SBRT vs. moderate hypofractionation: same or better quality of life

We have seen in several randomized clinical trials of external beam treatment of primary prostate cancer that moderately hypofractionated IMRT (HypoIMRT) treatment (accomplished in 12-26 treatments or fractions) is no worse than conventionally fractionated IMRT treatment (in 40-44 fractions).  We recently saw in a randomized clinical trial from Scandinavia that SBRT (in 5 fractions) is no worse than conventional IMRT (see this link) in long-term quality-of-life outcomes, even though they used inferior technology. The missing piece of the puzzle is to answer the question of whether SBRT is any worse than HypoIMRT.

We don’t yet have a definitive answer (which would require a randomized clinical trial), but an analysis of pooled data from 5 different clinical trials, suggests that SBRT is no worse and may be better than HypoIMRT in its urinary, rectal, and sexual outcomes. Johnson et al. pooled SBRT data from clinical trials among 534 men at 3 institutions (UCLA, Georgetown, and 21^st Century Oncology) and HypoIMRT data from clinical trials among 378 men at Fox Chase Cancer Center and the University of Wisconsin. All patients were treated between 2002 and 2013 at those top institutions, with state-of-the-art equipment in the context of carefully controlled clinical trials. Because of this, all outcomes are probably better than those achieved in everyday community practice. The only significant difference in patient characteristics was that SBRT patients were about 5 years older (69 vs. 64 years of age for HypoIMRT). We expect older men to have more natural deterioration in urinary and sexual function.

The following table shows the percent of men receiving each treatment who suffered from at least the minimally detectable difference in patient-reported scores on validated quality-of-life questionnaires with respect to urinary, rectal, and sexual function. Numbers in bold typeface represent a statistically significant difference.

	SBRT	HypoIMRT	Odds Ratio (adjusted)
Urinary	14%	33%	0.24
Rectal	25%	37%	0.66
Sexual	33%	39%	0.73

The data support the following conclusions:

• Urinary and rectal problems at 2 years were experienced by fewer of the men who had SBRT.
• Urinary and rectal problems improved after 2 years compared to 1 year post-treatment. For SBRT, they approached baseline values.
• Sexual issues did not improve at 2 years.
• While we expected the SBRT patients to experience greater deterioration owing to their age, the opposite occurred.

(update: 4/11/2020) Kwan et al. reported on 78 patients randomized to SBRT (36.25 Gy in 5 weekly treatments) or moderate hypofractionation (70 Gy in 28 treatments). After at least 6 months of follow-up:

• there were no statistically significant differences in grade 2+ or grade 3 toxicities
• there were no minimally important differences in patient-reported quality of life on incontinence, irritative/obstructive urinary issues or bowel issues.

Why were the SBRT outcomes better?

SBRT is not just a high-dose-per-fraction version of IMRT, although it is that too. When the linear accelerator is delivering only 2 Gy per fraction, missing the beam target by a little bit is not likely to make much difference – it will average out in the long run. Because a geographic “miss” of the beam target has much greater consequence for SBRT, where the dose per fraction can be 8 Gy, much more care is taken to achieve pinpoint accuracy. This includes such steps as:

• Fiducials/transponders aligned within each treatment and not just between treatments.
• Fast linear accelerators that minimize the time during which the prostate can move.
• No treatment if the bowel is distended or the bladder is not full.
• Tighter margins: as low as 0 mm on the rectal side and 2 mm on the front side. This compares to margins of 0.5-1 cm for IMRT.
• Narrower dose constraints for organs at risk, including the bladder, rectum, urethra, femurs and penile bulb.
• More care taken to find a plan that optimizes prostate dose relative to organs at risk.

It is entirely possible that IMRT outcomes might be equivalent to SBRT outcomes if the same factors were incorporated into IMRT planning and delivery. But fractionation probably has an effect as well. To understand why, we must look at the radiobiology of prostate cancer. Prostate cancer has been found to respond remarkably well to fewer yet higher doses of radiation. This is reflected in a characteristic called the “alpha/beta ratio (α/β).” The α/β of prostate cancer is very low, at about 1.5. It is lower, in fact, than that of surrounding healthy tissues. Many of those healthy tissues have an early response, which is responsible for acute toxicity, typically within 3 months of treatment (α/β = 10.0). Rectal mucosal tissue is an example. This means that a hypofractionated dosing schedule will kill relatively more cancer cells, while preserving more of the cells in the nearby organs.

