3.5 Anthropometrics & Body Composition Analysis (DEXA, BIA, CT)

Anthropometrics & Body Composition Analysis (DEXA, BIA, CT)

Accurate interpretation of anthropometric and body composition data in oncology requires looking far beyond total scale weight. Fluid accumulation, tumor mass, and altered muscle-to-fat ratios frequently obscure severe muscle depletion. Specialized body composition analysis and functional strength testing are critical diagnostic tools for the Board Certified Specialist in Oncology Nutrition (CSO).

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Unintentional Weight Loss Calculations & Timeframes

Percentage of unintentional weight loss evaluates the velocity and severity of tissue loss:

$\text{\% Weight Loss} = \left( \frac{\text{Usual Body Weight} - \text{Current Body Weight}}{\text{Usual Body Weight}} \right) \times 100$

Clinical severity is classified based on time elapsed:

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Adjusting for Fluid Retention (Edema & Ascites)

Fluid accumulation (pedal edema, sacral edema, pleural effusion, ascites) artificially inflates scale weight and masks tissue wasting. Clinicians must estimate dry body weight by subtracting estimated fluid weight:

• Peripheral Edema Deductions:
  • Mild ($1+$ pitting, ankle): Deduct $\sim 1 - 2\text{ kg}$ ($\sim 5%$ body weight).
  • Moderate ($2 - 3+$ pitting, to knee): Deduct $\sim 5 - 7\text{ kg}$ ($\sim 10%$ body weight).
  • Severe ($4+$ pitting / Anasarca): Deduct $\sim 10 - 15+\text{ kg}$ ($\ge 15%$ body weight).
• Ascites Deductions:
  • Mild Ascites: Deduct $\sim 3 - 5\text{ kg}$.
  • Moderate Ascites: Deduct $\sim 7 - 10\text{ kg}$.
  • Severe / Tense Ascites: Deduct $\sim 12 - 15+\text{ kg}$.

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Sarcopenic Obesity in Oncology

Sarcopenic Obesity is defined as the co-occurrence of high adiposity (BMI $\ge 30\text{ kg/m}^2$) and severe skeletal muscle depletion. It is particularly prevalent in breast, prostate, colorectal, and pancreatic cancer patients receiving adjuvant chemotherapy or hormonal therapies.

Pharmacokinetic & Clinical Implications

• Chemotherapy Toxicity: Systemic chemotherapy dosing is historically calculated using Body Surface Area (BSA), calculated via the Mosteller formula: $\text{BSA (m}^2\text{)} = \sqrt{\frac{\text{Height (cm)} \times \text{Weight (kg)}}{3600}}$ In sarcopenic obesity, high body weight inflates BSA, yielding a higher absolute chemotherapy dose. However, hydrophilic chemotherapeutic agents (e.g., 5-fluorouracil, carboplatin, cisplatin, doxorubicin) distribute primarily into lean muscle mass. Because lean muscle mass is severely contracted, drug concentration per unit of muscle mass becomes excessively high, triggering severe grade 3–4 hematologic and gastrointestinal toxicities, forced dose reductions, and premature treatment discontinuation.
• Prognostic Impact: Sarcopenic obese patients suffer higher surgical complication rates, longer hospitalizations, and significantly reduced overall survival compared to non-sarcopenic obese controls.

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Advanced Body Composition Assessment Modalities

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|                 BODY COMPOSITION MODALITIES IN ONCOLOGY               |
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| 1. COMPUTED TOMOGRAPHY (CT) AT L3 (Gold Standard)                     |
|    - Measures: Cross-sectional Skeletal Muscle Area (SMA in cm²)      |
|    - Normalizes for height: Skeletal Muscle Index (SMI = SMA / m²)    |
|    - Sarcopenia Cutoffs: < 55 cm²/m² (Men), < 39 cm²/m² (Women)       |
| 2. BIOELECTRICAL IMPEDANCE ANALYSIS (BIA)                             |
|    - Measures: Resistance (R) & Reactance (Xc) to electrical current  |
|    - Derives: Phase Angle = arctan(Xc / R) x (180 / pi)               |
|    - Prognostic Cutoff: Phase Angle < 4.5° to 5.0° indicates cell     |
|      membrane breakdown and shortened overall survival                |
| 3. DUAL-ENERGY X-RAY ABSORPTIOMETRY (DEXA)                            |
|    - Measures: Appendicular Lean Mass (ALM)                           |
|    - Cutoffs: ALM Index < 7.0 kg/m² (Men), < 5.5 kg/m² (Women)        |
| 4. HANDGRIP DYNAMOMETRY (Functional Strength)                         |
|    - EWGSOP2 Cutoffs: Handgrip < 27 kg (Men), < 16 kg (Women)         |
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1. Computed Tomography (CT) Scans at L3 (Gold Standard)

