. Response was determined by an independent and blinded radiology review using modified RECIST 1.1 criteria without confirmatory scans; patients without radiologic assessment were considered non-responders. Duration of therapy was defined from initiation of therapy until treatment discontinuation, and for patients continuing on therapy, times were censored at date of last visit; analysis was performed using Kaplan-Meier estimates.

Results: We identified 23 patients treated on protocol-based therapy with single-agent TDM-1 and report on the 20 patients who had discontinued protocol, and hence, T-DM1 therapy. All patients received trastuzumab in the metastatic setting prior to initiation of T-DM1, with a median number of 6 (range 1-14) prior regimens. Most (75%) were taken off study secondary to progressive disease and the remainder discontinued therapy for toxicity. Of these 20 patients, 75% (15 of 20) received further therapy with or without anti-HER2 agents after concluding T-DM1; reasons for not receiving additional treatment after T-DM1 include death (3) and interruption of therapy due to physician or patient request (2). The majority (12 of 15; 80%) of patients treated beyond T-DM1 received a regimen containing either trastuzumab and/or lapatinib at some point during their course. Partial response to either first-or second-subsequent line(s) of therapy was seen in 5 of 15 (33%) treated patients, including 33% (4 of 12) who received a regimen containing trastuzumab and/or lapatinib (Figure 1). Three patients did not have radiographic assessments; however, all 3 demonstrated clinically stable disease (as determined by review of clinical data) to first treatment after T-DM1. Median duration of therapy to first-subsequent regimen after T-DM1 was 5.5 months. Of the 9 patients that received a second-subsequent regimen, the median duration of therapy to the second-subsequent treatment was 6.4 months.

Conclusions: In heavily pretreated HER2-positive MBC patients, prior exposure to T-DM1 does not exhaust the potential benefit of ongoing anti-HER2 therapy with trastuzumab and/or lapatinib-based regimens.

Maximum Decrease in Target Lesion Diameter

Abstract PD08-09: Impact of a Telephone-Based Exercise Intervention on Physical Activity Behaviors and Fitness in a Cooperative Group Setting

Observational studies have demonstrated a 30-50% lower risk of disease-specific and overall mortality in physically active breast and colorectal cancer patients as compared to sedentary individuals. However, there have been no randomized trials looking at the impact of physical activity on cancer outcomes, and the optimal design of such a trial is not yet well-defined. The Active After Cancer Trial (AACT) is a multicenter feasibility study designed to evaluate the ability of a telephone-based intervention to increase physical activity in patients with breast and colorectal cancer. Methods: Sedentary (reporting less than 60 minutes/week of recreational activity) individuals with stage I-III breast or colorectal cancer were eligible for enrollment after completion of all adjuvant chemotherapy and radiation. Participants were randomized 1:1 to a centralized telephone coaching intervention, with a target goal of 180 minutes/week of physical activity, or to a usual care control group. Intervention participants received an average of 10 telephone contacts over 16-weeks. Initial calls focused upon building self-efficacy and later calls concentrated upon relapse prevention and maintenance of exercise behaviors. Participants underwent assessment of physical activity behaviors (7-Day Physical Activity Recall), fitness (6-Minute Walk Test), physical functioning (EORTC QLQ C-30), fatigue (FACIT) and exercise self-efficacy at baseline and 16 weeks after enrollment.

Results: One hundred and twenty-one patients were enrolled through 10 Cancer and Leukemia Group B (CALGB) institutions; 100 patients had breast cancer and 21 had colorectal cancer. Average age was 54.3, 74% of patients had received chemotherapy and mean time since completion of adjuvant treatment was 24 months. Participants randomized to the exercise group experienced significant improvements in fitness and physical functioning as compared to controls.

Table. Baseline measures and change (post-minus pre) scores in intervention and control patients (±SD)

Intervention participants also reported a doubling in minutes of weekly physical activity, but this was not a significant increase compared to controls.

Conclusions: Sedentary breast and colorectal cancer survivors can be enrolled in a physical activity intervention. A centralized telephone coaching intervention successfully increased fitness and physical functioning, although self-reported exercise time was not significantly changed. A large-scale clinical trial within the co-operative groups is feasible.

