SBRT has excellent outcomes for intermediate risk patients

Stereotactic Body Radiation Therapy (SBRT, or sometimes SABR or SHARP or CyberKnife) has had excellent 7-year outcomes in an update of the consortium study. Amar Kishan presented the results of his analysis at the ASTRO meeting today.

The consortium consisted of

1 Department of Radiation Oncology, University of California, Los Angeles, Los Angeles, CA, USA
2 Department of Urology, University of California, Los Angeles, Los Angeles, CA, USA
3 Flushing Radiation Oncology Services, Flushing, NY, USA
4 21st Century Oncology, Fort Myers, FL, USA
5 Department of Radiation Oncology, Georgetown University, Washington, DC., USA
6 Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA, USA
7 Division of Genesis Healthcare Partners Inc., CyberKnife Centers of San Diego Inc., San Diego, CA, USA
8 Swedish Radiosurgery Center, Seattle, WA, USA.
9 Department of Radiation Oncology, Odette Cancer Centre, Sunnybrook Health Sciences Centre, Toronto, ON,
Canada.
10 Section of Radiation Oncology, Virginia Mason Medical Center, Seattle, WA, USA
11 Department of Radiation Oncology, Stanford University School of Medicine, Stanford, CA, USA
12 Department of Radiation Oncology, University of Michigan
13 Scripps Health, 11025 North Torrey Pines Road, La Jolla, CA, USA
14 Virginia Hospital Center, 1701 N. George Mason Dr, Arlington, VA, USA

The meta-analysis covers 2,142 low (n=1,185) and intermediate-risk men treated with SBRT between 2003 and 2012. Intermediate risk men were further subdivided into "favorable intermediate risk" (n=692) and "unfavorable intermediate risk" (n=265) per the NCCN definition.

After a median follow-up of 6.9 years, the 7-year biochemical recurrence-free survival was:

• low risk: 95.5%
• favorable intermediate risk: 91.4%
• unfavorable intermediate risk: 85.1%
• all intermediate risk: 89.8%

Low risk patients and some of the favorable intermediate risk patients would probably be diverted to active surveillance today. The 7-year intermediate risk biochemical recurrence-free survival compares favorably with (note: this is not a randomized comparison, which is the only valid way of comparing):

• Surgery: favorable intermediate risk (PSA=6.0, T1c, GS 3+4, 33% cancerous cores): 79% (mean of 5 and 10-yr Progression-free survival) (1)
• Surgery: unfavorable intermediate risk (PSA=6.0, T1c, GS 4+3, 67% cancerous cores): 46% (mean of 5 and 10-yr Progression-free survival) (1)
• Hypofractionated IMRT (5 year):  85% (2)
• Conventional IMRT (5 year): 85% (2)
• Low dose rate brachytherapy: favorable intermediate risk (avg of 5 and 10-yr): 87% (3)
• Low dose rate brachytherapy: unfavorable intermediate risk (5-year): 81% (3)
• Brachy boost therapy: unfavorable intermediate risk (10 year): 92% (4)

7-year metastasis-free survival was:

• low risk: 99.9%
• favorable intermediate risk: 98.3%
• unfavorable intermediate risk: 97.0%
• all intermediate risk: 98.0%

There were no prostate cancer-related deaths.

Use of ADT and higher doses (doses ranged from 33 Gy to 40 Gy in 4 or 5 treatments) did not affect recurrence.

Acute (within 3 months of treatment) toxicity was low:

• Urinary toxicity Grade 2: 8.8% Grade 3: 0.6%
• Rectal toxicity Grade 2: 3.2% Grade 3: 0.1%

Late-term cumulative toxicity was low:

• Urinary toxicity Grade 2: 9.4% Grade 3+: 2.1%
• Rectal toxicity Grade 2: 3.9% Grade 3+: 0.4%

Late-term grade 3 or greater urinary toxicity of 2.1% compares favorably to other radiation monotherapies reported in other studies. For example:

• Low dose rate brachytherapy: 7.6% (5)
• High dose rate brachytherapy (3 fractions):11% (6)
• Hypofractionated IMRT (70 Gy/28 fx): 3.5% (7)
• Conventionally fractionated IMRT: 2.3% (7)
• Brachy boost therapy: 19% (8)

Late-term grade 3 or greater rectal toxicity of 0.4% compares favorably to other radiation monotherapies reported in other studies. For example:

