Melanoma is one of the most dangerous forms of skin cancer and occurs when pigment-producing cells called melanocytes mutate and begin to multiply uncontrollably. The size of a melanoma lesion is one of the key factors doctors use to determine how advanced the cancer is and the prognosis for the patient.
What determines the size and growth of melanoma?
Melanoma size is measured by the maximal diameter of the lesion in millimeters (mm) or centimeters (cm). Several factors influence the growth rate and size of melanomas:
- Breslow thickness – The vertical depth of invasion into the skin layers
- Mitotic rate – The rate of cell division
- Ulceration – Breaking of the skin over the melanoma
- Anatomic location – Some sites like the head and neck grow faster
- Genes and proteins expressed – Certain mutations lead to more rapid growth
In general, the deeper the invasion and the faster the mitotic rate, the larger the lesion is likely to become. Melanomas tend to grow in an outward, radial pattern initially. Over time, they begin invading deeper into the dermis and subcutaneous tissue which leads to rapid increases in size.
Tumor Size and Stage at Diagnosis
Both the American Joint Committee on Cancer (AJCC) and the World Health Organization (WHO) staging systems for melanoma incorporate tumor size as a key criterion:
- Stage 0 – Melanoma in situ, not invading deeper layers
- Stage I – Tumor 1mm or less in thickness, no ulceration or lymph node spread
- Stage II – Tumor 1-2mm thick, or with ulceration
- Stage III – Tumor 2-4mm thick, into deeper layers, lymph node involvement
- Stage IV – Tumor thicker than 4mm, widespread metastases
As shown, once a melanoma reaches a size greater than 1-2mm thick, it is considered at least a stage II tumor with a higher risk of recurrence. Melanomas larger than 4mm have often metastasized to other organs.
The Impact of Size on Prognosis
Numerous studies have shown a clear correlation between increasing melanoma size and reduced patient survival. Some key research findings include:
- 10 year survival rates decreased from 95% for melanomas under 1mm to 50% for thicknesses over 4mm in one study of 1,600 patients.
- 5 year survival was 98% for thin melanomas under 1mm but only 50% for thicknesses over 4mm in another study.
- Risk of recurrence after excision is only 1-2% for thin melanomas but over 50% for thicknesses over 4mm.
In most studies, melanoma thickness is one of the strongest independent predictors of outcomes when accounting for other factors such as location, mitotic rate, and genetic mutations.
How is melanoma size measured accurately?
To determine the maximal tumor diameter, a full skin examination and biopsy are required. Following are some of the techniques used:
Clinical Skin Exam
Visual inspection and palpation of the suspicious lesion by a dermatologist allows measurement of the visible size and estimation of depth based on morphology. However, clinical exam alone cannot fully evaluate the microscopic extent.
Dermoscopy
The use of a dermatoscope provides 10x magnification and lighting to reveal characteristics beneath the skin surface. This allows better assessment of size than the naked eye.
Total Body Photography
Specialized cameras can take detailed high-resolution images of the entire skin surface to monitor suspicious lesions over time. Serial photography allows early detection of size changes.
Histopathological Analysis
A punch biopsy or excisional biopsy of the entire lesion is performed for microscopic measurement of the depth and lateral extent of melanoma cells. This provides the most accurate assessment of tumor size.
Imaging Tests
If deeper invasion is suspected, ultrasound, CT scans or MRI can determine the size of deeper tumor tissue and evidence of metastasis to lymph nodes or organs that are not visible on the skin.
How does melanoma size guide treatment options?
The size of the primary tumor directly impacts the type of treatment selected. Key factors determined by size include:
- Surgical Margins – Small thin melanomas can be treated with a 0.5-1 cm margin excision while larger tumors require 2-3 cm margins due to subclinical spread.
- Lymph Node Evaluation – Sentinel lymph node biopsy is recommended for melanomas >1 mm to assess for metastasis.
- Adjuvant Therapy – Larger size may prompt additional treatments like immunotherapy, radiation, or chemotherapy after surgery.
- Clinical Trials – Size criteria often determine eligibility for enrollment into clinical trials of new drugs.
- Follow-up Schedule – Patients with thicker melanomas require more frequent follow-up visits to monitor for recurrence.
National Treatment Guidelines Based on Size
Melanoma size guides all major national treatment guidelines. A few examples:
- For melanomas under 1 mm, the National Comprehensive Cancer Network (NCCN) recommends 0.5-1 cm surgical margins.
- The American Academy of Dermatology (AAD) recommends sentinel lymph node biopsy for melanomas 1-4 mm thick.
- The American Society of Clinical Oncology (ASCO) advises consideration of adjuvant immunotherapy for stage III melanomas >4 mm thick.
What is the typical size at diagnosis?
