Obesity in radiation oncology

Obesity may negatively influence cancer treatment and outcomes for a variety of reasons. Clinical and radiation oncologist, Dr Mia Hugo, unpacks this for our understanding.

Patients who need to undergo radiation may come with a variety of co-morbidities that must be considered, with obesity being an important one. 

A healthy weight is imperative for breast cancer survivors. Obesity is associated with higher breast cancer-specific and overall mortality. Among a cohort of patients from a breast cancer registry, each 5kg gain was associated with a 12% increase in all-cause mortality and a 13% increase in breast cancer-specific mortality after adjustment for other factors. This correlation remained consistent regardless of pre-diagnosis BMI.1 

Colon cancer patients also showed a higher risk for recurrence, with patients with a BMI higher than 35 showing a 38% increased risk of cancer recurrence compared to normal weight survivors.2 

Looking at prostate cancer patients treated with external beam radiation therapy (EBRT), multiple studies have shown a correlation between obesity and inferior outcomes, with different mechanisms proposed, such as difficulty with daily set-up and an increased likelihood of shifts in tumour location among obese patients.1 A study found that men who were moderately or severely obese were at double the risk for biochemical failure than other men who weren’t, suggesting a link to the biology of this tumour.3 

All cancers aren’t the same though and an interesting finding was that in patients with head and neck cancer, a higher BMI was independently associated with improved overall survival.4

In terms of imaging for diagnosis and follow-up of cancer survivors, excess body fat also results in large increases in the effective radiation from diagnostic radiologic procedures compared with the doses to lean patients from the same examinations. For this reason, the effective doses to overweight and obese patients should be monitored carefully.5

Side effects worsened

Not only is cancer outcome and recurrence affected by obesity in some cancer types, but side effects from treatment are also often worse for obese patients. Women with an elevated BMI at diagnosis have worse quality of life after completing radiation for breast cancer and experienced significantly higher rates of dry desquamation and moist desquamation during radiation.6 Obese patients undergoing prostate radiation may experience incontinence more often than normal weight patients. Cervical cancer patients with a high BMI also experience increased treatment-related  toxicities compared with normal weight patients.6 

Specialised techniques may reduce some of the side effects in obese patients. One such study compared the outcomes of stereotactic body radiation (SBRT) in prostate cancer patients and found that at 24-months post-SBRT, obese men experienced only borderline clinically significant decrease in sexual function compared to non-obese patients but fatigue was still significantly higher in obese patients compared to non-obese patients at 18-months post-SBRT.7

Weight limits

Weight limits are applicable to radiation treatments due to the capacity of the couches. New radiation couches can manage a weight up to 200kg; many of the private units in JHB have weight limits of 110-120kg. Certain procedures, such as stereotactic radiosurgery, may also have a weight limit depending on the machine capacity. Due to the weight limit, a radiation oncologist may recommend weight loss to a patient before planning can commence. 

Positioning obese patients for treatment can be challenging for radiotherapists as their surface anatomy is more mobile, resulting in greater difficulty in setting up accurately to their marks, and imaging quality may also be affected by increased body habitus. With modern techniques, accuracy in daily set-up is of utmost importance to ensure no deviation from the highly conformal planned volumes.8 Different techniques or additional safety checks may be necessary to ensure that obese patients get safe, effective radiation.

Loss of weight intervention trials

A healthy weight is also important for cancer survivors in terms of the other long-term health risks of obesity, such as diabetes, hypertension, and cardiovascular disease. Weight loss intervention trials among breast cancer survivors are emerging and demonstrate that active intervention after treatment can lead to significant weight loss and decreased insulin resistance.1 

Maintaining a healthy weight and exercising can improve one’s quality of life, lower the risk of lymphoedema, boost physical functioning, lessen fatigue and reduce depression after a cancer diagnosis. 

A useful booklet one can download is the ASCO guide Managing Your Weight After a Cancer Diagnosis.

Dr Mia (neé Erasmus) Hugo is a clinical and radiation oncologist and works as a consultant in the Department of Radiation Oncology at Charlotte Maxeke Johannesburg Academic Hospital and the University of the Witwatersrand. She also works in limited private practice at Wits Donald Gordon Medical Centre and 200 Rivonia Medical Centre.


Dr Mia (neé Erasmus) Hugo is a clinical and radiation oncologist and works as a consultant in the Department of Radiation Oncology at Charlotte Maxeke Johannesburg Academic Hospital and the University of the Witwatersrand. She also works in limited private practice at Wits Donald Gordon Medical Centre and 200 Rivonia Medical Centre. 


  1. Ross KH, Gogineni K, Subhedar PD, Lin JY, McCullough LE. Obesity and cancer treatment efficacy: Existing challenges and opportunities. Cancer. 2019;125(10):1588-92.
  2. Dignam JJ, Polite BN, Yothers G, Raich P, Colangelo L, O’Connell MJ, et al. Body Mass Index and Outcomes in Patients Who Receive Adjuvant Chemotherapy for Colon Cancer. JNCI: Journal of the National Cancer Institute. 2006;98(22):1647-54.
  3. Strom SS, Kamat AM, Gruschkus SK, Gu Y, Wen S, Cheung MR, et al. Influence of obesity on biochemical and clinical failure after external-beam radiotherapy for localized prostate cancer. Cancer. 2006;107(3):631-9.
  4. Hicks DF, Bakst R, Doucette J, Kann BH, Miles B, Genden E, et al. Impact of obesity on outcomes for patients with head and neck cancer. Oral oncology. 2018;83:11-7.
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  8. Winters E, Poole C. Challenges and impact of patient obesity in radiation therapy practice. Radiography. 2020;26(3):e158-e63.