• Low dose rate brachytherapy: 0.8% (5)
• High dose rate brachytherapy (3 fractions):1% (6)
• Hypofractionated IMRT (70 Gy/28 fx): 4.1% (7)
• Conventionally IMRT: 2.6% (7)
• Brachy boost therapy: 9% (8)

This 7-year analysis on a large group of patients from multiple sites, should make intermediate risk patients comfortable in choosing SBRT, especially if they are favorable intermediate risk. For patients who are unfavorable intermediate risk, brachy boost therapy affords incomparable oncological control, but at the risk of much higher late term urinary and rectal toxicity.

SBRT Dose Escalation

Is there an optimum treatment dose for SBRT? At the low end of the spectrum, Alan Katz found that 35Gy in 5 fractions gave equivalent oncological outcomes with less toxicity compared to 36.5 Gy. At the other end of the dose spectrum, a clinical trial pushed the dose as high as 50 Gy in 5 fractions with disastrous consequences (see this link).  A trial of high dose rate brachytherapy, which is radiologically similar to SBRT, failed to find an optimum dose.

But radiation safety is not only just about dose. We saw that two treatment schedules using the same prescribed dose (40 Gy in 5 fractions) had disparate toxicity outcomes (see this link). In fact, the 12 month toxicity outcomes of Dr. King's high-risk study were recently presented and look excellent (see this link). It's also worth noting once again the outcomes of the 5-year multi-institutional SBRT clinical trial that used 40 Gy in 5 fractions and had excellent oncological and toxicity outcomes (see this link).

Helou et al. reported the outcomes of their SBRT (they call it SABR, but it's the same thing) trials at the Sunnybrook Health Sciences Centre in Toronto, Canada. There were sequential trials conducted from 2006-2014:

• 35 Gy/5 fractions/29 days - 82 low risk men only
• 40 Gy/5 fractions/11 days or 29 days - 177 low and intermediate risk men

A few (12) men had up to 6 months of androgen deprivation to shrink their prostates prior to radiation.

As an early measure of oncological effectiveness, they used PSA at 3 years (PSA3Y) after radiation. After correcting for the other variables like age, baseline PSA, T stage, and ADT use, the dose received remained the biggest predictor of PSA3Y. Median PSA3Y was:

• 0.64 ng/ml in those who received 35 Gy
• 0.27 ng/ml in those who received 40 Gy
• The difference was significant in both low risk men and intermediate risk men

The use of PSA3Y as a surrogate endpoint for biochemical recurrence is controversial. Because prostate cancer progresses very slowly and radiation, at the very least, reduces the cancer burden, it can take at least 5 years, and as long as 10 years, before we start to see concrete evidence that such therapy is curative. Also, a longer time until the nadir is achieved has been found to be correlated with failure-free survival (see this link). Nadir PSA has been proven to be a strong predictor of a lasting cure (see this link), but no one can tell when the nadir will be reached. In a recent study comparing the PSA at 1000 days after SBRT or HDR brachytherapy to the PSA at 1000 days after conventional IMRT, Kishan et al. reported that the PSA was lower for SBRT/HDR-BT. While the downward slope was about the same for the first 1000 days, the slope was steeper afterwards for SBRT/HDR-BT, indicating that a lower nadir would be achieved.

After correcting for confounders like age, baseline urinary function, and time between treatments, late term urinary toxicity of grade 2 or higher was 17 times greater among those who received 40 Gy compared to those who received 35 Gy.

The authors previously reported late term rectal toxicity. After 2 years, the cumulative probability of  grade 2 or higher rectal toxicity was suffered among:

• 5% of the men who received 35 Gy with 4mm margins
• 27% of the men who received 40 Gy with 5 mm margins
• 42% of the men who received 40 Gy with 5 mm margins +  30 Gy to seminal vesicles received 

Grade 3 and 4 rectal toxicity was especially high (10%) in the group that had their seminal vesicles irradiated. There were 3 cases of fistulas that may be attributable to rectal biopsies. [Patients should be very careful about the use of any kind of instrumentation within at least 6 months of radiation. That includes cystoscopies and colonoscopies.] Since this study, the authors have changed their radiation planning to include faster (VMAT) linacs and improved rectal dose constraints. Other changes that might mitigate rectal toxicity may include use of intrafractional tracking, rectal immobilization, and a rectal spacer.

