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Stereotactic Body Radiation Therapy for the Definitive Treatment of Early Stage Kidney Cancer: A Survival Comparison With Surgery, Tumor Ablation, and Observation

Open AccessPublished:January 21, 2020DOI:https://doi.org/10.1016/j.adro.2020.01.002

      Abstract

      Purpose

      Partial nephrectomy is the preferred definitive treatment for early stage kidney cancer, with tumor ablative techniques or active surveillance reserved for patients not undergoing surgery. Stereotactic body radiation therapy (SBRT) has emerged as a potential noninvasive alternative for patients with early stage kidney cancer not amenable to surgery, with early reports suggesting excellent rates of local control and limited toxicity.

      Methods and Materials

      The national cancer database from 2004 to 2014 was queried for patients who received a diagnosis of T1N0M0 kidney cancer. Treatments were categorized as surgery (partial or total nephrectomy), tumor ablation (cryoablation or thermal ablation), SBRT (radiation therapy in 5 fractions or less to a total biological effective dose [BED10] of 72 or more), or observation. A propensity score was generated by multinomial logistic regression. A Cox proportional hazards model was fit to determine association between overall survival and treatment group with propensity score adjustments for patient, demographic, and treatment characteristics.

      Results

      A total of 165,298 received surgery, 17,196 underwent tumor ablation, 104 underwent SBRT, and 18,241 were observed. Median follow-up was 51 months. On multivariable analysis, surgery, tumor ablation, and SBRT were associated with a decreased risk of death compared with observation, with hazard ratios of 0.25 (95% confidence interval, 0.24-0.26, P < .001), 0.36 (0.35-0.38, P < .001), and 0.56 (0.39-0.79, P < .001), respectively. When stratifying by BED10 and compared with observation, hazard ratio for risk of death for patients treated with SBRT to a BED10 ≥100 (n = 62) and a BED10 <100 (n = 42) was 0.34 (0.19-0.60, P < .001) and 0.90 (0.58-1.4, P = .64), respectively.

      Conclusions

      In this population-based cohort, patients undergoing high-dose SBRT (BED10 ≥100) for early stage kidney cancer demonstrated longer survival compared with patients undergoing observation. This may be a promising noninvasive treatment option for nonsurgical candidates with prospective efficacy and safety assessments meriting study in future clinical trials.

      Introduction

      Partial nephrectomy is the preferred definitive treatment for early stage kidney cancer, with tumor ablative techniques or active surveillance reserved for patients not undergoing surgery.
      • Ljungberg B.
      • Bensalah K.
      • Canfield S.
      • et al.
      EAU guidelines on renal cell carcinoma: 2014 update.
      With an increase in the number of incidentally diagnosed kidney cancers and in an increasingly elderly population who may not tolerate invasive procedures,
      • King S.C.
      • Pollack L.A.
      • Li J.
      • King J.B.
      • Master V.A.
      Continued increase in incidence of renal cell carcinoma, especially in young patients and high grade disease: United States 2001 to 2010.
      ,
      • Saad A.M.
      • Gad M.M.
      • Al-Husseini M.J.
      • et al.
      Trends in renal-cell carcinoma incidence and mortality in the United States in the last 2 decades: A SEER-based study.
      stereotactic body radiation therapy (SBRT) has emerged as a potential noninvasive alternative for patients not amenable to surgery, with early reports suggesting excellent rates of local control and limited toxicity.
      • Funayama S.
      • Onishi H.
      • Kuriyama K.
      • et al.
      Renal cancer is not radioresistant: Slowly but continuing shrinkage of the tumor after stereotactic body radiation therapy.
      • Ponsky L.
      • Lo S.S.
      • Zhang Y.
      • et al.
      Phase I dose-escalation study of stereotactic body radiotherapy (SBRT) for poor surgical candidates with localized renal cell carcinoma.
      • Siva S.
      • Pham D.
      • Kron T.
      • et al.
      Stereotactic ablative body radiotherapy for inoperable primary kidney cancer: A prospective clinical trial.
      • Siva S.
      • Kothari G.
      • Muacevic A.
      • et al.
      Radiotherapy for renal cell carcinoma: renaissance of an overlooked approach.
      • Correa R.J.M.
      • Louie A.V.
      • Zaorsky N.G.
      • et al.
      The emerging role of stereotactic ablative radiotherapy for primary renal cell carcinoma: A systematic review and meta-analysis.
      • Siva S.
      • Louie A.V.
      • Warner A.
      • et al.
      Pooled analysis of stereotactic ablative radiotherapy for primary renal cell carcinoma: A report from the International Radiosurgery Oncology Consortium for Kidney (IROCK).
      This study uses a large national cancer registry to assess patterns of care and survival outcomes in patients with stage I kidney cancer treated with SBRT.

