Jennifer M Munson, Adrian C Shieh
6 min
As solid tumors grow, they fundamentally alter their surrounding tissue, creating a unique microenvironment characterized by increased stiffness, elevated interstitial pressure, and altered fluid dynamics. While cancer research has historically focused on biochemical signaling, this paper highlights the critical role of biophysical forces. As tumors expand in confined spaces, they generate significant pressure gradients between the tumor bulk and healthy stroma. This pressure drives interstitial fluid flow (IF) from the tumor into the surrounding tissue, a phenomenon now recognized as a hallmark of disease progression.
Interstitial flow originates from a combination of vascular irregularities and physical constraints. Tumors induce angiogenesis, creating leaky, disorganized blood vessels that increase fluid influx into the tumor stroma. Simultaneously, the lack of effective lymphatic drainage within the tumor core prevents fluid clearance, exacerbating the pressure buildup. This environment is further complicated by the remodeling of the extracellular matrix (ECM) and the proliferation of cancer cells, which physically compress the stroma and increase hydraulic conductivity, facilitating the outward flow of fluid toward the invasive front.
Evidence suggests that interstitial flow is a potent, pro-migratory stimulus for cancer cells. Through mechanisms such as autologous chemotaxis—where cells create and sense their own chemokine gradients—and the activation of surface mechanosensors like the glycocalyx, cancer cells use fluid flow to guide their migration and invasion into healthy tissue. Furthermore, flow influences the tumor stroma by activating fibroblasts into myofibroblasts, which in turn secrete cytokines and remodel the ECM to further support tumor growth and metastasis. This creates a self-reinforcing cycle where fluid flow promotes the very conditions that drive malignancy.
Understanding interstitial flow is essential for improving cancer therapy. High interstitial pressure acts as a physical barrier that hinders the delivery and penetration of therapeutic agents. By identifying how fluid flow contributes to therapeutic resistance and disease recurrence, researchers can develop strategies to modulate the tumor microenvironment, potentially normalizing fluid dynamics to improve drug efficacy and patient outcomes.
As cancer progresses, a dynamic microenvironment develops that creates and responds to cellular and biophysical cues. Increased intratumoral pressure and corresponding increases in interstitial flow from the tumor bulk to the healthy stroma is an observational hallmark of progressing cancers. Until recently, the role of interstitial flow was thought to be mostly passive in the transport and dissemination of cancer cells to metastatic sites. With research spanning the past decade, we have seen that interstitial flow has a promigratory effect on cancer cell invasion in multiple cancer types. This invasion is one mechanism by which cancers can resist therapeutics and recur, but the role of interstitial flow in cancer therapy is limited to the understanding of transport of therapeutics. Here we outline the current understanding of the role of interstitial flow in cancer and the tumor microenvironment through cancer progression and therapy. We also discuss the current role of fluid flow in the treatment of cancer, including drug transport and therapeutic strategies. By stating the current understanding of interstitial flow in cancer progression, we can begin exploring its role in therapeutic failure and treatment resistance.
Sam: Like a swimmer feeling which way the river current is pulling them.
Alex: Exactly. That layer of antennae is called the "glycocalyx"—a coating of proteins and sugars on the cell surface. When fluid flows past, it creates friction on that coating. Cells can detect that friction, which researchers call "shear stress," and use it to figure out which direction the fluid is moving.
Sam: And if the fluid is moving outward, away from the tumor...
Alex: Then the cells follow it. The tumor's own pressure gradient is essentially giving cancer cells a navigation system. It points them toward the surrounding tissue, which is exactly the direction they need to go to spread to other parts of the body.
Sam: So the same physics that blocks the drug also guides the cancer's spread. It's doing two harmful things at once.
Alex: That's the key insight from the paper. The physical environment of the tumor—this pressurized, outward-flowing fluid system—both shields it from treatment and actively assists its invasion of healthy tissue. One mechanism, two serious consequences.
Sam: How confident are we that this is actually happening inside human patients, though? It's one thing to model this on paper.
Alex: That's an important caveat. Mathematical models of tumor fluid dynamics often rely on simplified assumptions—for instance, treating the tumor as a perfect sphere with uniform properties throughout. Real tumors are irregular, jagged, and heterogeneous. The flow patterns in an actual tumor could be quite different from what a clean model predicts.
Sam: So if the model assumes a smooth sphere but the real tumor has uneven edges and pockets, the predicted flow might not match what's actually happening.
Alex: Exactly. And direct measurements of fluid pressure and flow inside human tumors are technically difficult to obtain. Much of the supporting data comes from animal studies or lab-grown tissue models, which may not fully replicate the human situation. The paper is clear that this is an area where more measurement work is needed.
Sam: So the mechanism is well-reasoned, but the evidence in living human patients is still limited.
Alex: That's a fair summary. The theoretical framework is solid and supported by experimental work, but translating those findings into a complete picture of what happens inside a patient in real time remains an open challenge.
Sam: You mentioned the glycocalyx—those antennae on the cell surface. If we could interfere with those, could we stop the cells from following the flow?
Alex: The paper suggests that's a plausible direction. When researchers removed or disrupted those surface structures in experimental settings, the cells became less able to sense the fluid flow, and their migration slowed. That makes the glycocalyx a potential target—if you can block the cell's ability to read the signal, you might be able to interrupt the navigation system that helps cancer spread.
Sam: That's a different kind of target than what most cancer drugs go after. Most treatments try to kill the cell or stop it from dividing. This would be more like jamming its compass.
Alex: That's a useful way to think about it. And it connects to a broader shift in how some researchers are approaching cancer—not just asking "how do we destroy the cell?" but "how do we disrupt the physical environment that protects it and helps it move?" Targeting the mechanics, not just the biology.
Sam: So we're not just fighting the tumor's size or its genetic mutations. We're fighting its ability to navigate the body and resist treatment through physical means.
Alex: Exactly. And that reframing has real implications for how treatments are designed. If the pressure gradient is part of the problem, then therapies that reduce tumor pressure—or that work with the flow rather than against it—might reach the core more effectively than conventional drug delivery.
Sam: It's a reminder that biology doesn't happen in a vacuum. The physical forces shaping a tumor are just as much a part of the disease as the cells themselves.
Alex: Well put. The paper makes the case that understanding those forces isn't just an academic exercise—it's a necessary step toward treatments that can actually reach their target. Thanks for listening to ResearchPod.