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Fluorescence guided resection (FGR): A primer for oncology
Accepted Manuscript Title: Fluorescence guided resection (FGR): A primer for oncology Author: Ron R. Allison PII: DOI: Reference:
S1572-1000(15)30051-X http://dx.doi.org/doi:10.1016/j.pdpdt.2015.11.008 PDPDT 718
To appear in:
Photodiagnosis and Photodynamic Therapy
Received date: Revised date: Accepted date:
24-9-2015 25-11-2015 26-11-2015
Please cite this article as: Allison Ron R.Fluorescence guided resection (FGR): A primer for oncology.Photodiagnosis and Photodynamic Therapy http://dx.doi.org/10.1016/j.pdpdt.2015.11.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fluorescence Guided Resection (FGR): A Primer for Oncology Ron R. Allison, M.D.* [email protected] Medical Director, 21st Century Oncology *Corresponding author.
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Highlights ‐Fluorescent guided resection (FGR) is a type of Image Guided Surgery ‐FGR in Oncology allows the surgeon better tumor visualization for improved resection ‐FGR as currently practiced uses applied fluorescing agents, such as Photosensitizers, to image tumor ‐Improved outcomes have been seen with FGR for neurologic and urologic tumors
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Abstract Curative treatment for most cancer patients requires surgical removal of the tumor. Often though, residual disease is left behind negatively impacting tumor control and survival. Florescent Guided resection (FGR) is one type of Image Guided Surgery that offers the potential to improve outcomes for these patients. Currently, during FGR, a probe is preoperatively applied and allowed to concentrate in the tumor bed. At surgery appropriate light is applied to generate fluorescence which allows the surgeon to better visualize tumor extent. This improved visualization has translated both into enhanced rates for resection but also diminished rates of morbidity as less normal tissue need be removed to achieve negative margins. This paper will summarize the theory and practice of FGR as currently applied in clinical oncology including select outcomes and limitations of technique and technology. Keywords: Fluorescence; Fluorescent Guided Resection; photosensitizers; Image Guided Surgery; review
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Fluorescence Guided Resection (FGR): A Primer for Oncology
Surgery is the prime and essential ingredient in the curative treatment for
the majority of cancer patients. This remains true today due to a continuum of enhancement in surgical techniques and technology [1] . The surgical oncologist’s clinical goal is to remove both gross and microscopic tumor to render the patient disease free. Achieving this outcome generally has a direct effect to enhance the chance for survival [2] . Therefore clinical tools that may increase the chance for a successful resection are in great demand and widely sought out. One such innovative procedure is termed “fluorescent guided resection” (FGR) which allows direct illumination and enhanced visualization of tumor and its tendrils [3] . Potentially this may not only allow the surgeon a greater chance of complete lesion resection but also prevent or minimize resection of normal tissues, which generally do not light up. Essentially a fluorescent probe is applied and if successful, this probe will concentrate in the tumor [4] . An external light source is then used to illuminate the surgical field and through the phenomena of florescence the tumor literally lights up (Fig 1). Normal surrounding tissue, as it does not contain the fluorescent probe, does not. Therefore FGR has the outstanding ability to allow the surgeon enhanced visualization of the tumor bed which can improve tumor resection and tumor control while simultaneously minimizing normal tissue injury. This likely explains the rapid growth and spread of this technology around the globe [5] . FGR
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FGR is one of many image guided surgical techniques currently undergoing
evaluation both clinically and pre clinically [6] . Currently, in contrast to many other image guided surgical technology, FGR has already achieved clinical success [7] . Further, FGR has commercially available fluorescent probes, detectors and associated tools to allow for relatively rapid and widespread clinical introduction. Critically, FGR does not appreciably change the surgical procedure or require significant additional operating room time, both important concepts to allow for widespread use in all operating theatres [8] . Most importantly FGR appears to be a reliable and reproducible procedure with very little, if any, morbidity attributed to the actual fluorescence or fluorescent probe [8] .