      Methods and Materials

      The national cancer database was queried from 2004 to 2014 for patients who received a diagnosis of T1N0M0 kidney cancer (7 cm or less with no nodal or distant metastases). Both clinical and pathologic staging was used to determine patient inclusion. Histologic subtypes were limited to clear cell carcinoma, papillary carcinoma, renal cell not otherwise specified (NOS), and carcinoma NOS. Exclusion criteria is listed in Figure 1. Treatments were categorized as surgery (including partial and total nephrectomy), tumor ablation (including cryoablation and thermal ablation), SBRT, or observation. SBRT was defined as radiation therapy in 5 fractions or less to a total biological effective dose (BED10) of 72 or more assuming a tumor α/βvalue of 10. Although there is limited research into the radiobiology of kidney SBRT, studies from non-small cell lung cancer suggest a BED10 of approximately 70 corresponds to the lower limit of what may be considered an acceptable tumor control probability.
      • Mehta N.
      • King C.R.
      • Agazaryan N.
      • Steinberg M.
      • Hua A.
      • Lee P.
      Stereotactic body radiation therapy and 3-dimensional conformal radiotherapy for stage I non-small cell lung cancer: A pooled analysis of biological equivalent dose and local control.
      • Macià I.
      • Garau M.
      Radiobiology of stereotactic body radiation therapy (SBRT).
      • Park S.
      • Urm S.
      • Cho H.
      Analysis of biologically equivalent dose of stereotactic body radiotherapy for primary and metastatic lung tumors.
      Because a BED10 of 100 has been shown to be an important cut point for outcomes in multiple other disease sites,
      • Ohri N.
      • Tomé W.A.
      • Méndez Romero A.
      • et al.
      Local control after stereotactic body radiation therapy for liver tumors.
      ,
      • Onishi H.
      • Araki T.
      • Shirato H.
      • et al.
      Stereotactic hypofractionated high-dose irradiation for stage I nonsmall cell lung carcinoma: Clinical outcomes in 245 subjects in a Japanese multiinstitutional study.
      patients receiving SBRT were dichotomized by those treated to a BED10 of <100 or ≥100. This project was reviewed by our institutional review board and found to be exempted.
      Figure thumbnail gr1
      Figure 1Consort diagram displaying. Abbreviations: NCDB = National Cancer Database; SBRT = stereotactic body radiation.
      A propensity score was generated by multinomial logistic regression, and a Cox proportional hazard model was fit to determine association between treatment group and overall survival (OS) with propensity score adjustments for patient, demographic, and treatment characteristics, including age at diagnosis, race, sex, year of diagnosis, Charlson-Deyo comorbidity score, tumor size, laterality, histology, grade, insurance plan, rurality, median income, education, academic hospital, and distance traveled for treatment. The proportional hazard assumption was visually checked. To reduce lead time bias, patients were excluded if they died or were lost to follow-up before 2.67 months from diagnosis, corresponding to the time in which 90% of subjects had started definitive treatment. Approximately 3.5% of all patients (1.9% of surgery patients, 1.3% of tumor ablation patients, no SBRT patients, and 16.8% of observation patients) were excluded from analysis by this follow-up time constraint.