A phase I study of an autologous GM-CSF-secreting breast cancer vaccine

Abstract #4124

Background: Vaccination with tumor cells engineered to secrete granulocyte-macrophage colony stimulating factor (GM-CSF) generates potent, specific, and long-lasting anti-tumor immunity in multiple tumor models. At DF/HCC, over 100 patients with melanoma, ovarian cancer, and lung cancer have been vaccinated. These studies have led to the identification of targets of immune-mediated tumor destruction. Here, we present analysis on the safety, feasibility, and biologic activity of an autologous GM-CSF-secreting breast cancer vaccine.
 Methods:27 patients with metastatic breast cancer have undergone tumor procurement for vaccine from malignant pleural effusions (23), ascites (1), tumor nodules (2), lymph node (1), and 12 patients have received vaccine. Cells were transduced with a replication defective adenoviral vector encoding GM-CSF, irradiated, and GM-CSF secretion was measured by ELISA. Patients were required to have received at least one prior chemotherapy for metastatic disease and have an ECOG performance status 0-1. Vaccine was delivered s.c. and i.d. weekly for three weeks, then every other week. Immune monitoring included skin biopsies of vaccine sites, measurement of leukocyte populations, and proteomic-based assessment of antibody responses.
 Results: Sufficient viable tumor cell yields for vaccination were obtained from all patients, with cellular yields ranging from 1.1-679 x 106 cells and viabilities ranging from 71-100%. Dose levels were based on cellular yield, ranging from 105 -107 cells/dose. The average yield of GM-CSF was 838 ng/106 cells/24 hrs (range: 24.4-5696), higher than the average yields for lung cancer and melanoma. Vaccinated patients were 34-69 years, and received 3-23 vaccines. 9 patients had progressive disease after 3-10 vaccines (1-4 months). One patient had stable disease after 23 vaccines (11 months), resumed vaccination at 16 months for progression and remains on study. One other patient remains on study, and one patient did not have measurable disease. Toxicities related to treatment include grade I/II local injection-site reactions, grade I/II inflammation at tumor sites as well as grade I/II fatigue, fever, nausea, and edema. Skin biopsies of vaccination sites revealed mild-moderate infiltration of lymphocytes, granulocytes, and macrophages.
 Conclusion: Breast cancer cells can be harvested in sufficient number for autologous vaccine production from solid tumor as well as from malignant effusions. Autologous vaccination can induce coordinated immune responses with limited toxicity. The proteomic-based identification of antigen-specific immune responses following vaccination will be presented.

Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 4124.

The development of a protoplanetary disk from its natal envelope

Class 0 protostars, the youngest type of young stellar objects, show many signs of rapid development from their initial, spheroidal configurations, and therefore are studied intensively for details of the formation of protoplanetary disks within protostellar envelopes. At millimetre wavelengths, kinematic signatures of collapse have been observed in several such protostars, through observations of molecular lines that probe their outer envelopes. It has been suggested that one or more components of the proto-multiple system NGC 1333-IRAS 4 (refs 1, 2) may display signs of an embedded region that is warmer and denser than the bulk of the envelope. Here we report observations that reveal details of the core on Solar System dimensions. We detect in NGC 1333-IRAS 4B a rich emission spectrum of H2O, at wavelengths 20-37 microm, which indicates an origin in extremely dense, warm gas. We can model the emission as infall from a protostellar envelope onto the surface of a deeply embedded, dense disk, and therefore see the development of a protoplanetary disk. This is the only example of mid-infrared water emission from a sample of 30 class 0 objects, perhaps arising from a favourable orientation; alternatively, this may be an early and short-lived stage in the evolution of a protoplanetary disk.

Sensitivity study on hydraulic well testing inversion using simulated annealing

Cluster variable aperture (CVA) simulated annealing has been used as an inversion technique to construct fluid flow models of fractured formations based on transient pressure data from hydraulic tests. A two-dimensional fracture network system is represented as a filled regular lattice of fracture elements. The algorithm iteratively changes element apertures for a cluster of fracture elements in order to improve the match to observed pressure transients. Aperture size is chosen randomly from a list of discrete apertures. The cluster size is held constant throughout the iterations. Since hydraulic inversion is inherently nonunique, it is important to use additional information. We investigated the relationship between the scale of heterogeneity and the optimal cluster size and shape to enhance convergence of the inversion and improve the results. In a spatially correlated transmissivity field, a cluster size corresponding to about 20 % to 40 % of the practical range of the spatial correlation is optimal. Inversion results of the Raymond test site data are also presented and based on an optimal cluster size; the practical range of the spatial correlation is estimated to be 5 to 10 m.