There was clearly a trade-off between SBRT dose and late-term side effects of treatments. Perhaps we will one day be able to identify those cancers that are curable with a lower dose, and treat only those with the more radio-resistant cancers with a higher dose. Some believe that such techniques as simultaneous integrated boosts or heterogeneous planning may cure the cancer in the prostate better with less damage to organs at risk. But they remain to be proved in randomized clinical trials.

Note: Thanks to Dr. Andrew Loblaw for allowing me to review the full text of the study.

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.

Is there an optimal treatment schedule for high dose rate brachytherapy?

Protocols for high dose rate brachytherapy (HDR-BT) monotherapy vary. In recent years, practitioners have adopted various schedules for patient and physician convenience. Jawad et al. reported on the HDR-BT experience at William Beaumont Hospital. They treated 494 favorable risk patients using three different treatment schedules. Their definition of “favorable risk” was a Gleason score ≤7 and stage≤T2b and PSA≤15 ng/ml. The 3 treatment schedules they utilized, the number of patients who received each, and the relative biologically effective dose  (BED) were as follows:

1. 38 Gy in 4 fractions (n=319) – 1.29 relative BED 
2. 24 Gy in 2 fractions (n=79) – 1.00 relative BED 
3. 27 Gy in 2 fractions (n=96) – 1.25 relative BED

Dose schedules #1 and #3 delivered much higher relative dose compared to dose schedule #2. The questions addressed by the study are whether the higher dose is justified by greater cancer control, and whether dose increased at the expense of increased side effects.

After 5.5 years median followup for schedule #1, 3.5 years for schedule #2, and 2.5 years for schedule #3, the toxicity outcomes were as follows:

• No difference in clinical outcomes (cancer control) among the 3 treatment schedules.
• Acute (appearing in less than 6 months) and chronic (appearing 6 months or more after treatment) grade ≥2 genitourinary (GU) and gastrointestinal (GI) side effects were similar for all treatment schedules.
• Grade 2 acute GU toxicities:

o   Frequency/urgency: 14%

o   Dysuria (painful urination): 6%

o   Retention: 7%

o   Incontinence: 1.5%

o   Hematuria (blood in urine): 1.5%

• ·      Grade 2 chronic GU toxicities:

o   Frequency/urgency: 20%

o   Dysuria (painful urination): 7%

o   Retention: 4% (Urethral stricture: 2%)

o   Incontinence: 2%

o   Hematuria (blood in urine): 7%

• ·      There was minimal grade 3 GU toxicity
• ·      Grade 2 acute GI toxicities:

o   Diarrhea: 1%

o   Rectal pain/tenesmus: <1%

o   Rectal bleeding: 0%

o   Proctitis: <1%

• ·      Grade 2 chronic GI toxicities:

o   Diarrhea: 1%

o   Rectal pain/tenesmus: 0.5%

o   Rectal bleeding: 2%

o   Proctitis: 1%

• ·      No Grade 3 or higher GI toxicity
• ·      Time to maximal appearance of symptoms was similar across treatment schedules
• ·      They did not report ED rates, which are typically low for HDR-BT.

Given the equivalence of cancer control and toxicity with treatment schedule, and the lack of any effect due to increasing the biologically equivalent dose, there seems to be little basis, other than cost and convenience, for choosing among these treatment schedules, at least with the available follow-up reported here.

Aspects of treatment scheduling that affect the convenience of HDR-BT are the number of implantations of the catheters, and the time frame in which the fractions are delivered.  William Beaumont Hospital uses a single implantation of catheters for all treatment schedules. Schedule #1 involves a longer (overnight) hospital stay because they wait for several hours between fractions for healthy tissue to recover. It also means that anesthesia must be administered over a longer period.

The California Endocurietherapy Center at UCLA has typically used a different protocol. They deliver 42 Gy in 6 fractions, with 3 fractions delivered one week and 3 fractions delivered a week later. This involves 2 overnight hospital stays, with anesthesia each time. Recently, they added a protocol where they deliver 27 Gy in 2 fractions (similar to schedule #3), but those fractions are still inserted a week apart. While this is certainly a cost reduction for the patient, who can now be treated as an outpatient, the patient is inconvenienced by having to go through the full procedure twice. It is a convenience for the treatment team that no longer has to attend the patient over an extended timeframe.