For these, and reasons to follow, FGR is becoming widespread in surgical
oncology. This paper will highlight the fundamental components of fluorescence and Florescent guided resection in oncology. The clinical success and limitations are then highlighted. Finally, means to improve outcomes are offered. FGR Fundamentals: To achieve a successful procedure the following are critical and needed components: 1) Fluorescence: Many natural and synthetic structures have the ability to emit light when activated. When this light is emitted as fluorescence these structures are termed fluorophores [9] . Fluorescence phenomena occur when a photon of incoming light energy interacts with an electron of a fluorophore. The incident light photon may transfer its energy to the resting electron of the fluorophore bringing this electron to an excited higher energy state. Often, when this excited electron returns to its ground state, energy is
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lost. Usually the energy is lost as detectable light of a longer wavelength (fluorescence) (Fig 2). This florescence may be detected. If a naturally occurring fluorophore (such as melanin) is activated this is termed autofluorescence. Very few tumors will naturally autofluoresce. In most cases a fluorophore is applied to the tumor as a fluorescent probe (see below). 2) Fluorescent probes: These structures require numerous qualities to allow for clinical success. They may be introduced orally, intravenously or topically. Clinically an ideal probe would have most of the following characteristics. 1) Specificity: The probe should essentially target the tumor exclusively [10] . This allows the surgeon to feel confident that all fluorescing tissue is malignant. 2) High Quantum Yield: A distinct and reliable difference in fluorescence between tumor and normal tissue is needed. This difference in fluorescence allows the surgeon to differentiate tumor from normal tissue. Conceptually this would be similar to a high signal to noise ratio [11] . 3) Non toxic: The probe should not be toxic to normal tissue. 4) Short half‐life: The probe should clear tissues in hours or days. 5) Amphylicity: The ability to travel via the bloodstream without a carrier is useful.
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6) Activation wavelength: The infrared (600 – 900 nm) wavelength of light travels through tissue and is of clinical benefit (see text) 7) Detection: Commercially available detection devices for the fluorescence is critical. 8) Regulatory approval: This allows for widespread use. 9) Elementary procedure: The key to clinical introduction is a reliable procedure that does not add complexity to the surgery. Currently many fluorescent probes are commercially available, but few have been designed and tested in clinical trials [12] . Most agents were developed for other purposes (i.e. Photodynamic Therapy) and have now been repurposed for FGR [13] . These agents have the benefit of being found clinically safe based on their previous lives. Two major families of probes exist: non targeted and targeted probes. New families based for example on nanoparticles are expected [14,15] . a) Non targeted fluorescent probes: These agents are not tumor specific [5,16] . Rather they appear to concentrate in rapidly proliferating tissues such as wounds. As tumors often are with proliferating regions these probes have found a role in FGR. As these probes have been available for decades (for other uses) they are considered generally safe and importantly, are commercially available. In this category are Toluidine Blue, Indocyanine Green, Fluorescein, Violet Acetate and Lugol’s Iodine among others [4] . While not specifically designed for FGR, these agents have many of the characteristics needed to be successful fluorescent probes. Still enhanced probes with more specificity are more likely to become the standard for this surgery.
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b) Targeted Fluorescent Probes
These agents appear to specifically concentrate in tumor for various reasons
[17] . Most commonly they are poorly cleared from malignancy as compared to surrounding normal tissue. In addition upregulated receptors (such as lipoprotein) found in tumors may allow for these probes to concentrate in the malignancy. The most common and successful targeted probes are generally photosensitizers developed for photodynamic therapy [18] . They have characteristics that allow for both fluorescence and to allow for singlet oxygen production (the photodynamic reaction) which is destructive to the tumor. Therefore these probes may have dual function‐to allow for fluorescent guided surgery and PDT. An underappreciated concept.
The clinically successful photosensitizers have been shown to concentrate in
tumor and clear normal tissue. Given their fluorescent capability many photosensitizers are now used for FGR. This list includes among others Photofrin, Foscan, Verteporfin and Texaphyrin.