There are fewer types of tissue in the pelvic area that have a delayed response to radiation, and those tissues, like nerve cells, tend to be radio-resistant. This is why late-term toxicity is relatively low. Some of the late-term effects we do see are due to cumulative responses to radiation, like the buildup of scar tissue and other reactive responses in vasculature, along the urethra, and in the rectum. Late responding tissue has an α/β of about 3.5

We can compare the biologically effective dose (BED) of the various dosing schedules to see the effect that hypofractionation would theoretically have in killing cancer cells and preserving healthy tissue.

	BED for cancer control	Relative BED for cancer control	BED for acute side effects	Relative BED for acute side effects	BED for late side effects	Relative BED for late side effects
80 Gy in 40 fractions	187 Gy	1.00	96 Gy	1.00	126 Gy	1.00
60 Gy in 20 fractions	180 Gy	0.96	78 Gy	0.81	111 Gy	0.89
40 Gy in 5 fractions	253 Gy	1.35	72 Gy	0.75	131 Gy	1.05

So the kind of fractionation used in SBRT theoretically has about 35% more effective cancer-killing power than conventional fractionation, while its ability to generate acute toxic side effects is reduced by 25%, and its late-term side effects would be similar.

Why isn’t everyone who elects to have primary treatment with external beam radiation treated with SBRT?

It’s one thing to make predictions based on theory, but it’s quite another to determine whether it works as well in clinical practice. So far, non-randomized trials like the ones examined in this study have shown excellent oncological and quality-of-life outcomes for SBRT with up to 9 years of follow-up. We await the oncological results of randomized trials comparing SBRT to IMRT. The oncological outcomes from the randomized Scandinavian trial are expected any time now. There are several others that are ongoing.

With SBRT, the patient enjoys the obvious benefits of appreciably lower cost and a more convenient therapy regimen. Medicare and most (but far from all) insurance companies now cover SBRT. There is considerable resistance from radiation oncologists in private practice who would get reduced revenues, and would have to learn the new techniques and gain adequate experience in using them.

5-year SBRT trial: high cancer control, low toxicity

(9/10/2018)
Meier et al reported the results of a 5-year multi-institutional trial, (also reported at the 2017 ASTRO meeting), finding that SBRT had high rates cancer control and low toxicity.

This was a prospective clinical trial in which all 21 institutions treated 309 patients according to the same protocol. The institutions were community, regional and academic hospitals across the US. All patients were low (56%) or intermediate risk (44%). Of the 137 intermediate risk patients,  61% were favorable and 39% were unfavorable intermediate risk. The treatment was:

• 40 Gy in 5 treatments to the prostate
• 36.25 Gy to the seminal vesicles in intermediate risk patients
• No concurrent or adjuvant androgen deprivation therapy was allowed.

At five years after SBRT treatment, the following oncological outcomes were reported:

• 97.1% had no biochemical progression; that is, no increases in PSA to over 2 ng/ml from the lowest value achieved 
  o 97.3% for low risk patients, compared to 92.3% for IMRT historically
  o 97.1% for intermediate risk patients, compared to 91.3% for IMRT historically
       - 100% among favorable intermediate risk
       - 93.1% among unfavorable intermediate risk

By five years after SBRT treatment, the late toxicity outcomes were reported:

• No grade 3 (serious) rectal side effects
• Grade 2 rectal side effects in 2%

• Grade 3 (serious) urinary side effects in 4 of the 309 patients (1.3%)
• Grade 2 urinary side effects in 12%

These are certainly excellent outcomes, and are in-line with or better than retrospective SBRT studies that have previously been reported. So far, the longest running SBRT single institution study has been reported by Alan Katz (see this link). I’ve heard that a ten-year update is in the works. That will be as long and larger than the longest running IMRT trial.

SBRT is about half the cost of IMRT, and at only 5 treatments, is certainly a lot less bother for the patients. It has excellent outcomes even without adjuvant ADT in unfavorable intermediate risk patients. With large long-term studies now available, it is difficult to understand why some insurance companies still don’t cover it.

Toxicity equal for SBRT and conventional external beam radiation

There has been some question as to whether the toxicity of delivering very high doses of external beam radiation per treatment (or fraction) in fewer fractions (called “extreme hypofractionation” or SBRT) would be high compared to conventional dose rates per fraction. While SBRT practitioners have reported very low toxicity rates (see table in this link), there has always been some doubt because there may have been some bias in how patients were selected in the various studies.

