Figure 1. SAbR of a left lower pole kidney tumor. The isodose plan in three planes showing conformal dose distribution and adequate sparing of nearby bowel. Kidney Cancer Journal 73 essentially utilizes an infinite number of beams, further reduces the fraction of doses in in the path of radiation, intensifying it to the tumor. With even more advancement of technology such as image guided radiation and appropriate tumor motion assessment and management, it is now possible to precisely target even a moving tumor in the kidney that exhibits respiratory motion with pinpoint accuracy. As the experience grows with SAbR in various RCC settings—not just primary but metastatic as well—we need benchmark data regarding changes in size and rate of growth. There is now growing evidence supporting the use of this technique as it becomes more widely adapted clinically and its potential therapeutic benefit is realized. Primary Renal Tumors: Effect of SAbR on Growth Kinetics From the histological evidence gathered in case reports to a meta-analysis on the use of SAbR in the treatment of primary renal tumors, there are numerous papers documenting favorable responses. Based on a review of recent literature, these trends have emerged: • A systematic review5 reported a weighted rate of local control for the treatment of RCC with SAbR in 126 patients to be 93.9%. • Reported toxicities after SAbR appear to be tolerable with severe toxicity of 3.8% and a weighted rate of minor toxicity of 21.4%. • SAbR has been reported to provide local control with preservation of adequate renal function in solitary kidneys and in patients with preexisting chronic kidney disease.26 • The consensus of preliminary results supports a role of SAbR as an alternative treatment option for patients with primary RCC and comorbidities that exclude total or partial nephrectomy.27 Even in the setting of surgical candidacy, it makes sense to consider SAbR since in the rare event that the tumor progresses after SAbR, partial or radical nephrectomy may still be a possibility. Formation of scar tissue in the radiated field has been the concern for surgeons in performing surgery after radiation. However, with SAbR delivering extremely focused doses of radiation, the extent of scar tissue will be limited to regions immediately surrounding the tumor which may keep even the partial nephrectomy options option. Data are clearly lacking in these clinical settings. The trends of SAbR treatment for primary RCC were delineated by Sun et al in their report on the effect of SAbR on the growth kinetics and enhancement pattern of primary renal tumors. Even though this is a retrospective study, the majority of the patients included in this study are from the phase I dose escalation study that escalated doses of 7Gy in 3 fractions all the way to 16Gy in 3 fractions designed by McBride et. al and first reported in the 2013 ASTRO conference.28 In their retrospective study of SAbR over a 5-year period involving 41 renal tumors from 40 patients they found that the mean pretreatment tumor growth rate of 0.68 cm/y decreased to -0.37 cm/y post treatment (P<0.0001), and the mean tumor volume growth rate of 21.2 cm3/per year before treatment decreased to -5.35 cm3/y after treatment (P=0.002). Local control defined as less than 5 mm of growth—was achieved in 38 of 41 tumors. Interestingly, the three failures in this study were already reported in the 2013 ab-
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