Beyond dose distribution: Gut microbiome as a new biological determinant in modern radiotherapy – Asia-Oceania Federation of Organizations for Medical Physics

Prof. Arun Chougule PhD, FIUPESM, FIOMP, FAMS
Past President of AFOMP and Immediate past Chair of ETC IOMP
Vice President of Indian Society of Radiation Biology (ISRB)
arunchougule11@gmail.com

Introduction: From Physical Precision to Biological Precision

Over the last two decades, radiation oncology has witnessed remarkable technological advances. Techniques such as intensity-modulated radiotherapy (IMRT), stereotactic body radiotherapy (SBRT), and image-guided radiotherapy (IGRT) have allowed medical physicists to sculpt dose distributions with millimetric precision. Yet, despite similar dose–volume histograms (DVHs), clinicians frequently observe significant inter-patient variability in treatment response and normal tissue toxicity.

This inconsistency raises an important question: Are physical dose parameters alone sufficient to explain radiotherapy outcomes? Emerging evidence suggests that the gut microbiome, the complex community of microorganisms residing in the gastrointestinal tract may be a critical, previously overlooked biological modifier of radiotherapy response and toxicity. Gut microbiota may be an important player in modulating tumour microenvironment, ultimately affecting treatment efficacy. The gut microbiome can influence both the effectiveness of cancer treatment and the severity of cancer treatment induced gastrointestinal toxicities

Why should Medical Physicists care about the Gut Microbiome?

At first glance, the gut microbiome may appear far removed from beam modelling, inverse planning, or quality assurance. However, the microbiome directly influences several processes that are central to radiotherapy:

Radiation-induced inflammation
DNA damage response and repair
Immune activation and suppression
Normal tissue regeneration

For pelvic, abdominal, and even extra abdominal radiotherapy, these biological effects can significantly alter the therapeutic ratio, irrespective of how well the dose is optimized.

Radiotherapy induced Dysbiosis: An unintended Biological Effect

Ionizing radiation does not affect only tumour cells and normal tissues, it also alters gut microbial ecosystems. Radiotherapy can induce dysbiosis, characterized by:

Reduced microbial diversity
Loss of beneficial short-chain fatty acid (SCFA) producing bacteria
Expansion of pro-inflammatory and pathogenic species

These changes are particularly pronounced during:

Pelvic IMRT (cervical, prostate, rectal cancers)
Abdominal SBRT
Chemoradiation protocols

Importantly, dysbiosis may persist long after treatment, contributing to chronic radiation toxicity.

IMRT and the microbiome: more than bowel dose constraints

IMRT has significantly reduced high-dose exposure to bowel loops, yet acute diarrhoea and chronic radiation proctitisremain common. Recent clinical studies indicate that:

Patients with low baseline gut microbial diversity experience higher grade gastrointestinal toxicity
Loss of Faecalibacterium prausnitzii (a butyrate producing bacterium) correlates with mucosal inflammation
Increased Proteobacteria abundance is associated with severe proctitis

This suggests that two patients with identical DVHs may respond very differently due to underlying microbiome differences, a paradigm familiar to physicists working in Radiogenomics, now extending into Radiomicrobiomics.

Figure 1. Schematic representation of gut major pathways on microbiome alterations during
radiotherapy response

SBRT, High Dose per Fraction, and Immune Modulation

SBRT introduces unique radiobiological effects:

High dose per fraction
Vascular damage
Strong immune activation

The gut microbiome modulates these effects by shaping systemic immune responses. Preclinical and early clinical evidence suggests that:

Favourable microbiome profiles enhance immune mediated tumour control
Antibiotic induced microbiome depletion reduces SBRT efficacy
The microbiome influences the likelihood of observing the abscopal effect

For medical physicists involved in SBRT planning, this reinforces the idea that biological context matters as much as geometric accuracy.

Pelvic and abdominal radiotherapy: A Microbiome-critical site

Pelvic and abdominal radiotherapy represents the most clinically relevant interface between radiation and the gut microbiome. Radiation induced changes in gut flora contribute to:

Clinical Effect	Microbiome Contribution
Acute diarrhoea	Inflammatory dysbiosis
Chronic proctitis	Persistent microbial imbalance
Late enteropathy	Impaired epithelial regeneration

Increased toxicity burden may lead to treatment interruptions and therefore, from a workflow perspective, this highlights the importance of integrated supportive care, including dietary counselling and microbiome preserving strategies during treatment.

Figure 2. Major biological pathways linking the gut microbiome to radiotherapy response.
Radiotherapy induced dysbiosis amplifies oxidative stress, DNA damage, inflammatory cytokine release, and immune dysregulation. Altered antigen presentation and imbalance between regulatory (Treg) and effector (Th17/CD8⁺) immune cells influence both tumour control and normal tissue injury. These pathways operate independently of physical dose parameters and help explain inter-patient variability in outcomes.

The microbiome immune radiotherapy axis

With the deeper understanding of radiobiology underlying radiotherapy management of cancer, it is now recognized that radiotherapy works as an immune modulating treatment. The gut microbiome acts as a biological amplifier of this effect by:

Enhancing antigen presentation
Promoting CD8+ T-cell activation
Regulating cytokine signalling

This triad is especially relevant in combined radiotherapy with immunotherapy protocols, where microbiome composition can determine treatment success or failure.

Clinical translation: what can be done today?

While microbiome guided radiotherapy planning is not yet routine, several practical considerations are already relevant:

Avoid unnecessary antibiotics during radiotherapy
Encourage high fibre diets to support SCFA production
Consider probiotics cautiously in selected patients
Recognize unexplained toxicity as a possible biological and not as dosimetric phenomenon

Future integration may include baseline microbiome profiling alongside imaging and dosimetric data.

Figure 3 : Future radiation oncology workflows may incorporate baseline microbiome profiling alongside imaging, DVH analysis, and Radiogenomics. Microbiome informed supportive care and adaptive strategies could improve therapeutic ratio by reducing toxicity and enhancing tumour response without altering prescribed dose.

Implications of understanding of microbiome effect by Medical Physicists

The evolution of radiotherapy has progressed from conventional dose delivery to advanced technics such as CRT, IMRT, IGRT and the next frontier can be biology guided radiotherapy, where microbiome data complements Radiogenomics, functional imaging, and AI-driven planning. For medical physicists, this represents an opportunity to expand the scope of precision from shaping dose distributions to understanding biological variability.

Conclusion

The gut microbiome is emerging as a significant biological determinant of radiotherapy response and toxicity, particularly in IMRT, SBRT, and pelvic radiotherapy. While advanced technology has optimized physical dose delivery, biological heterogeneity continues to shape clinical outcomes. Integrating microbiome science into radiation oncology offers a promising pathway toward truly personalized radiotherapy where physics and biology work in harmony to improve patient care.