Of note, one particular photosensitizer, aminolevulinic acid (ALA), has moved
to the forefront of FGR [19,20] . ALA is a pro drug and part of the heme synthetic pathway. Excess (applied) ALA is converted to Protoporphyrin IX (PPIX) both a potent fluorescent agent and photosensitizer. The conversion to PPIX is favored in malignant and premalignant cells making ALA an excellent targeted probe. By hexalating ALA (HAL), a stronger fluorescent agent is created [21] . c) Other Probe Concepts
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Nanoparticles may be created to target specific lesions and may well serve as the probes of the future [5] . Probes designed to fluoresce when attached to specific receptors also have been created [22] . A particularly interesting group includes VEG‐F and folate receptor based probes, as these receptors are commonly over expressed in malignancy [23] . 3) Fluorescence detection: The photon of light energy emitted as fluorescence can be detected, measured and localized [24] . If intense enough the fluorescence may be directly visible. Often the florescence is weak requiring amplification. This may be easily done by a CCD‐camera which readily detects even very weak levels of fluorescing light. Clinical Concepts for Successful Fluorescence Guided Surgery
A successful FGR would allow the surgeon a clear identification of the
fluorescing tumor to allow for complete resection. The clinical reality of this is clouded by a number of limitations. This requires an understanding of the interaction of light with tissue.
Ideally the tumor would autofluoresce and not require any applied probes.
While autofluorescence is a possibility in some tumors of the endo‐bronchus most tumors do not autofluoresce. Further autofluorescence phenomena in tumor is difficult to clinically detect [25] . The inherent fluorophores in the body (melanin for example) do not appear to be concentrated to a great degree in most tumors. Further, inherent fluorophores are found in most normal cells. Therefore weak auto fluorescence of normal tissue and tumor is expected, making differentiation of the
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two difficult. Yet subtle differences in inherent fluorophore concentration between tumor and normal tissue do exist. The difference often is slight and results in a low signal to noise ratio, meaning it is difficult by direct visualization to differentiate normal from neoplastic tissue. A number of physics based techniques and algorithms are in the developmental stage to better enhance auto‐fluorescence detection [4,8,11,25] . Despite these enhancements, it remains unclear if auto‐ fluorescent techniques are yet reliable enough on their own to successfully guide surgery [8] . Currently almost all FGR starts with an applied florescent probe that has been allowed to accumulate to a far greater degree in the tumor than in surrounding normal tissue [25] . This high signal (tumor) to noise (normal tissue) ratio should then allow visual detection of the florescent signal from the tumor to guide the surgeon’s resection. As normal tissue should not significantly fluoresce this may translate into a better ability to spare nonmalignant tissue from being resected. Clinically, removing less normal tissue should translate into fewer side effects.
To achieve this clinical outcome the fluorescent probe must be in the target
and the fluorescence from the probe must be detected. As will be shown this is much harder than it sounds.
When a photon of light interacts with tissue many pathways may be possible
[26] . Essentially the light itself may be absorbed or scattered, usually multiple combinations occur (Fig 3). For simplicity we will separate these events (absorption and scattering) with the understanding that both occur.
