Which of the following melanomas do not have an in situ growth phase?
The core concept here is the classification of melanomas based on their growth patterns. The main types are superficial spreading, nodular, lentigo maligna, and acral lentiginous. Each has different growth phases. The in situ phase refers to the radial growth phase where the melanoma is confined to the epidermis without invading deeper layers.
Superficial spreading melanoma typically has an in situ phase. Nodular melanoma is known for being more aggressive and often presents without a long in situ phase, progressing quickly to invasive growth. Lentigo maligna also has an in situ phase, often seen in older adults on sun-exposed skin. Acral lentiginous melanoma is similar to lentigo maligna but occurs on non-sun-exposed areas and also has an in situ phase.
So the correct answer should be nodular melanoma since it lacks a prolonged in situ phase. The other options (superficial spreading, lentigo maligna, acral lentiginous) all have in situ phases. The distractors are the other melanoma types that do have in situ phases. The key point here is that nodular melanoma tends to grow vertically from the start, making it more dangerous as it can metastasize earlier. The clinical pearl is to remember that nodular melanoma's lack of in situ phase correlates with its aggressive nature and poorer prognosis.
**Core Concept**
Melanomas are classified by their growth patterns. The *in situ* growth phase refers to radial epidermal spread without dermal invasion. Nodular melanoma lacks this phase, progressing directly to vertical growth.
**Why the Correct Answer is Right**
**Nodular melanoma** arises abruptly with vertical growth into the dermis, bypassing the radial/in situ phase. It is associated with aggressive behavior, rapid metastasis, and poor prognosis due to early invasion. Unlike other types, it lacks the prolonged in situ phase seen in superficial spreading or lentigo maligna melanomas.
**Why Each Wrong Option is Incorrect**
**Option A:** *Superficial spreading melanoma* has a distinct in situ phase (radial growth phase).
**Option B:** *Lentigo maligna melanoma* progresses through an in situ lentigo maligna stage.
**Option D:** *Acral lentiginous melanoma* also exhibits an in situ phase on palms/soles.
**Clinical Pearl / High-Yield Fact**
Nodular melanoma accounts for ~15-20% of cases but has a higher mortality rate due to its lack of an in situ phase. Remember: "Nodular means no radial—go straight to vertical!"
**Correct Answer: B. Nodular melanoma**