      Results

      A total of 200,839 patients were included, of whom 165,298 received surgery (median follow-up 57 months), 17,196 underwent tumor ablation (median follow-up 50 months), 104 underwent SBRT (median follow-up 37 months), and 18,241 were observed (median follow-up 19 months; Table 1). The most common fractionation schemes for patients receiving SBRT were 40 Gy in 5 fractionation for the BED10 <100 cohort (42 patients) and 48 Gy in 3 fractionation for the BED10 ≥100 cohort (62 patients; Table 2).
      Table 1Patient and tumor characteristics
      TreatmentP value
      SurgeryTumor ablationSBRTObservation
      165,298(%)17,196(%)104(%)18,241(%)
      Age (y)<.001
       Median61697574
       18-4933,31220.2%11656.8%11.0%11226.2%
       50-6467,87041.1%486028.3%2423.1%390221.4%
       >6464,11638.8%11,17165.0%7976.0%13,21772.5%
      Race<.001
       White122,02473.8%13,23076.9%7572.1%12,97271.1%
       Black18,67111.3%176510.3%1514.4%274915.1%
       Hispanic20,05512.1%182810.6%109.6%206911.3%
       Other45482.8%3732.2%43.8%4512.5%
      Sex<.001
       Female65,64439.7%641137.3%3735.6%765542.0%
       Male99,65460.3%10,78562.7%6764.4%10,58658.0%
      Year of diagnosis<.001
       200411,5977.0%4362.5%00.0%10345.7%
       200512,4007.5%7354.3%00.0%12476.8%
       200613,4888.2%11616.8%11.0%13197.2%
       200714,3948.7%14168.2%54.8%14698.1%
       200814,7538.9%176210.2%1110.6%15548.5%
       200915,6009.4%190511.1%98.7%16308.9%
       201015,1909.2%191711.1%1817.3%17169.4%
       201116,2029.8%186410.8%1413.5%17869.8%
       201216,99110.3%192811.2%1211.5%193910.6%
       201317,18310.4%196411.4%2120.2%233012.8%
       201417,50010.6%210812.3%1312.5%221712.2%
      Charlson-Deyo comorbidity score<.001
       0115,16569.7%11,55167.2%8379.8%12,17866.8%
       137,41022.6%402923.4%1817.3%368520.2%
       293875.7%11826.9%21.9%15558.5%
       333362.0%4342.5%11.0%8234.5%
      Tumor stage<.001
       T1a110,01466.6%15,18888.3%6360.6%11,83764.9%
       T1b55,28433.4%200811.7%4139.4%640435.1%
      Tumor size (mm)<.001
       Median34.926.535.832.7
       0-2557,24534.6%939754.6%2927.9%829445.5%
       26-5081,52449.3%752743.8%6562.5%780742.8%
       51-7026,52916.0%2721.6%109.6%2,14011.7%
      Laterality<.001
       Left79,98748.4%840748.9%4644.2%877148.1%
       Right85,08051.5%877551.0%5855.8%928550.9%
       Unknown2310.1%140.1%00.0%1851.0%
      Histology<.001
       Clear cell carcinoma92,15755.8%677939.4%3735.6%269314.8%
       Papillary carcinoma26,45716.0%246514.3%1312.5%10886.0%
       Renal cell carcinoma NOS46,38228.1%754543.9%4745.2%12,79270.1%
       Carcinoma NOS3020.2%4072.4%76.7%16689.1%
      Tumor grade<.001
       123,58014.3%268515.6%1110.6%9545.2%
       284,04050.8%433025.2%2322.1%15458.5%
       330,16118.2%6123.6%00.0%3602.0%
       426231.6%350.2%00.0%500.3%
       Unknown24,89415.1%953455.4%7067.3%15,33284.1%
      Academic treatment facility<.001
       Yes65,41039.6%736242.8%4947.1%659336.1%
       No90,19754.6%956055.6%5451.9%11,34662.2%
       Unknown96915.9%2741.6%11.0%3021.7%
      Abbreviations: NOS = not otherwise specified; SBRT = stereotactic body radiation therapy.
      Table 2Patient and tumor characteristics by radiation dose
      SBRT doseP value
      BED <100BED≥100
      42(%)62(%)
      Age (y).47
       Median7573
      Charlson-Deyo comorbidity score.83
       03378.6%5080.7%
       1819.1%1016.1%
       212.4%11.6%
       300.0%11.6%
      Tumor stage.32
       T1a2354.8%4064.5%
       T1b1945.2%2235.5%
      Tumor size (mm).06
       Median39.233.5
      Laterality.57
       Left2047.6%2641.9%
       Right2252.4%3658.1%
      Histology.16
       Clear cell carcinoma2047.6%1727.4%
       Papillary carcinoma37.1%1016.1%
       Renal cell carcinoma NOS1740.5%3048.4%
       Carcinoma NOS24.8%58.1%
      Most common fractionation (fx) schemesN/A
       40 Gy in 5 fx1331.0%
       39 Gy in 3 fx921.4%
       36 Gy in 3 fx921.4%
       48 Gy in 3 fx2946.8%
       45 Gy in 3 fx914.5%
       50 Gy in 5 fx812.9%
      Abbreviations: BED = biological effective dose; NOS = not otherwise specified; SBRT = stereotactic body radiation therapy.
      At a median follow-up of 51 months, 40,489 patients (20.2%) had died with 5-year OS estimate shown in Table 3 and Figure 2. On multivariable analysis with propensity score adjustment, patients undergoing surgery, tumor ablation, and SBRT were associated with a decreased risk of death compared with patients undergoing observation, with a hazard ratio (HR) of 0.25 (95% confidence interval [CI] 0.24-0.26, P < .001), 0.36 (0.35-0.38, P < .001), and 0.56 (0.39-0.79, P < .001), respectively. Compared with observation, HR for risk of death for SBRT patients treated to a BED10 <100 and a BED10 ≥100 was 0.90 (0.58-1.4, P = .64) and 0.34 (0.19-0.60, P < .001), respectively (Table 4). A sensitivity analysis using Cox regression with propensity score adjustment stratified into quintiles provided similar results (Table 5).
      Table 3Unadjusted 5-year overall survival estimates by treatment group
      Patients

