Defining normoxia, physoxia and hypoxia in tumours-implications for treatment response

Area of Science:

• Oncology
• Cancer Biology
• Physiology

Background:

• Tumour hypoxia is a critical factor negatively affecting cancer treatments and promoting malignant progression.
• Current research often uses normoxia (20% oxygen) as a benchmark, which does not reflect physiological conditions.

Purpose of the Study:

• To clarify relevant oxygen levels in cancer research.
• To differentiate between normoxia, physiological oxygen (physoxia), and tumour oxygenation.
• To highlight the implications of inaccurate oxygen level comparisons in studies.

Main Methods:

• Review of existing literature on oxygen levels in normal tissues and tumours.
• Analysis of oxygen concentration ranges under physiological conditions and in various cancer types.
• Discussion of the impact of therapeutic interventions on tumour oxygenation.

Main Results:

• Normal tissues exhibit "physoxia" with oxygen levels averaging around 5% (3–7.4%).
• Untreated tumours show significantly lower median oxygen levels, often below 2% (0.3–4.2%), with notable exceptions in prostate and pancreatic cancers.
• Therapy can induce unpredictable longitudinal changes in tumour oxygenation.

Conclusions:

• Using normoxia as a comparator for tumour hypoxia is inappropriate and can lead to flawed conclusions.
• Accurate representation of physiological and tumour oxygen levels (approx. 5% and 1%, respectively) is crucial for understanding treatment response and malignant progression.
• In vitro studies comparing to human tumours must account for these oxygen disparities.