Variables Associated With Bone Biopsy Tumor Yield in mCRPC
Variables Associated With Bone Biopsy Tumor Yield in mCRPC
The need to interrogate the cellular and molecular nature of cancer is becoming increasingly important to understand emerging resistance mechanisms. Difficulties in obtaining tumor tissue from bone metastases have blunted efforts to characterize the genomic landscape in mCRPC. Given the importance of quality specimen collection, challenges associated with biopsy and patient effort, we sought to describe variables that may predict increased tumor yield on bone biopsy. This analysis was exploratory in nature, but importantly sets the stage for future efforts to collect tissue for projects that aim to molecularly characterize mCRPC tumors to direct individual therapy.
We hypothesized that imaging variables associated with increased ease of biopsy would be associated with increased yield. In our analysis, lesion size and distance from the skin to the lesion were the two imaging variables which most strongly correlated with biopsy yield. This is consistent with prior data demonstrating that bone and soft-tissue lesions >5 cm result in increased diagnostic yield. Lesion size and distance from the skin to the lesion warrant consideration when selecting the optimal bone biopsy site. We anticipated that biopsy of vertebral body lesions, which requires passage through the vertebral pedicle, would be more technically challenging resulting in decreased tumor yield. However, in our analysis, only five spine biopsies were performed, likely related to selection bias by the performing radiologist, limiting our ability to detect meaningful differences between groups. On the basis of prior studies, we predicted that sclerotic lesions would result in lower yield. Although we did not show a statistically significant correlation between attenuation and tumor yield, likely related to the small sample size of this cohort, lesions with greater attenuation (>400 HU) did have decreased PPC. In a retrospective analysis of 410 CT-guided biopsies of osseous lesions, imaging appearance was associated with biopsy accuracy (accuracy for sclerotic, lytic or mixed lytic–sclerotic was 76%, 93% and 92%, respectively). This may be secondary to decreased cellularity associated with sclerotic lesions and increased risk of crush artifact, hampering diagnostic yield. In addition, we hypothesized that increased radiotracer uptake on bone scan, suggestive of higher osteoblastic turnover and active disease, would be associated with increased yield. Our analysis did suggest such a trend and, we believe, is the first to evaluate the value of this variable.
Furthermore, we hypothesized that peripheral lesion sampling, as opposed to central sampling, would result in increased tumor acquisition, given presumed increased cellularity at the stromal–tumor interface. However, in our limited cohort we did not observe this correlation. Of the six biopsies with only peripheral sampling, 3/15 cores contained tumor. Explanations include difficulty of peripheral targeting and inability to ascertain the exact location of sampling retrospectively. A previous study demonstrated that peripheral lesion sampling may be associated with increased yield (78% versus 52% for peripheral versus central sampling, P=0.24).
In addition, in our study, increased sampling seemed to correlate with decreased tumor yield. The exact reason for increased sampling was not specified in the procedure note; however, difficulty of biopsy could have resulted in increased biopsy attempts. Wu et al. previously described that the cumulative diagnostic yield for bone biopsy reached a plateau at the third specimen.
With regard to soft tissue, all biopsies had at least one core that was positive for tumor. Although we did not review the variables associated with tumor yield for soft tissue lesions, overall pathologic yield was greater than that observed with bone. Given such, we recommend soft tissue as the preferred site of biopsy for molecular analyses, understanding that the biology of soft tissue and bone metastasis may differ and remains an unanswered question to be explored in future studies.
We hypothesized that clinical parameters associated with higher disease burden would predict increased yield, including increased disease sites, PSA, alkaline phosphatase and calcium, and lower hemoglobin and platelets. Higher calcium seemed to correlate with increased biopsy yield; however, a correlation was not observed across the remaining clinical parameters. The lack of correlation may be, at least in part, due to our relatively small sample size and the fact that men were enrolled on a clinical trial requiring normal organ function and good performance status, limiting our ability to detect small differences between groups. In a trial of undirected bone marrow biopsies in mCRPC, lower hemoglobin and higher alkaline phosphatase and lactate dehydrogenase were associated with an increased likelihood of a positive biopsy. In addition, survival was shorter in men with a positive biopsy, suggesting these individuals had more advanced disease.
Given that men with prostate cancer are susceptible to skeletal complications secondary to treatment-associated bone loss and bone metastases, osteoclast-targeting agents are frequently utilized. Bisphosphonates, analogues of pyrophosphate, have an inhibitory effect on osteoclasts, which are responsible for bone resorption. Denosumab, a monoclonal antibody against the receptor activator of nuclear factor-κβ ligand, prevents osteoclast function. We hypothesized that treatment with an osteoclast-targeting agent would result in decreased tumor yield, given the risk of osseous sclerosis associated with therapy. We observed that longer duration of therapy seemed to correlate with decreased yield. In a prior analysis, zoledronic acid therapy for ≥1 year resulted in a decreased number of positive bone biopsies (50%) compared with patients who did not receive therapy or received <1 year of treatment (74%; P=0.27). The direct effect of osteoclast-targeting agents in patients undergoing bone biopsy needs to be further explored.