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Absorption: The introduced light photon may be partially or completely absorbed by the tissue it interacts with. Since the target (tumor) is only a small portion of these tissues most of the light photons are absorbed by non target tissue. The most abundant tissue component is water which preferentially absorbs virtually all applied light above 1100 nm in wavelength [27] . Hemoglobin and melanin, also quite common in tissue, will absorb most light in the 400 nm (blue) to 500 nm (green) wavelengths [28] . This leaves the red to infrared wavelength to not be absorbed and penetrate tissue (and also allows the red tinge to lighter colored skin). As red light can penetrate tissue to perhaps 1 cm this is currently the limitation in depth for clinical fluorescent detection and FGR. Scattering: If a photon is not completely absorbed by tissue it may scatter [26] . The most extreme form of scatter is reflection where the photon does not even penetrate tissue. Usually multiple scatterings occur, with loss of light energy at each event. Another important concept with scattering is refraction. Here the light photon changes direction at the interface of different tissues including air. With all the scattering and absorption in normal tissue only a relatively tiny number of introduced light photons actually interact with the fluorescent probe. Fluorescent Imaging: The small portion of photons absorbed by the probe may be energetic enough to create fluorescence. The fluorescent photon will now also either be absorbed by tissue or scatter further. It is these scattered light photons that may eventually be detected and imaged by a fluorescent detector [29,30] . The incident light and scattered light may also create autofluorescence. Essentially naturally occurring fluorophore [5] exist in the body including hemoglobin which may under
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appropriate circumstance also fluoresce. Of particular interest is that scar tissue may autofluoresce and be mistaken for tumor. Tumor that has undergone radiation and chemotherapy often forms scar like tissue which may autofluoresce making for a difficult clinical situation to determine scar or viable tumor. This is a not uncommon clinical situation. These are potentially serious limitations of FGR. Fluorescent Detection Tools
To achieve clinical acceptance the tools for detection should fit well within
the usual parameter of the surgery [31,32] . Introducing unique, time‐consuming and expensive tools will make FGR difficult and hinder widespread introduction. Fortunately, many of the commonly used operating room and procedure room equipment can be designed or retrofitted for FGR. Currently white operating room light is a critical component to allow the surgeon to see the tumor bed. So light based surgery including therapy using fluorescence is not a radical change. During FGR the surgical tools for resection remain the same. What is added is the fluorescent probe and an ability to detect fluorescence. This may be as simple as a pair of polarized goggles to assist in visualizing the fluorescing field [31] . The surgical field is now illuminated by two light sources. The usual white light allows for detection of normal anatomy. A second light source, filtered to the wavelength that allows for fluorescence for the particular fluorescent probe is also employed. Detectors can then be used to overlay the fluorescent region on the white light image. Generally these detectors are CCD‐ cameras. In addition high resolution spectroscopy tools and fiberoptic probes can be
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hand guided over the anatomy in question to give even more specific information on the concentration of the fluorescent probe and it's fluorescence. This allows for very precise localization of abnormal tissue. The tools to detect this fluorescence are often off the shelf in origin. More recently, commercially available and regulatory agency approved devices have become widespread. Neurosurgical operating microscopes, commonly employed for tumor resection, now come with fluorescent imaging and detection capability. The surgeon can change this illumination source on or off with a switch. The ease of use has made FGR a straightforward part of many neurological procedures [33] . Similarly the cystoscope used by urologists to assess the lower urinary tract can often illuminate with fluorescence detection [34] . Again, as this same tool is used for standard white light procedures and for fluorescent guided procedures the use of FGR in urology continues to grow. The commonly used endoscopes for pulmonary and GI procedures can also be equipped for fluorescence [35] . As the operator can easily switch between white and filtered light FGR endoscopic procedures are quite amendable with these tools. Clinical Outcomes and Limitations: Currently most surgery and medical procedures are undertaken with intense white light (room light) which allows the operator good visualization of the operating field with minimal shadowing. Unfortunately most tumors and lesions appear visually the same as surrounding normal tissue [4,36] . The mass of the tumor may be visualized but where the tumor ends (margins) and “normal” tissue
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starts is generally anything but clear [37] . The surgeon through judgment and experience will attempt to achieve tumor free margins of resection. As no one wishes to intentionally leave tumor behind the margins can be generous. This still does not guarantee tumor free resection but may, in fact, remove too much normal tissue and result in functional morbidity. This is not the intention of the procedure.