The William Beaumont Hospital experience demonstrates that HDR-BT treatment schedules can be constructed so as to lower costs and increase convenience for patients and doctors, without sacrificing cancer control or quality of life.

Why toxicity was higher with hypofractionation in Dutch trial

Aluwini et al. have published the toxicity outcomes of a randomized clinical trial (HYPRO) designed to test whether a hypofractionated external beam (EBRT) regimen compared to conventional fractionation. They will report on the oncological outcomes at a later date.

Between 2007 and 2010, 782 intermediate and high-risk patients were treated at 4 Dutch centers. About half were treated with the hypofractionated regimen, half with conventional dosing as follows:

• ·      Hypofractionation: 19 fractions of 3.4 Gy each
• ·      Conventional fractionation: 39 fractions of 2.0 Gy each
• ·      The relative biologically effective dose is 16% higher for the hypofractionated regimen.
• ·      Both groups were treated with conformal EBRT (3D-CRT and IMRT).

After a median followup of 60 months, the 3-year late-term toxicity outcomes were as follows:

• ·      Genitourinary toxicity, grade 2 or higher: 41% among the hypofractionated group vs. 39% for conventional fractionated.

o   Hazard ratio: 1.16 (Non-inferiority threshold: 1.11)

• ·      Genitourinary toxicity, grade 3 or higher: 19% among the hypofractionated group vs. 13% for conventional fractionated.

• ·      Gastrointestinal toxicity, grade 2 or higher: 22% among the hypofractionated group vs. 18% for conventional fractionated.

o   Hazard ratio: 1.19 (Non-inferiority threshold: 1.13)

• ·      Gastrointestinal toxicity, grade 3 or higher: 3% among the hypofractionated group vs. 3% for conventional fractionated.

Because the toxicity difference slightly exceeded the pre-established thresholds, the authors conclude that the hypofractionated regimen was not non-inferior to the conventionally fractionated regimen in terms of late term toxicity.

Because the hypofractionated regimen was a higher biologically effective dose, we might expect toxicity to be somewhat higher. Several recent major trials showed that hypofractionated IMRT was non-inferior to conventional fractionation in terms of both oncological control and late-term toxicity (see this link and this one, and this one). The lesson we learn from this study is that hypofractionation carries increased risk of toxicity. To avoid that, it is important to use well-planned IMRT or SBRT regimens. 3D-CRT is probably not the optimal platform for such treatment.

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 Boost Therapy

Recently we have seen evidence of improved cancer control in high-risk patients treated with external beam radiotherapy with a brachytherapy boost to the prostate. This has been demonstrated with both HDR brachytherapy boost and with LDR brachytherapy boost. Can the same cancer control be obtained with IMRT and an SBRT boost to the prostate?

Anwar et al. reported the outcomes of 48 intermediate and high-risk patients treated with SBRT boost therapy between 2006 and 2012 at UCSF. 71% (34 patients) were high risk, 39% (14 patients) were intermediate risk.

The treatment consisted of:

• ·      IMRT: 45-50 Gy in 25 fractions to the entire pelvis if the risk of lymph node involvement was > 15%, otherwise with a 1 cm margin.
• ·      SBRT boost: 9.5 or 10.5 Gy in 2 fractions to the prostate, seminal vesicles + a 2 mm margin, 0 mm on the rectal side.
• ·      Heterogeneous planning was used to mimic HDR brachytherapy dosimetry.
• ·      Gold fiducials were used for daily (IMRT) and intra-fractional (SBRT) image tracking.
• ·      Intermediate risk patients had 4-6 months of adjuvant hormone therapy.
• ·      High-risk patients had up to 2 years of adjuvant hormone therapy

After a median of follow-up of 42.7 months, they reported the following results:

• ·      5-yr  biochemical no evidence of disease: 90%
• ·      PSA nadir (median): 0.05 ng/ml
• ·      2 patients had a PSA bounce over 2 ng/ml, which declined with longer followup
• ·      4 patients had a clinical recurrence outside of the radiation field
• ·      Local control (within the radiation field) was 100%.
• ·      Acute toxicity:

o   Urinary, grade 2: 17%

o   Rectal, grade 2: 10%

• ·      Late toxicity:

o   Urinary, grade 2: 25%; grade 3: 1 patient

o   Rectal, grade 2 or higher: none

Clearly, these are excellent results for cancer control.  The table below shows outcomes in similar trials of SBRT boost treatments and of SBRT monotherapy.