Given the exploratory nature of our analysis, several limitations should be highlighted. Although samples were collected prospectively, imaging and procedural variables were analyzed retrospectively. In addition, our sample size was relatively small, limiting our ability to identify small differences between the variables analyzed. Biopsy site selection, needle gauge and number of cores obtained were at the discretion of the radiologist performing the biopsy, potentially resulting in selection bias. Our analysis was not comprehensive of all possible factors that can influence tumor yield. Other metrics to be considered include biopsy of 'new' (within 3–6 months) versus 'old' lesions, total number of needle passes, volume of tissue collected and evidence of necrosis. Lastly, we do not report nucleic acid quality/quantity or success of molecular analysis, which are important and should be included in future analyses. Unlike diagnostic biopsies, biopsies performed for tumor molecular analysis require special handling, given that bone is not decalcified during processing and occasionally laser capture microdissection is performed to increase sample purity. These additional steps have the potential to impact the success of molecular sequencing.
As molecular characterization of metastatic tumors becomes increasingly important, there is a pressing need to establish guidelines to improve biopsy yield. In this exploratory analysis, we identified several variables associated with increased tumor yield on bone biopsy. As we embark on a large multi-institutional project that aims to molecularly characterize mCRPC tumors, we have developed the following guidelines to optimize tumor yield in our patients undergoing biopsies as part of this endeavor:
Larger studies are needed to develop the optimal algorithm for successful bone biopsy, specifically biopsies with the goal of downstream molecular analysis. Given such, we have incorporated a prospective analysis of the variables described here as part of a SU2C multi-institutional project. Furthermore, a critical next step is correlating these parameters with successful genome sequencing, which is the ultimate goal of these efforts as we enter the era of personalized medicine.
Discussion
The need to interrogate the cellular and molecular nature of cancer is becoming increasingly important to understand emerging resistance mechanisms. Difficulties in obtaining tumor tissue from bone metastases have blunted efforts to characterize the genomic landscape in mCRPC. Given the importance of quality specimen collection, challenges associated with biopsy and patient effort, we sought to describe variables that may predict increased tumor yield on bone biopsy. This analysis was exploratory in nature, but importantly sets the stage for future efforts to collect tissue for projects that aim to molecularly characterize mCRPC tumors to direct individual therapy.
We hypothesized that imaging variables associated with increased ease of biopsy would be associated with increased yield. In our analysis, lesion size and distance from the skin to the lesion were the two imaging variables which most strongly correlated with biopsy yield. This is consistent with prior data demonstrating that bone and soft-tissue lesions >5 cm result in increased diagnostic yield. Lesion size and distance from the skin to the lesion warrant consideration when selecting the optimal bone biopsy site. We anticipated that biopsy of vertebral body lesions, which requires passage through the vertebral pedicle, would be more technically challenging resulting in decreased tumor yield. However, in our analysis, only five spine biopsies were performed, likely related to selection bias by the performing radiologist, limiting our ability to detect meaningful differences between groups. On the basis of prior studies, we predicted that sclerotic lesions would result in lower yield. Although we did not show a statistically significant correlation between attenuation and tumor yield, likely related to the small sample size of this cohort, lesions with greater attenuation (>400 HU) did have decreased PPC. In a retrospective analysis of 410 CT-guided biopsies of osseous lesions, imaging appearance was associated with biopsy accuracy (accuracy for sclerotic, lytic or mixed lytic–sclerotic was 76%, 93% and 92%, respectively). This may be secondary to decreased cellularity associated with sclerotic lesions and increased risk of crush artifact, hampering diagnostic yield. In addition, we hypothesized that increased radiotracer uptake on bone scan, suggestive of higher osteoblastic turnover and active disease, would be associated with increased yield. Our analysis did suggest such a trend and, we believe, is the first to evaluate the value of this variable.
Furthermore, we hypothesized that peripheral lesion sampling, as opposed to central sampling, would result in increased tumor acquisition, given presumed increased cellularity at the stromal–tumor interface. However, in our limited cohort we did not observe this correlation. Of the six biopsies with only peripheral sampling, 3/15 cores contained tumor. Explanations include difficulty of peripheral targeting and inability to ascertain the exact location of sampling retrospectively. A previous study demonstrated that peripheral lesion sampling may be associated with increased yield (78% versus 52% for peripheral versus central sampling, P=0.24).