The HYPO-RT-PC trial was the first trial ever to randomly assign patients to one kind of radiation or the other. Between 2005 and 2015, they enrolled 1200 intermediate-risk patients in Scandinavia to receive either:

1.  Conventional fractionation: 78 Gy in 39 fractions
2. SBRT: 42.7 Gy in 7 fractions

The biologically effective dose is 19% higher for SBRT in terms of cancer control. The biologically effective doses are equivalent in terms of toxicity.

There were a few differences from some US practices:

• “Intermediate risk” was defined as one or two of the following 3 risk factors:

1. Stage T1c-T3a (T3a is a high risk factor in the commonly used US definition)
2. PSA> 10 ng/ml (PSA> 20 ng/ml is a high risk factor in the commonly used US definition)
3. Gleason score ≥7 (Gleason scores greater than 7 are a high risk factor in the commonly used US definition)

• 80% of the men were treated with a technology called 3D-CRT, which is seldom used for external beam therapy anymore at major tertiary care centers. It is never used for SBRT in the US because it is considered not precise enough, and too toxic.

• SBRT is usually delivered in 4 or 5 fractions in the US. CyberKnife and VMAT are the most common technologies in use, and use of sophisticated image guidance throughout each treatment is a common practice.

The toxicity results are based on 866 patients who had 2-year follow-up results. There were some differences in acute toxicity:

• Acute urinary toxicity was 27.6% for the SBRT group and 22.8% for the conventional fractionation group, but the difference was not statistically significant.
• Acute rectal toxicity was 9.4% for the SBRT group and 5.3% for the conventional fractionation group. The difference was statistically significant, but narrowed by 3 or 6 months.

Neither physician-reported toxicity nor patient-reported late-term toxicity differed by the fractionation schedule they received. By two years:

• Late-term urinary side effects were reported by 5.4% of the SBRT group and 4.6% of the conventional fractionation group. The difference was not statistically significant.
• Late term rectal side effects were reported by 2.2% of the SBRT group and 3.7% of the conventional fractionation group. The difference was not statistically significant.
• Impotence was reported by 34% of both groups, up from 16% at baseline.
• Patient-reported bother from urinary, rectal and sexual side effects were not different.

Given their use of the largely outmoded 3D-CRT technology, it was not surprising that acute toxicity would be elevated. I’m frankly surprised that late-term toxicity was not higher for SBRT.

They plan to present their findings on oncological outcomes at a future time.

Safety limits of SBRT dose escalation

In a recent commentary, we saw that the lack of a standard of care for SBRT dose escalation may put patients at risk when dose limits are pushed beyond what is customarily considered effective and safe. Hannan et al. have now published their efficacy findings. Further details of the IRB-approved clinical trial specs are available here.

Between 2006 and 2011, the researchers at several institutions conducted a dose escalation trial utilizing SBRT on 91 men treated for low and intermediate risk prostate cancer. Among those men:

• ·      64% were intermediate risk, defined as:

o   Either GS 6 and PSA between 10 and 20 ng/ml , or

o   GS 7 with PSA≤ 15 ng/ml and clinical stage ≤ T2b

• ·      36% were low risk by the NCCN definition.

All patients received 5 treatments or fractions. The first 15 patients were treated with 45 Gy, the next 15 with 47.5 Gy, the next 15 with 50 Gy. Because that last group did not exhibit their predefined “maximally tolerated dose” in the short term, an additional 47 patients also received the 50 Gy dose.

The cancer control was excellent. At 5 years after treatment:

• ·      98.6% were free from biochemical failure
• ·      100% were free from metastases
• ·      None had died of prostate cancer
• ·      Overall survival was 89.7%

Toxicity was another matter. There were no reports of serious acute urinary toxicity. However, late-term urinary toxicity of grade 3 or greater was reported in 5.5% of patients. For the purposes of their analysis, acute toxicities were those observed within 9 months of treatment, and late-term toxicities were those observed between 9 and 18 months.

Rectal toxicity was reported in detail earlier by Kim et al. and merit a closer look:

• ·      Among those who received 45 Gy there was no serious (grade 3 or higher) acute or late term toxicity.

o   No acute grade 2 toxicity was observed.

o   Late-term grade 2 toxicity was observed in 1 patient (of 15).