Even when removing gross tumor (gross total resection) microscopic tumor
too small to visualize may remain. More often small satellite tumors near the primary exist but cannot be visualized by the surgeon. Of as much consequence are small regional metastasis that may exist but are too small to visualize under the routine white light surgery, which is our current standard of care. In all instances the patient’s outcome will not be as optimal as wished for due to the consequences of residual tumor or undue normal tissue resection. It is in these instances that FGR can truly shine. As the following clinical highlights attest FGR may improve tumor resection rates while minimizing normal tissue resection [4‐6] . This can translate into clinically improved outcomes. Brain/Spinal Cord
Among the original clinical sites tested for FGR include the brain and spinal
cord [38] . As these tumors rarely spread, local control of disease often directly translates into improved survival rates. This is also why treatment is so aggressive involving surgery, radiation and chemotherapy as every attempt for local control is usually employed. This is also likely why morbidity from treatment can be so high too. Any intervention that could improve the chance of tumor control and/or
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morbidity reduction would likely be welcomed. This is why FGR has spread quickly into the world of neurosurgery as it can increase gross resection rates while simultaneously decreasing resection of normal brain tissue, an important combination [39] . Important too is that the FGR procedure can be readily accommodated in the neurosurgical operating room suite [40] . Currently once the brain or spinal cord is exposed an operating room microscope is brought into place so the surgeon can magnify and visualize these critical structures. During FGR the operating room lights (microscope lights) are switched from white light to blue light (literally via a simple on off switch). The blue light allows for the fluorescence of the applied probe to literally light up the tumor (Fig 1). The extent of the tumor and its spread are now much more discernible to the surgeon’s eye. Resection then follows. Non fluorescing tissue is considered normal and is spared. Clinically this is a much more precise surgery.
Multiple papers have shown that the extent of fluorescence correlates well
with the extent of tumor [41] . Further the fluorescence much more accurately shows disease then even the most state of the art imaging (MRI, PET/ CT) [42] . This allows for a more accurate surgery. What is also critical is the FGR is real time, simply switching light sources allows the surgeon to see the extent of disease in real time during surgery. This is a very time efficient process. For example intraoperative MRI, done during surgery, is not real time as imaging takes many minutes and may add hours to an already long procedure. Further intraoperative MRI scanners are rare around the world, while fluorescent capable operating microscopes are common.
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Some of the earliest work started as part of photodynamic therapy [43] .
Often prior to PDT the photosensitizer (generally Photofrin) was fluoresced confirming location of residual disease which was then followed by PDT as part of planned treatment. Subsequently, the photosensitizer ALA was also employed to enhance fluorescence detection of tumor [44] . Currently the use of ALA as the fluorophore for FGR as a stand alone treatment has moved to the forefront [45] .
The pro drug ALA is given orally or intravenously several hours before
surgery. ALA is enzymatically converted to PPIX a potent fluorescing agent (and photosensitizer). ALA crosses the blood brain barrier and concentrates up to 10x greater in malignant tissue compared to normal tissues. This means a very bright differentiation between normal brain and tumor. With training the surgeon can easily visualize this difference under blue light. The tumor essentially appears red while normal tissue appears blue.
FGR has been undertaken in various primary brain and spinal tumors.
Overall morbidity with the procedure appears low [46] . The actual surgical procedure does not appear more morbid then white light surgery. As more normal tissue can be spared, the surgery may be less morbid in that regard. The probe itself may have some morbidity. Though ALA is well tolerated, [47] when given orally a small percent of patients experience mild nausea. A small percentage may also have temporary elevation of liver functions. Enough ALA systematically circulates that a 72 hour window of sunlight photosensitivity is expected. Room light is of minimum consequence but sunlight may lead to burn. During these 72 hours daylight should be avoided.
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As Glioblastoma (GBM) is a common and extremely aggressive primary brain
tumor multiple studies of FGR have been conducted for this diagnosis. In particular as survival even with resection, chemotherapy and radiation produces relatively short survival (months) with a high morbidity expected this is a diagnosis needing innovative therapy.