In addition, in our study, increased sampling seemed to correlate with decreased tumor yield. The exact reason for increased sampling was not specified in the procedure note; however, difficulty of biopsy could have resulted in increased biopsy attempts. Wu et al. previously described that the cumulative diagnostic yield for bone biopsy reached a plateau at the third specimen.
With regard to soft tissue, all biopsies had at least one core that was positive for tumor. Although we did not review the variables associated with tumor yield for soft tissue lesions, overall pathologic yield was greater than that observed with bone. Given such, we recommend soft tissue as the preferred site of biopsy for molecular analyses, understanding that the biology of soft tissue and bone metastasis may differ and remains an unanswered question to be explored in future studies.
We hypothesized that clinical parameters associated with higher disease burden would predict increased yield, including increased disease sites, PSA, alkaline phosphatase and calcium, and lower hemoglobin and platelets. Higher calcium seemed to correlate with increased biopsy yield; however, a correlation was not observed across the remaining clinical parameters. The lack of correlation may be, at least in part, due to our relatively small sample size and the fact that men were enrolled on a clinical trial requiring normal organ function and good performance status, limiting our ability to detect small differences between groups. In a trial of undirected bone marrow biopsies in mCRPC, lower hemoglobin and higher alkaline phosphatase and lactate dehydrogenase were associated with an increased likelihood of a positive biopsy. In addition, survival was shorter in men with a positive biopsy, suggesting these individuals had more advanced disease.
Given that men with prostate cancer are susceptible to skeletal complications secondary to treatment-associated bone loss and bone metastases, osteoclast-targeting agents are frequently utilized. Bisphosphonates, analogues of pyrophosphate, have an inhibitory effect on osteoclasts, which are responsible for bone resorption. Denosumab, a monoclonal antibody against the receptor activator of nuclear factor-κβ ligand, prevents osteoclast function. We hypothesized that treatment with an osteoclast-targeting agent would result in decreased tumor yield, given the risk of osseous sclerosis associated with therapy. We observed that longer duration of therapy seemed to correlate with decreased yield. In a prior analysis, zoledronic acid therapy for ≥1 year resulted in a decreased number of positive bone biopsies (50%) compared with patients who did not receive therapy or received <1 year of treatment (74%; P=0.27). The direct effect of osteoclast-targeting agents in patients undergoing bone biopsy needs to be further explored.
Given the exploratory nature of our analysis, several limitations should be highlighted. Although samples were collected prospectively, imaging and procedural variables were analyzed retrospectively. In addition, our sample size was relatively small, limiting our ability to identify small differences between the variables analyzed. Biopsy site selection, needle gauge and number of cores obtained were at the discretion of the radiologist performing the biopsy, potentially resulting in selection bias. Our analysis was not comprehensive of all possible factors that can influence tumor yield. Other metrics to be considered include biopsy of 'new' (within 3–6 months) versus 'old' lesions, total number of needle passes, volume of tissue collected and evidence of necrosis. Lastly, we do not report nucleic acid quality/quantity or success of molecular analysis, which are important and should be included in future analyses. Unlike diagnostic biopsies, biopsies performed for tumor molecular analysis require special handling, given that bone is not decalcified during processing and occasionally laser capture microdissection is performed to increase sample purity. These additional steps have the potential to impact the success of molecular sequencing.
As molecular characterization of metastatic tumors becomes increasingly important, there is a pressing need to establish guidelines to improve biopsy yield. In this exploratory analysis, we identified several variables associated with increased tumor yield on bone biopsy. As we embark on a large multi-institutional project that aims to molecularly characterize mCRPC tumors, we have developed the following guidelines to optimize tumor yield in our patients undergoing biopsies as part of this endeavor:
Discuss the optimal lesion for biopsy with the collaborating radiologist.
Select soft tissue over bone lesions when possible.
Avoid biopsy of a previously irradiated lesion.
Use a larger needle for bone biopsies (11 gauge preferred for bone biopsies and 18 gauge preferred for soft tissue biopsies).
For bone biopsy, select lesions with larger size, low HU, short distance from the skin to the lesion and increased radiotracer activity on bone scan.
Minimize use of osteoclast-targeting therapy preceding bone biopsy when possible.
Larger studies are needed to develop the optimal algorithm for successful bone biopsy, specifically biopsies with the goal of downstream molecular analysis. Given such, we have incorporated a prospective analysis of the variables described here as part of a SU2C multi-institutional project. Furthermore, a critical next step is correlating these parameters with successful genome sequencing, which is the ultimate goal of these efforts as we enter the era of personalized medicine.
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