• ·      Among those who received 47.5 Gy there was no serious (grade 3 or higher) acute or late term toxicity.

o   Acute grade 2 toxicity was observed in 4 of 15 patients (27%)

o   Late-term grade 2 toxicity was observed in 5 of 15 patients (33%).

• ·      Among the 61 patients who received 50 Gy there was:

o   One case of serious (grade 3) acute toxicity and one case of life-threatening (grade 4) acute toxicity.

o   3 cases (5%) of serious (grade 3) late-term toxicity and 2 cases (3%) of life-threatening (grade 4) late-term toxicity.

o   2 of the patients developed rectourethral fistulae, and 5 required diverting colostomies.

We note that even at the lowest dose level given in this trial (45 Gy), they were delivering much more than the customary SBRT dose of 36.25 Gy. Because this study began with such a high dose, it did not succeed in its objective of finding an optimal dose. It did, however, find the dose that created dose-limiting toxicity. At 50 Gy, they were delivering a dose that is bioequivalent to more than twice the customary and safe IMRT dose (80 Gy in 40 fractions). This is especially troubling when we realize that 36% were low-risk patients who might have delayed treatment with active surveillance.

There are many aspects of this study that are hard to understand. It’s hard to understand why they didn’t start at a more reasonable dose level. Dr. Alan Katz reported excellent cancer control with extremely low toxicity using only 35 Gy (see this link). With the sharp increase in acute grade 2 toxicities at 47.5 Gy, it’s hard to understand why the researchers did not pull the plug before patients were seriously harmed. It’s also hard to understand how the internal review board (IRB) did not question the ethics of this study.

(Update 2/6/2019) In a small (n=26) prospective dose-finding study of 40 Gy (n=9), 45 Gy (n=10) and 50 Gy (n=7) among low and intermediate risk patients, Potters et al. reported freedom from biochemical failure of 92%, 100% and 100% respectively with 67 months of follow-up. There were no Grade 3 toxicities, and toxicity was about equal in all groups. Quality of life returned to baseline in all groups within 2 years.

We have observed (see this link) that there is a lot more to SBRT safety than simply setting the prescribed dose. Careful planning, image guidance and accurate delivery are equally important. In the right hands, SBRT is among the safest and most effective of all radiation therapies, with excellent convenience and relatively low cost. In fact, I chose it for myself.

SBRT Registries

Patient registries are potentially a rich source of information with which to evaluate outcomes. They often include patient characteristics, details of the therapies they received, and outcomes tracked over time. They provide full population data of all patients treated at participating centers, and can provide very large amounts of data over time.

Like a clinical trial, there are specific and uniform definitions used in capturing patient and treatment data, allowing for comparability on a variety of variables. Registries and clinical trials are internal review board (IRB) approved for ethical standards and must comply with HIPAA laws (patients must consent, and patient names are not entered in). In the US, they both have an insurance advantage as well: Medicare, Medicaid and insurance companies may cover the costs of clinical trials and registries for treatments that they would not ordinarily cover. In some situations, they will only provide coverage if the patient is enrolled in a registry or clinical trial.

Unlike a clinical trial, there are usually no detailed patient inclusion and exclusion criteria, and the treatments may vary from center to center and from patient to patient. Because patients are not excluded from the database, registries are capable of providing very large databases for analysis. There is no randomization, so there is selection bias – patients who received different treatments may have been selected for specific reasons. The quality of the data is only as reliable as the clinician entering it, and it is not necessarily subject to peer review as publication of clinical trial results are. As with other large database analyses, it may be possible to find matched cases for control, but that is not the same as randomization. While clinical trials have a hypothesis to be proved or disproved, a registry provides data for quality improvement and for generating hypotheses.

Registries are difficult and expensive to establish and maintain. The American Board of Radiology attempted to create a national brachytherapy registry, but abandoned those efforts in 2015 when issues in its development and implementation “proved to be more daunting and costly than initially anticipated.” In 2012, the American Society of Radiation Oncologists (ASTRO) announced plans to implement a National Radiation Oncology Registry (NROR) with Prostate Cancer as its first focus. A pilot was completed in June 2015, and there are plans for expansion.

The Registry for Prostate Cancer Radiosurgery (RPCR) was established in 2010. There are 45 participating sites in the US, and the database included nearly 2000 men as of 2014. They collect three kinds of data for each patient: screening, treatment, and follow-up.