      N
      Events

      N
      5-year estimated OS(95% CI)P value
      All200,83940,4890.82(0.81, 0.82)
      Surgery165,29826,7680.86(0.86, 0.86)<.001
      Tumor ablation17,19641800.77(0.76, 0.77)
      SBRT, BED <10042200.42(0.25, 0.59)
      SBRT, BED ≥10062120.73(0.56, 0.84)
      Observation18,24195090.43(0.42, 0.43)
      Abbreviations: BED = biological effective dose; SBRT = stereotactic body radiation therapy.
      Figure thumbnail gr2
      Figure 2Kaplan-Meier survival curves of overall survival by treatment groups with SBRT cohorts combined (a and b) or separated by BED10 (c and d). Abbreviations: Abl = tumor ablation; Obs = observation; SBRT = stereotactic body radiation therapy; SRG = surgery.
      Table 4Cox proportional hazards regression for overall survival with propensity score adjustments
      Models additionally adjusted for age at diagnosis, race, sex, year of diagnosis, Charlson-Deyo comorbidity score, tumor size, laterality, histology, grade, insurance plan, rurality, median income, education, academic hospital, distance traveled for treatment and propensity score.
      HR(95% CI)P value
      Treatment
       Observation1
       Surgery0.25(0.24, 0.26)<.001
       Tumor ablation0.36(0.35, 0.38)<.001
       SBRT0.56(0.39, 0.79)<.001
       BED <1000.9(0.58, 1.4).64
       BED ≥1000.34(0.19, 0.6)<.001
      Age (y)
       18-491
       50-641.75(1.66, 1.84)<.001
       >642.85(2.71, 2.99)<.001
      Race
       White1
       Black0.99(0.96, 1.02).5
       Hispanic0.88(0.85, 0.91)<.001
       Other0.8(0.74, 0.86)<.001
      Sex
       Female1
       Male1.16(1.13, 1.18)<.001
      Year of diagnosis
       2004-20091
       2010-20120.89(0.87, 0.91)<.001
       2013-20140.88(0.85, 0.92)<.001
      Charlson- Comorbidity Score
       01
       11.38(1.35, 1.41)<.001
       21.97(1.91, 2.04)<.001
       32.56(2.45, 2.68)<.001
      Tumor size
       0-25 mm1
       26-50 mm1.21(1.19, 1.24)<.001
       51-70 mm1.56(1.52, 1.61)<.001
      Laterality
       Left1
       Right0.98(0.96, 1.00).09
      Histology
       Clear cell carcinoma1
       Papillary carcinoma1.01(0.98, 1.04).67
       Renal cell carcinoma NOS1.13(1.11, 1.16)<.001
       Carcinoma NOS1.33(1.25, 1.42)<.001
      Grade
       11
       21.05(1.01, 1.08).007
       3, 41.271.23-1.33<.001
      Abbreviations: BED = biological effective dose; OS = not otherwise specified; SBRT = stereotactic body radiation therapy.
      Models additionally adjusted for age at diagnosis, race, sex, year of diagnosis, Charlson-Deyo comorbidity score, tumor size, laterality, histology, grade, insurance plan, rurality, median income, education, academic hospital, distance traveled for treatment and propensity score.
      Table 5Cox proportional hazards regression for overall survival by propensity score stratification in quintiles
      Models additionally adjusted for age at diagnosis, race, sex, year of diagnosis, Charlson-Deyo comorbidity score, tumor size, laterality, histology, grade, insurance plan, rurality, median income, education, academic hospital, distance traveled for treatment and propensity score.
      HR(95% CI)P value
      Treatment
       Observation1
       Surgery0.20(0.19, 0.20)<.001
       Tumor ablation0.32(0.31, 0.33)<.001
       SBRT0.52(0.37, 0.74)<.001
       BED <1000.85(0.55, 1.32).64
       BED ≥1000.32(0.18, 0.56)<.001
      Abbreviations: BED = biological effective dose; CI = confidence interval; HR = hazard ratio; NOS = not otherwise specified; SBRT = stereotactic body radiation therapy.
      Models additionally adjusted for age at diagnosis, race, sex, year of diagnosis, Charlson-Deyo comorbidity score, tumor size, laterality, histology, grade, insurance plan, rurality, median income, education, academic hospital, distance traveled for treatment and propensity score.

