Editorial: Stereotactic radiotherapy for primary renal cell carcinoma: time for larger-scale prospective studies

A number of important trends in kidney cancer diagnosis have emerged in recent decades, including the increasing detection of renal tumours in older patients with more comorbidities. In the UK in 2012–2014, 50% of new cases were diagnosed in people aged 70 years and over. Whilst many of these lesions are incidental small renal masses suitable for active surveillance, the dilemma of how to manage the higher-risk lesion (rapid growth kinetics, larger size, symptomatic lesion) is increasingly encountered. Surgical management may pose an unacceptable risk of morbidity, mortality or dialysis, yet these patients may live long enough to experience the consequences of disease progression. Thermal ablation is an option for small cortical tumours (≤3 cm), but there are limitations for larger or centrally located tumours.

In this issue of BJUI, Siva and colleagues [1] report promising early efficacy and toxicity data using stereotactic ablative body radiotherapy (SABR) for the treatment of primary RCC in this difficult cohort. SABR is a non-invasive treatment that delivers very high doses of radiation over one to five outpatient sessions. It uses advanced motion management, radiation planning and image guidance techniques to ensure delivery of an ablative dose with millimetre precision. Survival benefits with stereotactic radiosurgery have been demonstrated in patients with solitary brain metastases [2], and SABR is now an accepted standard of care for patients with medically inoperable early-stage lung cancer [3]. Randomized phase III trials are currently under way, testing SABR against standard of care in primary prostate (clinicaltrials.gov ID NCT01584258) and liver cancer (NCT01730937) and in the oligometastatic setting (NCT02759783). Historically considered radio-resistant, both pre-clinical and clinical data now support the sensitivity of RCC to high-dose per fraction radiotherapy, as used in SABR [4].

The study by Siva and colleagues is one of the largest, early-phase, prospective studies of SABR for primary RCC to date, accruing 37 patients with cT1a–cT2a RCC not suitable for other therapies. Importantly, this was not a cohort of incidentally detected small renal masses. The majority (65%) of tumours were >4 cm (median 4.8 cm), were growing on surveillance or symptomatic, and were biopsy-proven. The inclusion of enlarging T1a tumours is not unreasonable. A recent analysis of patients with localized T1a kidney cancer from the Surveillance, Epidemiology and End Results (SEER) Medicare data reported an excess of kidney cancer deaths for non-surgically managed patients aged >75 years, highlighting how difficult it can be to find the right balance between active and expectant management in this group [5]. Indeed 11% of patients in the present study by Siva et al. developed distant metastases by 2 years.

In the present study, tumours <5 cm received a single 26-Gy fraction of SABR, whilst tumours >5 cm received 42 Gy over three fractions. Whilst acknowledging a number of uncertainties in modelling, this should equate to an equivalent biological dose in excess of 100 Gy. The authors found that delivering this SABR regimen was feasible and well tolerated with one grade 3 toxicity (transient fatigue) and no grade 4–5 toxicities. Most patients sustained only transient minor side effects (78%) or no treatment-related side effects (18%). The mean baseline estimated GFR was 55 mL/min, which decreased to 44 mL/min at 1 year, and was maintained for those with 2 years follow-up.

Similarly, short-term efficacy appears promising. With a median follow-up of 24 months, freedom from local progression at 2 years was 100%, with one patient subsequently progressing locally with concurrent distant metastases 28 months after treatment. Local progression was defined using Response Evaluation Criteria In Solid Tumors (RECIST) v1.1. This is a pragmatic definition and takes into account the challenges in interpreting standard imaging after SABR and the difficulties in obtaining and interpreting repeat biopsies in this cohort. Similarly to the study by Sun et al. [6] it appears that stable or partial radiological responses will predominate in the early years after SABR and, unlike thermal ablation, changes in enhancement patterns can be very slow to evolve.

Longer-term follow-up is required to confirm these promising tumour control and nephron preservation rates, in addition to evaluating longer-term late effects. To that end, the present study has provided a platform for the authors to launch an international phase II clinical trial under the auspices of the TransTasman Radiation Oncology Group (TROG 15.03 FASTRACK, clinicaltrials.gov ID NCT02613819). Larger-scale prospective studies are essential to confirm the efficacy and safety of this non-invasive, nephron-sparing, ablative technique and provide further information to help refine patient selection and develop better biomarkers of response.