Screening data include age, performance status, rationale for radiosurgery, initial TNM stage, Gleason score, number of positive biopsy cores, use of hormonal therapy, and several baseline measures, including pre-treatment PSA, IPSS, International Index of Erectile Function (IIEF-5) score, Bowel Health Inventory score, and Visual Analog pain score.

Treatment data include radiation delivery device details, treatment dates, dosimetry (e.g., doses, schedules, targets, margins, including doses to specific organs at risk: rectum, bladder, penile bulb, and testicles), and how image tracking was performed.

Follow-up data include periodic tracking of the baseline data collected at screening, as well as physician-reported toxicity. RPCR encourages sites to record follow-up data every 3 months for the first 2 years following SBRT treatment and every 6–12 months thereafter, for a minimum of 5 years.

Some interim findings have been published by Freeman et al. So far, they have only reported 2-year data on 1,743 patients. Oncological control was reported as biochemical disease-feee survival:

·      Low Risk: 99% (n=111)

·      Favorable Intermediate Risk: 97% (n=435)

·      Unfavorable Intermediate Risk: 85% (n=184)

·      High Risk: 87% (n=168)

There was no severe late-term urinary toxicity, and one patient developed severe late-term rectal bleeding. Erectile function was preserved in 80% of men under 70 years of age, and 55% of men over 70.

The other SBRT registry is called the Radiosurgery Society Search Registry (RSSearch Registry) and includes data from 17 community centers treating prostate cancer patients. There were 437 prostate cancer patients enrolled between 2006 and 2015. The data collected is similar to the RPCR Registry. All patients in their first report were treated using the CyberKnife platform (this registry was originated by Accuray, the manufacturer of CyberKnife), although they allowed other platforms in later enrollments.

Davis et al. recently reported their interim findings. Oncological control was reported as 2-year biochemical disease-fee survival:

·      Low Risk: 99.0% (n=189)

·      Intermediate Risk: 94.5% (n=215)

·      High Risk: 89.8% (n=33)

There was no severe (grade 3) acute urinary or rectal toxicity, and very little grade 2. There was no severe (grade 3) late-term urinary or rectal toxicity. The highest incidence of grade 2 late term symptoms was 8% with urinary frequency, They did not collect baseline data on sexual function.

Both of these registries are administered by Advertek. The results of the RSSearch Registry were reported in Cureus, which is their own publication. RPCR results were published in Frontiers in Oncology, which is an independently peer-reviewed journal. It is important to note this because questions about the reliability of the data may arise.

If these data look a little too good to be true… well, let’s dig a little deeper. The biochemical disease-free survival figures only reflect 2 years of follow-up. In that short amount of time, many patients have not yet reached their nadir PSA let alone had time to rise 2 points above that nadir. Most of the low-risk patients and many of the intermediate-risk patients would not have had a rise of 2 points in their PSA even if they’d had no treatment.

The toxicity data are very suspect. Unlike a clinical trial where experienced researchers are carefully evaluating patients on a regular schedule, patient evaluations by community clinicians are haphazard. The clinicians may introduce affirmation bias into their assessments – they have incentive to make their numbers look good. The best way to evaluate toxicity is with patient-reported outcomes on validated, guided-response questionnaires, like EPIC. This was not done in either of these registries. 

I think SBRT is actually quite a good therapy (I chose it for myself!), but we have to look to other sources for more reliable data. With longer term follow-up, the cancer control data from these registries may become more reliable, and may help us generate better hypotheses about which treatment variants work best and on which patient groups.

SBRT has equivalent toxicity with 5 treatments and 12 treatments

Lukka et al. reported one-year outcomes of the RTOG 0938 trial designed to test whether SBRT done in 5 treatments of fractions has equivalent and acceptable toxicity compared to SBRT in 12 fractions.

This was a multi-institutional US /Canadian study among 246 low risk men. They were randomly assigned to one of two SBRT treatment regimens:

• Arm 1: 36.25 Gy delivered in 5 fractions twice a week for 2 ½ weeks.
• Arm 2: 51.6 Gy delivered in 12 fractions 5 days a week for 2 ½ weeks.

These doses are approximately equivalent in biologically effect for cancer control and in their expected effect on healthy tissues. Men were allowed to be treated on several different SBRT platforms, including CyberKnife, VMAT and protons.

