SAN DIEGO — Stereotactic ablative body radiotherapy (SABR) was an effective, noninvasive strategy for treating primary renal cell carcinoma (RCC) in patients who were not suitable for surgery, according to the FASTRACK II trial.

In the prospective, phase II study of 70 patients, none had local progression of kidney cancer during the trial lifetime (median follow-up of 43 months). In addition, cancer-specific survival was 100% over the same period, reported Shankar Siva, PhD, of the Peter MacCallum Cancer Centre in Melbourne, Australia.

Freedom from distant failure at 1 and 3 years was 99% (95% CI 90-100%), while overall survival at 1 and 3 years was 99% (95% CI 90-100%) and 82% (95% CI 70-89%), respectively.

“We think these study results would suggest that SABR is a new standard of care for primary kidney cancer not suited to surgery,” said Siva at the American Society for Radiation Oncology (ASTRO) annual meeting. “These outcomes support the design of a future randomized clinical trial of SABR versus surgery for primary kidney cancer,”

“The observed excellent efficacy is likely attributable to the potent biological doses used in this trial, as well as the rigorous quality assurance protocol,” he added.

Siva pointed out that RCC incidence has increased worldwide over the last 3 decades, with the most rapid increase observed in people, ages >70.

He noted that while surgery — both total and partial nephrectomy — is the standard of care, “there are some limitations for patients who are medically inoperable or those who may be high risk — for example, at risk of post-surgical dialysis.”

The advantage of SABR, also known as stereotactic body RT (SBRT), is that it avoids general anesthesia and has the capacity to treat perihilar and large tumors, unlike thermal ablation. Also, unlike both thermal ablation and surgery, it is noninvasive.

The TROG 15.03/ANZUP international multicenter FASTRACK II trial was conducted at seven Australian center and one Dutch center between July 2016 and February 2020.

Eligible patients had biopsy-confirmed diagnosis of primary RCC with a single lesion within a kidney, ECOG performance ≤2, estimated glomerular filtration rate (eGFR) >30 mLs/min, and were considered medically inoperable, high risk, or had declined surgery.

Patients’ median age was 77, 70% were male, median BMI was 32, and median Charlson comorbidity score was 7.

About one-third of patients had T1a disease, which is a population that may potentially be addressed with thermal ablation, Siva said. Most of the remaining population (56%) had T1b disease or greater.

The 23 patients with tumors ≤4 cm received a single fraction of 26 Gy, while those with tumors >4 cm (n=47) were prescribed 42 Gy in three fractions. Among all patients, the median tumor size 46 cm, while for those who got single fraction, the median tumor size was 3.3 cm, and for those who got three fractions, it was 5.3 cm.

Siva noted that the median tumor size of 4.6 cm is larger than that seen in randomized trials of partial nephrectomy versus radical nephrectomy. Despite this larger tumor size, renal function loss was comparable to that seen with partial nephrectomy in those trials of surgery, he stated.

Baseline mean eGFR (95% CI) was 61.1 mLs/min and reduced by -10.8 mLs/min by 1 year, by -14.6 mLs/min by 2 years, and plateaued thereafter. One patient underwent dialysis.

Regarding safety, Siva reported 10% of patients experienced grade 3 treatment-related adverse events (TRAEs), most of which involved either transient pain or nausea and vomiting.

“Prophylactic antiemetics or steroids were not mandated in this trial, but should be recommended and considered in the future,” he observed.

There were with no grade 4 or 5 TRAEs, while 11 (16%) patients reported no AEs.