It is clear that high‐grade tumors such as GBM intensely fluoresce and this
has allowed FGR to improve gross total resection rates [48,49] . As gross resection has direct impact on enhancing survival for GBM, FGR has shown this benefit [50] . An early study by Stummer randomized ALA FGR versus white light surgery [51] . A 20% increase in progression free survival was seen with FGR. The ability to increase complete resections by FGR is routinely 20% higher than under white light surgery [50] . When combined with surgical mapping gross resection rates improved by 80% or more can be achieved [52] . A doubling of complete resection rates may be achievable by FGR vs. white light. Similarly in the meta‐analysis reported by Eljamel, FGR allowed for a 75% gross total resection with an extraordinary six months average survival advantage over white light procedures [50] . These results include elderly patients, who make up a majority of this diagnosis [53] .
Attempts to enhance the ability of FGR by combining it with intraoperative
ultrasound as well as intra op MRI have shown great potential [54,55] .
Fluorescent guided resection has also found a niche in spinal tumors. Here
extraordinary precision in treatment, particularly resection, is required to prevent further damage to the spinal cord. Several studies show an enhanced ability to achieve tumor resection without additional spinal cord morbidity [56,57] . This
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enhanced ability to accurately resect while better sparing normal, uninvolved tissue, is now also spreading in application in the pediatric brain tumor population [58] . It is important to note that high grade tumors appear to routinely fluoresce, where low grade tumors may not [59] . For high grade lesions fluorescence may be .87 sensitive and 0.87 specific, both impressive figures [59] . However, tumors that have been radiated may not routinely fluoresce [60] . So still much work needs to be accomplished to best define the patients who FGR will benefit. The use of FGR followed by PDT as a combination therapy makes great oncologic sense and should be revisited [38,61] . GU
Another anatomic region with mature clinical outcome for FGR is in urology
[62] . The lower urinary tract is prone to development of malignancies. Due to the aggressive nature of these lesions, and a general lack of other successful treatment options, complete organ resection is common. In particular, when the bladder is removed this leads to life‐changing issues so any therapy that can maintain an intact and functioning bladder would be welcome [63] .
Currently the majority of bladder cancers, even insitu and noninvasive
lesions, eventually require cystectomy. This is due to the very aggressive and life‐ threatening behavior of these cancers. Local resection of tumor often fails as these lesions are multifocal and also difficult to discern from normal bladder urothelium [64] . FGR has found a growing application for these individuals. Simply, ALA based
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FGR has shown a track record of allowing for better identification of bladder cancers as they fluoresce intensely [65] . Further the florescence of the tumor, as compared to the non fluorescing urothelium has allowed for enhanced rates of complete resection [62,66] . A more intensely fluorescing version of ALA, Hexalated‐ALA, has become even more commonly employed, where available [21] .
Allowing for the widespread use of FGR in urology is that the white light
cytoscope employed to visualize and resect can easily be equipped with fluorescent generating light sources [67] . Essentially the urologic surgeons can easily switch between white light and FGR as needed without much additional training or equipment. This is a growth engine in FGR for urology.
FGR has allowed for increased detection and local control of early bladder
cancers. In a seminal trial FGR showed a 50% residual tumor rate following white light cystoscopy [68] . By adding FGR local tumor control may double. Similarly in another large trial FGR improved detection of insitu bladder cancer by over 30% as compared to white light [69] .
A meta‐analysis of randomized trials has shown that FGR increases local
tumor control, decreases local reoccurrence and improves the chance for clear or negative resection margin [62] . Additional work has shown that FGR may also allow for minimally invasive treatment of the ureter and the pelvic region of the kidney with retention of function [70] .
A very important consideration is that FGR in the GU track may translate into
significant cost savings [71,72] . Fewer patients need additional surgery due to
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increased tumor control rates of the initial FGR procedure. The additional added survival benefit is also a paramount and critical consideration [73] . Other Anatomic Regions
FGR has been explored clinically in select anatomic regions. As local control
also has direct impact on survival in head and neck cancer this seems a promising group of patients for clinical trial [74] . A phase one clinical trial explored the ability of FGR to improve the rate of clear resection margins in head and neck cancer in oral cancers [75] . Similarly, FGR improved the rate of clear margins as reported by Atallah [76] .