      Discussion

      In this analysis, we show that SBRT for primary kidney cancer is an uncommon treatment in the United States despite an increasing number of diagnosed patients, emerging evidence for the safety and efficacy of the treatment, and recent technical improvements in radiation delivery.
      • King S.C.
      • Pollack L.A.
      • Li J.
      • King J.B.
      • Master V.A.
      Continued increase in incidence of renal cell carcinoma, especially in young patients and high grade disease: United States 2001 to 2010.
      • Saad A.M.
      • Gad M.M.
      • Al-Husseini M.J.
      • et al.
      Trends in renal-cell carcinoma incidence and mortality in the United States in the last 2 decades: A SEER-based study.
      • Funayama S.
      • Onishi H.
      • Kuriyama K.
      • et al.
      Renal cancer is not radioresistant: Slowly but continuing shrinkage of the tumor after stereotactic body radiation therapy.
      • Ponsky L.
      • Lo S.S.
      • Zhang Y.
      • et al.
      Phase I dose-escalation study of stereotactic body radiotherapy (SBRT) for poor surgical candidates with localized renal cell carcinoma.
      • Siva S.
      • Pham D.
      • Kron T.
      • et al.
      Stereotactic ablative body radiotherapy for inoperable primary kidney cancer: A prospective clinical trial.
      We demonstrate that this is a recently adopted treatment, with no reported cases of primary kidney SBRT in 2004 or 2005 and only one case in 2006. Moreover, we show that patients treated with SBRT, and in particular, those with a BED10 ≥100, demonstrated an improved OS at 5 years compared with those who were observed, even after adjusting for patient and tumor characteristics. This outcome may in part reflect patient selection based on clinical factors not available or measured in covariates. For example, the median size of tumors in the higher BED cohort was 33.5 mm compared with 39.2 mm in the lower BED group, which suggests that BED may in part be a surrogate for tumor size. Still, the improved survival in patients treated with SBRT to a BED10 ≥100 versus BED10 <100 persisted after propensity-adjustments and generates the hypothesis that radiation treatment, particularly at highly ablative doses, may have the potential to significantly alter the disease course in treated patients. This analysis supports prior single center studies that generally explored SBRT for primary kidney cancer with highly ablative doses
      • Funayama S.
      • Onishi H.
      • Kuriyama K.
      • et al.
      Renal cancer is not radioresistant: Slowly but continuing shrinkage of the tumor after stereotactic body radiation therapy.
      • Ponsky L.
      • Lo S.S.
      • Zhang Y.
      • et al.
      Phase I dose-escalation study of stereotactic body radiotherapy (SBRT) for poor surgical candidates with localized renal cell carcinoma.
      • Siva S.
      • Pham D.
      • Kron T.
      • et al.
      Stereotactic ablative body radiotherapy for inoperable primary kidney cancer: A prospective clinical trial.
      and ongoing prospective clinical trials.
      • Siva S.
      • Chesson B.
      • Bressel M.
      • et al.
      TROG 15.03 phase II clinical trial of Focal Ablative STereotactic Radiosurgery for Cancers of the Kidney: FASTRACK II.
      For patients who are not ideal candidates for surgical resection, potential options include tumor ablation with cryotherapy or radiofrequency ablation, SBRT, or observation. SBRT may be an attractive treatment option for many patients for several reasons. First, SBRT is able to treat tumors larger than 4 cm or tumors located near the renal pelvis, criteria which are generally unsuitable for interventional radiology–guided tumor ablation.
      • Maria T.
      • Georgiades C.
      Percutaneous cryoablation for renal cell carcinoma.
      ,
      • Wah T.M.
      • Irving H.C.
      • Gregory W.
      • et al.
      Radiofrequency ablation (RFA) of renal cell carcinoma (RCC): Experience in 200 tumours.
      In this analysis, nearly 40% of SBRT tumors were >4 cm, compared with just 12% of ablated tumors. Second, SBRT is a noninvasive treatment with no associated anesthesia risk or prolonged recovery time. Third, SBRT is convenient for the patient, with treatment generally completed in 5 days or less or, in many cases, in a single day.