This is the planned 1-yr quality-of-life analysis, with future analyses to be performed after 2 and 5 years.  The EPIC questionnaire was used to assess bowel, urinary, and sexual quality-of-life.

•     Bowel changes > 5 points are considered clinically significant.

o   Any such change affecting ≤ 35% of men was considered to be acceptable.

o   Any such change affecting ≥ 55% of men was judged to be unacceptable.

•   Urinary changes  > 2 points are considered clinically significant.

o   Any such change affecting ≤ 40% of men was considered to be acceptable.

o   Any such change affecting ≥ 60% of men was considered to be unacceptable.

• Sexual score changes ≥ 11 points are considered clinically significant

After 1 year of follow-up, patient-reported clinically significant changes were noted in:

• Bowel changes were acceptable: 29.8% in Arm 1 and 28.4% in Arm 2
• Urinary changes were borderline acceptable: 45.7% in Arm 1 and 42.2% in Arm 2
• Sexual score changes: 32.9% in Arm 1 and 30.9% in Arm 2
• Disease-free survival at two years: 93.3% in Arm 1 and 88.3% in Arm 2
• None of the differences between Arm 1 and 2 were statistically significant

Physician-reported toxicities were as follows:

• Acute urinary: Grade 3 – 2 patients (1.7%)
• Acute rectal: Grade 3 – 2 patients (1.7%), Grade 4 – 1 patient (1.1%)
•  Late urinary: Grade 3 – 1 patient (0.8%)
•  Late rectal: Grade 3 – 2 patients (1.7%)

Both treatment regimens substantially met the study’s toxicity requirements, and confirm that 5 fractions are as toxicity-free as 12 fractions. These outcomes are in line with historical controls based on conventional IMRT treatment regimens. Of course, only a randomized clinical trial (like this one, which proved there were no differences in oncological or toxicity outcomes) can compare IMRT and SBRT.

9-year SBRT outcomes

Katz and Kang have posted their 9-year SBRT outcomes on 515 patients. This represents the longest tracking of SBRT outcomes -- just one year short of the IMRT tracking reported by Alicikus et al. on a starting cohort of 170 patients treated at Memorial Sloan Kettering Cancer Center.

The patients were treated between 2006-2010 using the CyberKnife platform.

• ·      324 were low risk, 139 intermediate risk, and 52 were high risk according to NCCN definitions.
• ·      70 patients received adjuvant ADT for up to one year.
• ·      158, all with Gleason score<4+3, received 35 Gy in 5 fractions.
• ·      357 received 36.25 Gy in 5 fractions
• ·      Median age was 69
• ·      Median PSA was 6.5 ng/ml

After a median followup of 84 months:

• ·      Oncological Control:

o   9-yr freedom from biochemical failure was:

§  95% for low-risk men

§  89% for intermediate risk men

§  66% for high-risk men

o   Median PSA nadir was .1 ng/ml

o   No difference in biochemical control for the lower vs. the higher radiation dose.

o   99.6% prostate cancer survival

o   86% overall survival

• ·      Toxicity:

o   Late rectal toxicity:

§  Grade 2: 4%

o   Late urinary toxicity:,

§  Grade 2: 9.5%

§  Grade 3: 1.9%

§  Grade 2 or 3: 6.9% for the lower radiation dose vs. 13.2% for the higher dose.

o   Patient-reported bowel and urinary quality-of-life (EPIC questionnaire) declined at one month then returned to baseline by 2 years. Sexual quality-of-life declined by 29% at last followup.

These are clearly excellent results for any kind of radical therapy. The authors conclude:

“These long-term results appear superior to standard IMRT with lower cost and are strikingly similar to HDR therapy.”

While it’s tempting to conclude that neither the higher dose of radiation, with its greater toxicity, nor the addition of ADT conferred any incremental benefit, that can only be proved with a randomized clinical trial. Until so proven, it must be understood as only a good hypothesis to be discussed by patients with their radiation oncologists. It is also worth noting that these reflect the outcomes of one very expert practitioner. There is an SBRT registry currently collecting data across many treatment centers.

The reported outcomes are nearly identical to those reported at 7 years (see this link and this link and this link), indicating very stable control and no additional late term toxicity with longer followup. In light of that, its low cost, convenience, and the fact that the standard of care, IMRT, has only one more year of follow-up on a much smaller sample size, it’s difficult to understand why some insurance companies still balk at covering SBRT for low and intermediate risk patients. Medicare does cover it.