Horizontal growth, in its most common medical usage, refers to the early phase of melanoma development where cancer cells spread outward across the skin’s surface rather than downward into deeper tissue. This stage, formally called the radial growth phase, is significant because melanomas caught during horizontal growth have an excellent prognosis. In a study tracking 161 patients with pure radial growth phase melanomas for a median of 13.7 years, every single patient remained free of metastasis.
How Horizontal Growth Differs From Vertical Growth
Melanoma progresses through a recognized sequence. It typically begins as an abnormal mole, advances through stages of increasing irregularity, and eventually becomes a primary melanoma. The first cancerous stage is the radial (horizontal) growth phase, where abnormal pigment-producing cells multiply sideways within the epidermis, the outermost layer of skin. The cells spread laterally, which is why the lesion gets wider and more irregular over time.
The critical turning point comes when the tumor shifts to vertical growth. During this transition, cancer cells break through the basement membrane (the thin boundary between the skin’s outer and inner layers) and push downward into the dermis. Once in the dermis, the tumor gains access to blood vessels and lymphatic channels, giving it the ability to spread to distant organs. Melanomas in the vertical growth phase have high metastatic potential, while those confined to horizontal growth have little or none.
The mechanisms that trigger this shift from horizontal to vertical are still not fully understood. What is known is that the cancer cells somehow escape the control of surrounding skin cells and penetrate the barrier separating the epidermis from the dermis.
Which Melanoma Types Start With Horizontal Growth
Not all melanomas go through a horizontal growth phase. The most common types that do include superficial spreading melanoma, lentigo maligna, and acral lentiginous melanoma. These account for the majority of melanoma diagnoses. Superficial spreading melanoma, in particular, is defined by its radial growth pattern, with tumor cells spreading in a flat, outward distribution across the skin surface.
Nodular melanoma is the notable exception. It skips the horizontal phase entirely and grows vertically from the start, which is why it tends to be thicker and more dangerous at the time of diagnosis. This distinction matters because nodular melanomas don’t follow the usual visual warning signs of a gradually widening, irregularly shaped spot.
What Horizontal Growth Looks Like on the Skin
During the horizontal phase, melanomas tend to grow outward at measurable rates that vary depending on their microscopic pattern. Research using dermoscopy (a magnified skin examination technique) found that melanomas displaying a “clod” pattern, which looks like clusters of rounded dots or globules, grew the fastest horizontally at about 10.4 square millimeters per year. Melanomas with a net-like (reticular) pattern grew at roughly 4.8 square millimeters per year. The slowest were those with a featureless or fine-dotted pattern, expanding at around 2.6 square millimeters per year.
To the naked eye, a melanoma in horizontal growth typically appears as a flat or barely raised spot that changes over time. It may develop uneven borders, multiple shades of brown or black, and an asymmetric shape. These are the classic features captured by the ABCD criteria (asymmetry, border irregularity, color variation, diameter) that doctors use for screening. Because horizontal growth melanomas fulfill these visual criteria more reliably than nodular types, they’re more likely to be caught early during routine skin checks.
Thickness and Staging
Breslow thickness, measured in millimeters from the skin surface to the deepest point of the tumor, is one of the most important numbers in melanoma staging. Melanomas still in the horizontal growth phase tend to be thinner. In one study comparing the two phases within superficial spreading melanoma, tumors in the radial phase averaged 0.48 mm thick, while those that had transitioned to vertical growth averaged 0.78 mm. That difference is clinically meaningful because thinner melanomas correspond to earlier stages and better outcomes.
The radial growth phase is also central to how pathologists classify melanoma. When a biopsy sample shows cancer cells spreading laterally within the epidermis without forming an expansive mass in the dermis, the tumor is categorized as radial growth phase. The presence or absence of cell division (mitosis) within the sample further helps distinguish between a more indolent radial phase tumor and one that may be on the verge of transitioning to vertical growth.
What Drives the Cancer Cells to Multiply
At the molecular level, horizontal growth in melanoma is frequently driven by overactivation of a cell-signaling chain called the MAPK pathway. This pathway normally helps cells respond to growth signals from their environment, but when key genes along the chain mutate, the pathway gets stuck in the “on” position, pushing cells to multiply uncontrollably.
The most common mutation involved is in the BRAF gene. BRAF mutations are especially associated with superficial spreading melanoma, the type most likely to exhibit horizontal growth. People who had significant sun exposure early in life and who are prone to developing moles are more likely to carry BRAF-driven melanomas. These patients tend to be diagnosed younger and often have tumors on the trunk. Understanding this molecular driver has led to targeted therapies that block BRAF signaling, though these treatments are primarily used for more advanced disease.
Treatment During the Horizontal Phase
When melanoma is caught in the horizontal growth phase, surgical removal is the standard treatment and is typically curative. For melanoma in situ (the earliest form, confined entirely to the epidermis), current guidelines recommend removing the tumor with a surrounding margin of 5 mm to 1 cm of healthy-looking skin to ensure no cancer cells are left behind.
Margins can be tricky in certain locations. On the head and neck, for example, a 5 mm margin for lentigo maligna (a type of in situ melanoma common on sun-damaged skin) often turns out to be insufficient, requiring wider excision. For the patient, this typically means a straightforward outpatient procedure with local anesthesia. Recovery depends on the size and location of the excision, but most people heal within a few weeks.
The prognosis after complete removal of a radial growth phase melanoma is excellent. The long-term follow-up data showing 100% metastasis-free survival in pure radial growth phase cases underscores why early detection matters so much. Catching melanoma while it’s still growing horizontally, before it transitions to vertical growth, is the single most important factor in achieving a cure.
Horizontal Growth Outside of Melanoma
The term “horizontal growth” also appears in other medical and scientific contexts. In craniofacial anatomy, horizontal growth refers to the lateral expansion of bones in the skull and jaw. The mandible and the upper jaw complex grow in three dimensions through cartilage conversion, deposition at suture lines, and remodeling of the bone surface. Orthodontists and oral surgeons use the concept of horizontal versus vertical growth patterns to plan treatments for jaw misalignment and facial development issues.
In business strategy, horizontal growth describes a company expanding by acquiring competitors or entering related markets at the same level of the supply chain, as opposed to vertical growth (integrating suppliers or distributors). If you searched this term in a business context, horizontal growth means scaling outward within your industry rather than moving up or down the production chain.