Illustration showing tumor inside prostate gland and closeup view of prostate cancer cells to the left of the image.
Credit: Dr_Microbe/Getty Images

Magnetic resonance-guided daily adaptive stereotactic body radiotherapy (MRg-A-SBRT) significantly lowers the risk for acute genitourinary and gastrointestinal toxicity in patients with prostate cancer relative to standard computed tomography-guided non-adaptive prostate SBRT (CT-SBRT), shows a pooled analysis of previously published data.

First author of the review, Jonathan Leeman, from the Dana‐Farber Cancer Institute and Brigham and Women’s Hospital, explained to Inside Precision Medicine that “MR-guided daily adaptive treatment involves use of a new radiation device called an MR-Linac which incorporates an MRI scanner into the radiation treatment machine.”

This device has three key benefits over the more commonly used CT-SBRT. Firstly, it can visualize anatomy more clearly due to the use of on-board MRI; secondly, monitors radiotherapy delivery with continuous real-time MRI imaging to ensure the treatment is on-target; and thirdly, it is capable of adaptive planning which involves adjustment of the radiotherapy plan each session to account for day-to-day changes in anatomy. Together, these elements allow for radiotherapy delivery with high precision and high confidence that can substantially limit the delivered dose and thus toxicity to the bladder, rectum, and urethra/bladder neck in patients with prostate cancer.

“Because the goal is to deliver the treatment with millimeter precision, we need to be able to account for very small changes in organ position and shape that can be unpredictable and this includes changes in bladder filling, changes in rectal filling due to gas or stool, or swelling of the prostate which can occur during treatment,” says Leeman.

He continues: “In a sense, MRg-A-SBRT is a paradigm of precision medicine or personalized medicine because the radiotherapy treatment is personalized to the patient’s anatomy for each treatment to deliver the most effective and safe radiation plan possible for the patient in that moment.”

Although MRg‐A‐SBRT offers technological advantages in precision of radiation dose delivery over CT‐SBRT, toxicity rates in people with prostate cancer have never been directly compared between the two techniques.

Leeman and colleagues from the Dana-Faber Cancer Institute therefore carried out a meta-analysis of 29 prospective studies that included a total of 2,547 men with prostate cancer, of whom 329 underwent MRg‐A‐SBRT and 2,218 received CT‐SBRT.

The follow-up period among the studies was typically 90 days and during this time the pooled incidence of acute grade 2 or higher (G2+) genitourinary toxicity was 16% with MRg-A-SBRT versus 28% with CT-SBRT. For G2+ gastrointestinal toxicity, the rates were a corresponding 4% and 9%.

After accounting for potentially confounding factors, the researchers found that individuals who received MRg-A-SBRT were 44% less likely to experience acute G2+ genitourinary toxicity and 60% less likely to develop gastrointestinal toxicity than those treated with CT-SBRT.

Writing in Cancer, Leeman et al. say that there are many potential reasons why MRg‐A‐SBRT is associated with a reduced risk of acute toxicity including the use of daily online adaptive planning, MRI‐based contouring that results in smaller treatment volumes, and MRI tracking. However, the data did not allow them to determine whether the reduction is related to MRI guidance versus CT guidance, or adaptive versus non‐adaptive treatment, or both.

They note, however, that among the CT‐SBRT studies, there was no observed difference in toxicity between those that used that used radiation guidance markers, known as intrafractional fiducial tracking, and those that did not. “This may potentially support the notion that the adaptive component of MRg‐A‐SBRT is driving the reduced toxicity findings rather than the use of intrafraction monitoring,” the authors write.

Considering the clinical value that MRg-A-SBRT offers to patients, Leeman acknowledges that although the analysis has shown that “MRg-A-SBRT carries important benefits in terms of side effect reduction, the practicality of how this treatment can be widely delivered is also critical to consider.”

“MRg-A-SBRT requires more time, more resources, and particular expertise regarding the treating physician and team. However, in its short history, we have already seen this treatment become more efficient and this trend is likely to continue as automation and artificial intelligence come to play more of a role in radiotherapy treatment planning and delivery. It has been common in our field that when new technologies arrive, the early experience requires more time, effort, and development. But because we are seeing real benefits for patients, effort should be placed into streamlining the technology as it becomes more mature and making it more accessible,” he notes.

The investigators conclude by saying that longer follow-up is now needed to determine whether late toxicity and cancer control rates differ with the use of MRg‐A‐SBRT compared with CT‐SBRT.

An accompanying editorial discusses the analysis’ findings, weighs the potential benefits and shortcomings of adopting this treatment strategy for patients, and questions the value of broad adoption.

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