	Anwar et al. SBRT boost	Katz & Kang SBRT boost	Katz & Kang SBRT monotherapy	Lin et al. SBRT boost
Risk levels treated (# of patients)	Intermediate (14) High (34)	High (45)	High (52)	High (41)
Relative BED*	1.27-1.52	1.13-1.17	1.06-1.13	1.17
ADT used	88%	62%	50%	100%
Biochemical Disease-free survival	90% at 5 years	70% at 5 years	68% at 5 years	92% at 4 years
Late-term urinary toxicity	27%	5%	12%	none

* Biologically Effective Dose for cancer control relative to 80 Gy in 40 fractions

Compared to these other small trials, Anwar et al. used significantly higher effective radiation doses and got perhaps better control (remembering that almost a third were intermediate risk), but late-term urinary toxicity was high. Lin et al. used lower doses, had similar control in their all high-risk group trial at 3 years, and none suffered from late-term urinary toxicity. Katz treated consecutive high-risk patients with SBRT boost and with monotherapy, respectively, but had the same cancer control in both groups, and the late-term urinary toxicity was not significantly different. Katz concluded that the SBRT boost accomplished nothing compared to the monotherapy, and also found that ADT use did not contribute to cancer control in his patients. He treated all subsequent high-risk patients with SBRT monotherapy only and without ADT.

We can also look at the Anwar outcomes next to those of a recent LDR brachy boost therapy trial and an HDR monotherapy trial in the table below.

	Anwar et al. SBRT boost	ASCENDE-RT LDRBT boost	Hoskin et al. HDR-BT monotherapy
Risk levels treated (# of patients)	Intermediate (14) High (34)	Intermediate (122) High (276)	Intermediate (103) High (86)
Relative BED*	1.27-1.52	1.21	1.21-1.35
ADT used	88%	100%	80%
Biochemical Disease-free survival	90% at 5 years	Int.Risk-94% High Risk-83% at 7 years	Int.Risk-95% High Risk-87% at 4 years
Late-term urinary toxicity	25% Grade 2 2% Grade 3	NA Grade 2 18% Grade 3	19% Grade 2 10% Grade 3

SBRT boost therapy seems to provide similar rates of cancer control, but with less late term urinary toxicity compared to brachy boost therapy or HDR-BT monotherapy.

In an interesting twist, Memorial Sloan Kettering Cancer Center is running a clinical trial of SBRT supplemented with an LDR-BT boost to the prostate in intermediate-risk men (NCT02280356). I would guess that this would have considerable toxicity, but the clinical trial will prove or disprove that hypothesis.

So far, trials of SBRT boost therapy are too small to draw anything but provisional conclusions. There is a larger trial nearing completion at Georgetown University Hospital next month. Based on these pilot studies, SBRT boost therapy seems to be capable of providing good cancer control in high-risk patients and may be able to accomplish that with less toxicity than brachytherapy-based treatments. As we’ve seen, SBRT monotherapy and HDR brachy monotherapy are emerging therapies for high-risk patients as well. It would certainly be a lot more convenient to accomplish the same cancer control, at lower cost, and with perhaps less toxicity using just 5 SBRT monotherapy treatments instead of 27 treatments with SBRT boost. Only a randomized comparison clinical trial can tell us whether one therapy is better than another. The most appropriate radiation dose level, dose constraints, the size of margins, lymph node treatment, and whether adjuvant ADT provides any benefit are variables yet to be determined.

This is an area of active investigation. If readers are interested in participating in a clinical trial of SBRT boost therapy, below is a list of open trials and their locations:

Fountain Valley, CA (NCT02016248)

Sacramento, CA (NCT02064036)

San Francisco, CA (NCT02546427)

Miami, FL (NCT02307058)

Park Ridge, IL (NCT01985828)

Boston, MA (NCT01508390)

Madison, WI (NCT02470897)

21^st Century Oncology- Scottsdale, AZ, Ft. Myers and Plantation, FL, Farmington Hills, MI, Myrtle Beach, SC (NCT02339948)

Sydney, Australia (NCT02004223)

Gliwice, Poland (NCT01839994)

Poznan, Poland (NCT02300389)