      Limitations include the small number of patients treated with SBRT compared with other cohorts and the potential for confounding factors. Without information on cancer specific mortality or cause of death, and in a disease where overall outcomes are expected to be favorable,
      • Gill I.S.
      • Aron M.
      • Gervais D.A.
      • Jewett M.A.
      Clinical practice. Small renal mass.
      it is unclear whether the observed differences are related to differences in treatment or patient selection. Our findings of improved OS in patients treated with BED10 ≥100 compared with <100 are surprising because the risk of distant metastases and cancer-specific death in patients with T1N0M0 kidney cancer is relatively low.
      • Crispen P.L.
      • Wong Y.N.
      • Greenberg R.E.
      • Chen D.Y.
      • Uzzo R.G.
      Predicting growth of solid renal masses under active surveillance.
      • Jewett M.A.
      • Mattar K.
      • Basiuk J.
      • et al.
      Active surveillance of small renal masses: Progression patterns of early stage kidney cancer.
      • McIntosh A.G.
      • Ristau B.T.
      • Ruth K.
      • et al.
      Active surveillance for localized renal masses: Tumor growth, delayed intervention rates, and >5-yr clinical outcomes.
      • Smaldone M.C.
      • Kutikov A.
      • Egleston B.L.
      • et al.
      Small renal masses progressing to metastases under active surveillance: A systematic review and pooled analysis.
      Indeed, prior single institutional studies of kidney SBRT demonstrated very low rates of local failure.
      • Funayama S.
      • Onishi H.
      • Kuriyama K.
      • et al.
      Renal cancer is not radioresistant: Slowly but continuing shrinkage of the tumor after stereotactic body radiation therapy.
      • Ponsky L.
      • Lo S.S.
      • Zhang Y.
      • et al.
      Phase I dose-escalation study of stereotactic body radiotherapy (SBRT) for poor surgical candidates with localized renal cell carcinoma.
      • Siva S.
      • Pham D.
      • Kron T.
      • et al.
      Stereotactic ablative body radiotherapy for inoperable primary kidney cancer: A prospective clinical trial.
      Even if higher BED10 leads to improved local control, it is unclear if this would drive a survival benefit in this population during this period of follow-up.
      Other limitations include the potential for discrepancies between staging technique between treatment cohorts (ie, patients treated with SBRT are staged only clinically, compared with those undergoing surgery who are staged pathologically). In addition, this analysis grouped together patients treated with both total and partial nephrectomy, although these are distinct treatments with likely distinct outcomes. Furthermore, we excluded any patients who received systemic therapy as a component of initial treatment, which may erroneously exclude patients receiving planned adjuvant systemic therapy after surgery, tumor ablation, or SBRT. Additionally, patients with less than 2.67 months of follow-up were excluded in an attempt to reduce lead time bias. Although such exclusion criteria may limit capture of perioperative or treatment-related mortality, perioperative mortality after nephrectomy or partial nephrectomy is low.
      • Pereira J.
      • Renzulli 2nd, J.
      • Pareek G.
      • et al.
      Perioperative morbidity of open versus minimally invasive partial nephrectomy: A contemporary analysis of the National Surgical Quality Improvement Program.
      Finally, this study does not include cases treated in the past several years given the nature of national cancer database reporting and the lag between treatment and data collection and distribution.

      Conclusions

      SBRT for early stage kidney cancer may be a promising noninvasive treatment option for nonsurgical patients. Despite the small number of patients treated with SBRT and potential for unmeasured confounding factors, a national registry study such as this may be the only current viable way to compare outcomes after SBRT in early stage kidney cancer given its extremely limited utilization at present. The efficacy and safety of this approach is being evaluated in ongoing prospective clinical trials.
      • Siva S.
      • Chesson B.
      • Bressel M.
      • et al.
      TROG 15.03 phase II clinical trial of Focal Ablative STereotactic Radiosurgery for Cancers of the Kidney: FASTRACK II.

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