Clinical Study
Mayo Clinic researchers have reported results from a clinical study of minibeam radiotherapy in patients with treatment-resistant cancers in or near the skin. The technique delivers very high radiation doses through closely spaced, extremely narrow beams while limiting radiation exposure to healthy tissue between the beams.
The study, published in the International Journal of Radiation Oncology*Biology*Physics, included 36 patients with 46 tumors. Forty tumors could be evaluated. Patients had advanced skin cancers, including melanoma and squamous cell carcinoma, or soft tissue sarcomas. Ninety percent had received at least two previous cancer therapies, and all tumors had been resistant to other treatments.
Among the 40 evaluable tumors:
- 22.5% disappeared
- 25% shrank
- 45% remained stable
- Symptoms improved in 86% of patients.
A Different Radiation Pattern
Mayo Clinic researchers have studied minibeam radiotherapy for five years, building on preclinical research. They developed a tungsten collimator that divides a conventional radiation beam into 0.5-millimeter beamlets spaced 1 millimeter apart from center to center.
This produces areas of high radiation exposure, or “peaks,” separated by areas of low exposure, or “valleys.” Researchers believe the pattern allows healthy tissue to repair and regenerate more effectively than with conventional radiation therapy.
Of the 46 tumors treated in the study, 39 received minibeam radiotherapy alone, while seven also received planned conventional radiation therapy. The treatment can be completed in two or three sessions instead of 10–30 sessions.
At a median follow-up of about 15 months, the estimated 12-month local control rate was:
- 89% overall
- 92% for tumors receiving minibeam radiotherapy alone
Scott Lester, M.D., co-author and a radiation oncologist at Mayo Clinic Comprehensive Cancer Center, said the approach is being investigated for tumors requiring high radiation doses and involving sensitive parts of the body. "Tumors that require high doses of radiation and have infiltrated sensitive parts of the body provide the most formidable challenge for us. Ideally, we could treat the tumor and preserve function," he says. "We want to eliminate the cancer and simultaneously promote healing to get the best functional outcome for our patients."
Observed Immune Response
The researchers also reported responses in cancer outside the area treated with minibeam radiation. In 11% of patients, tumors outside the treated area responded despite the patients receiving no other cancer treatments. The researchers describe this as an abscopal response.
According to the study researchers, the observation raises questions about how minibeam radiotherapy may interact with the immune system.
"This represents an important shift in how we think about radiotherapy because our results suggest that you don't necessarily need to treat all of the tumor with an ablative dose," says Robert Mutter, M.D., chair of research in the Department of Radiation Oncology. "We may be able to exploit the distinct ways that tumors die after minibeam radiotherapy to personalize treatment and activate the immune system to fight the cancer."
Source: Mayo Clinic