Indocyanine green was the probe employed by Yokoyama [77,78] . In a series
of reports this FGR allow for far greater sparing of normal functioning tissue when used in para‐pharyngeal tumor resection. Morbidity was reduced and functional outcome was improved. Also the surgeons noted that FGR did not significantly prolong surgery.
In addition to morbidity associated with primary tumor resection, neck
dissection can add significantly in that regard. As most advanced head and neck tumor spread to regional nodes neck dissection is often also employed. For these patients FGR may modify this process. In an interesting clinical trial FGR was used to fluoresce the primary (sentinel) node during resection of oral cavity tumors [79] . For patients with negative sentinel nodes, extensive neck dissection was avoided as was the associated significant functional morbidity of the procedure. The use of FGR to the primary tumor and sentinel node may become a doorway to a very innovative function sparing and far less morbid surgery for head and neck cancer patients.
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FGR has also been assessed in primary lung tumors. Several studies have
shown an ability to spare normal lung by tighter resection essentially of the fluorescing tumor with sparing of nonfluorescent lung [80,81] . This is critical as most individuals with lung cancer have poor pulmonary reserve, and sparing normal lung translates into improved pulmonary status. Further in these studies, unrecognized satellite metastatic lesions seen only under fluorescence would have been left behind in nearly 1 of 10 patients. These satellite tumors, not seen on preoperative imaging studies, fluoresced during surgery. A small group of patients have undergone FGR for liver metastasis [82,83] . Again results showed an increased ability to achieve clear resection margins and also an unexpectedly high number of metastatic but resectable lesions not visualized without fluorescence. Summary and Future Directions
Currently, many cancer patients will have improved local tumor control,
improved progression free survival and improved overall survival when the primary tumor is completely removed. As surgery is the mainstay of this process, techniques that improve complete resection have moved to the forefront of clinical surgery research. FGR has an emerging track record of achieving both enhanced local control by improving the chance of complete resection (clear margins) and also decreased morbidity as these margins are achieved without removal of undue volumes of functioning normal tissue.
The procedure itself is well tolerated and may not significantly prolong the
surgery or anesthesia, both critical factors. Importantly, FGR is a natural extension
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of the current white light surgical procedure, so rarely, are unique or expensive tools required, both of which are hindrances to worldwide introduction, particularly in the settings of limited or cost contained medical systems.
Ultimately though the question must be answered as to the true value of FGR
both in outcome and functional costs. These will be answered by a combination of well conducted clinical trials and continued innovation by those with hands on experience.
Much work is required in identifying and optimizing fluorescent probes.
Ones currently clinically available have shortcomings, but are with the advantage of being available and also allow for good clinical results. In reality an ideal probe may never pass financing for clinical trial, regulatory agency approval and then commercialization. Therefore to wait for the ideal probe (if ever available) would not benefit patients so continued efforts to maximize yields of current fluorophores is of great worth. Additional enhancements of fluorescent detection tools are also of great need. Conceivably one might create a simple to use goggle or face shield with heads up display. This could not only enhance visualization of the fluorescing tumor but also through virtual reality provide other anatomic or physiologic data to enhance the surgeon’s resection capabilities. An underappreciated aspect of FGR is that when Photosensitizers (PS) are employed as the fluorophore, they can also be activated to cyto‐destructive agents by the Type 2 Photodynamic reaction (Fig 2). This would mainly require more intense light of the appropriate wavelengths for the particular PS [18] . Therefore
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the surgeon could use FGR as an enhanced means to improve resection, followed immediately by Photodynamic Therapy to wipe up any residual tumor. This duality of the PS should be explored further.
FGR has great potential for both patient and operator. Only time and effort
will tell what it will truly achieve.