• Opportunistic Secondary Analysis: Utilizes routine diagnostic oncology CT imaging at the 3rd lumbar vertebra (L3) level.
• Tissue Segmentation: Software segments tissues based on Hounsfield Unit (HU) radiodensity thresholds: Skeletal muscle ($-29\text{ to } +150\text{ HU}$), Visceral adipose tissue ($-150\text{ to } -50\text{ HU}$), Subcutaneous adipose tissue ($-190\text{ to } -30\text{ HU}$).
• Skeletal Muscle Index (SMI): $\text{SMI (cm}^2\text{/m}^2\text{)} = \text{Skeletal Muscle Area (cm}^2\text{)} / [\text{Height (m)}]^2$.
• Diagnostic Cutoffs for Sarcopenia (Martin et al.):
  • Men: $\text{SMI } < 55\text{ cm}^2\text{/m}^2$ (if BMI $\ge 25$) or $< 43\text{ cm}^2\text{/m}^2$ (if BMI $< 25$).
  • Women: $\text{SMI } < 39\text{ cm}^2\text{/m}^2$ ($< 41\text{ cm}^2\text{/m}^2$ in female obesity).

2. Bioelectrical Impedance Analysis (BIA) & Phase Angle

• Mechanism: Measures electrical impedance—comprising resistance ($R$, opposition to current flow through body water) and reactance ($Xc$, capacitive storage by cell membranes)—as low-frequency alternating current passes through tissues.
• Phase Angle (PA): Derived via the mathematical relationship: $\text{PA (degrees)} = \arctan(Xc / R) \times (180 / \pi)$.
• Clinical Significance: Reflects cell membrane capacitance and cellular integrity. Normal PA is $5.0^\circ - 7.0^\circ$. A Phase Angle $<4.5^\circ \text{ to } 5.0^\circ$ indicates breakdown of cell membrane structure, fluid shift from intracellular to extracellular space, severe cellular exhaustion, and serves as an independent predictor of mortality in cancer.

3. Functional Muscle Strength: Handgrip Dynamometry

• Procedure: Measures isometric handgrip strength using a calibrated hydraulic dynamometer (best of 3 trials per hand).
• EWGSOP2 Diagnostic Sarcopenia Cutoffs:
  • Men: $< 27\text{ kg}$
  • Women: $< 16\text{ kg}$

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Detailed Worked Clinical Case Study

Patient Scenario

A 68-year-old male with metastatic colorectal cancer presents for body composition assessment prior to starting palliative chemotherapy. Scale weight = $84\text{ kg}$, Height = $178\text{ cm}$ (Scale BMI = $26.5\text{ kg/m}^2$). Usual weight 4 months ago was $95\text{ kg}$. Physical exam shows $3+$ pedal edema and moderate abdominal ascites.

Step-by-Step Assessment

1. Fluid Adjustment & True Dry Weight:
   • $3+$ pedal edema $\rightarrow$ Deduct $\sim 6\text{ kg}$.
   • Moderate ascites $\rightarrow$ Deduct $\sim 8\text{ kg}$.
   • Total fluid deduction = $14\text{ kg}$.
   • True Dry Weight: $84 - 14 = 70\text{ kg}$.
2. Percentage Weight Loss Calculation:
   • $[(95 - 70) / 95] \times 100 = (25 / 95) \times 100 = 26.3%$ unintentional weight loss over 4 months.
   • Interpretation: Severe Weight Loss ($>7.5%$ in 3 months / $>10%$ in 6 months).
3. Dry BMI Calculation:
   • $\text{Dry BMI} = 70 / (1.78)^2 = 22.1\text{ kg/m}^2$.
4. CT L3 Scan & BIA Evaluation:
   • CT L3 Skeletal Muscle Index = $42\text{ cm}^2/\text{m}^2$ ($<55\text{ cm}^2/\text{m}^2$ cutoff $\rightarrow$ Severe Sarcopenia).
   • BIA Phase Angle = $4.1^\circ$ ($<4.5^\circ$ cutoff $\rightarrow$ High Risk of Cellular Membrane Breakdown & Shortened Survival).
   • Handgrip Strength = $21\text{ kg}$ ($<27\text{ kg}$ cutoff $\rightarrow$ Low Muscle Strength).

Comprehensive Clinical Synthesis

Despite a scale BMI of $26.5\text{ kg/m}^2$ suggesting overweight status, fluid-adjusted analysis reveals $26.3%$ severe weight loss, severe sarcopenia, cellular membrane failure, and reduced muscle strength.