Based on cancer registry data, the breakdown of melanoma sizes at initial diagnosis is:
- Thickness ≤ 1.00 mm: 65% of cases
- 1.01-2.00 mm: 20% of cases
- 2.01-4.00 mm: 10% of cases
- > 4 mm: 5% of cases
The average thickness at diagnosis is around 1.5 mm. Encouragingly, the proportion of thin melanomas (≤1 mm) has increased over the past 30 years due to earlier detection. Public education efforts on self-skin exams and physician screening of high-risk patients has helped identify more small, early-stage lesions.
Trends in Melanoma Size Over Time
| Time Period | Median Thickness (mm) | % Thin (≤ 1mm) |
|---|---|---|
| 1970-1979 | 1.59 mm | 40% |
| 1980-1989 | 1.46 mm | 48% |
| 1990-1999 | 1.18 mm | 57% |
| 2000-2009 | 0.97 mm | 65% |
As shown in the table, both the median thickness and proportion of thicker melanomas has decreased over time, indicating improved early detection.
What does small size tell about melanoma risk?
The smaller the lesion, the lower the risk, however other factors must also be considered. Some key points about small melanomas:
- Melanomas < 1 mm are mostly in situ lesions with very low potential to spread.
- However, small size does not guarantee low risk – factors like mitotic rate, location, and genetics also contribute.
- Even melanomas < 0.5 mm can metastasize in rare cases.
- Completely excising small primary lesions is critical to prevent recurrence and metastasis.
- Regular skin exams are still required after excision to monitor for new primary melanomas over time.
In summary, while small size equates to better outcomes for most patients, continued monitoring and awareness are warranted as size alone does not always predict biological behavior.
Does melanoma begin as a small lesion?
In most cases, yes – melanoma begins as a small, localized tumor before spreading:
- Melanomas are thought to arise from a single mutated melanocyte that starts dividing and forms a tiny lesion.
- With radial growth, the lesion expands within the epidermis first before invading deeper.
- Only after the tumor reaches a vertical depth of ≥1 mm does it have significant metastatic potential.
- The transition from radial to vertical growth is associated with more rapid increases in size.
However, melanoma does not always follow this linear progression:
- Rarely, melanomas can metastasize early even when small.
- In some cases, deeper tissue invasion occurs first before any visible skin lesions.
- This emphasizes the importance of self-exams for ‘ugly duckling’ lesions or suspicious non-healing sites.
The ABCDEs of Early Melanoma
Public education aims to enhance early recognition of small, evolving lesions. The ABCDE criteria instruct people to look for:
- Asymmetry – One half different than other half
- Border – Irregular, indistinct edges
- Color – Variegated shades of brown/black
- Diameter – Size greater than 6 mm tip of pencil eraser
- Evolving – Changes in size, shape, symptoms
Lesions with these attributes, especially if new or changing, should prompt a prompt medical evaluation even when small.
Should all small melanomas be treated aggressively?
Most experts recommend complete excision of all melanoma lesions regardless of size to prevent recurrence and spread. The National Comprehensive Cancer Network (NCCN) clinical practice guidelines advise:
- Excising melanoma in situ (stage 0) with 0.5-1 cm margins to clear subclinical extension.
- Excising invasive melanomas ≤ 1mm with 1 cm margins.
- Assessing all sentinel lymph nodes for microscopic metastases, even for thin melanomas <1 mm thick.
However, some controversy exists about overtreatment of smaller lesions. Arguments include:
- Low risk melanomas < 0.5 mm rarely metastasize so may not require lymph node evaluation.
- Superficial shave biopsy alone may be sufficient for some in situ lesions without deep excision.
- Active surveillance could be considered for elderly or frail patients with small low-risk melanomas.
More research is needed to determine if less aggressive approaches can be utilized for the smallest melanomas without sacrificing outcomes.
Importance of Long-Term Follow-Up
Regardless of initial treatment, regular skin exams and monitoring for recurrence are critical for all melanoma patients. Studies show:
- New primary melanomas develop in 10% of patients within 5 years.
- Melanoma survivors have a 9-fold greater lifetime risk of new melanoma.
- Median time to recurrence is 2-3 years but can be much longer.
Vigilance is required lifelong as the risk of additional melanomas never fully resolves.
Conclusion
In summary, size represents a critical prognostic factor for melanoma, with increasing tumor thickness and diameter associated with higher risk of spread and mortality. Measuring size requires detailed skin and microscopic examination. While most melanomas arise as small localized tumors, they can still metastasize when tiny in rare cases. Aggressive treatment is still recommended for early stage small melanomas, however, controversy exists regarding overtreatment of some tiny low-risk lesions. Regardless of initial tumor size, long-term follow-up care is imperative for all melanoma patients given the lifelong risk of new primary lesions developing with time.