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Figure Captions
Figure 1: The fluorophore (ALA) concentrates in the tumor. It fluoresces Red when illuminated. The remaining brain tissue, not having ALA, remains blue. This allows the surgeon an enhanced ability to define and resect tumor while sparing normal surrounding tissue.
Photo courtesy of Sam Eljamel.
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Figure 2: Jablonski Diagram
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Figure 3: Incident light may reflect or be absorbed. If the incident light activates the fluorophore, fluorescence may then occur.
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S1572-1000(15)30051-X http://dx.doi.org/doi:10.1016/j.pdpdt.2015.11.008 PDPDT 718
To appear in:
Photodiagnosis and Photodynamic Therapy
Received date: Revised date: Accepted date:
24-9-2015 25-11-2015 26-11-2015
Please cite this article as: Allison Ron R.Fluorescence guided resection (FGR): A primer for oncology.Photodiagnosis and Photodynamic Therapy http://dx.doi.org/10.1016/j.pdpdt.2015.11.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fluorescence Guided Resection (FGR): A Primer for Oncology Ron R. Allison, M.D.* [email protected] Medical Director, 21st Century Oncology *Corresponding author.
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Highlights ‐Fluorescent guided resection (FGR) is a type of Image Guided Surgery ‐FGR in Oncology allows the surgeon better tumor visualization for improved resection ‐FGR as currently practiced uses applied fluorescing agents, such as Photosensitizers, to image tumor ‐Improved outcomes have been seen with FGR for neurologic and urologic tumors
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Abstract Curative treatment for most cancer patients requires surgical removal of the tumor. Often though, residual disease is left behind negatively impacting tumor control and survival. Florescent Guided resection (FGR) is one type of Image Guided Surgery that offers the potential to improve outcomes for these patients. Currently, during FGR, a probe is preoperatively applied and allowed to concentrate in the tumor bed. At surgery appropriate light is applied to generate fluorescence which allows the surgeon to better visualize tumor extent. This improved visualization has translated both into enhanced rates for resection but also diminished rates of morbidity as less normal tissue need be removed to achieve negative margins. This paper will summarize the theory and practice of FGR as currently applied in clinical oncology including select outcomes and limitations of technique and technology. Keywords: Fluorescence; Fluorescent Guided Resection; photosensitizers; Image Guided Surgery; review
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Fluorescence Guided Resection (FGR): A Primer for Oncology
Surgery is the prime and essential ingredient in the curative treatment for
the majority of cancer patients. This remains true today due to a continuum of enhancement in surgical techniques and technology [1] . The surgical oncologist’s clinical goal is to remove both gross and microscopic tumor to render the patient disease free. Achieving this outcome generally has a direct effect to enhance the chance for survival [2] . Therefore clinical tools that may increase the chance for a successful resection are in great demand and widely sought out. One such innovative procedure is termed “fluorescent guided resection” (FGR) which allows direct illumination and enhanced visualization of tumor and its tendrils [3] . Potentially this may not only allow the surgeon a greater chance of complete lesion resection but also prevent or minimize resection of normal tissues, which generally do not light up. Essentially a fluorescent probe is applied and if successful, this probe will concentrate in the tumor [4] . An external light source is then used to illuminate the surgical field and through the phenomena of florescence the tumor literally lights up (Fig 1). Normal surrounding tissue, as it does not contain the fluorescent probe, does not. Therefore FGR has the outstanding ability to allow the surgeon enhanced visualization of the tumor bed which can improve tumor resection and tumor control while simultaneously minimizing normal tissue injury. This likely explains the rapid growth and spread of this technology around the globe [5] . FGR
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FGR is one of many image guided surgical techniques currently undergoing
evaluation both clinically and pre clinically [6] . Currently, in contrast to many other image guided surgical technology, FGR has already achieved clinical success [7] . Further, FGR has commercially available fluorescent probes, detectors and associated tools to allow for relatively rapid and widespread clinical introduction. Critically, FGR does not appreciably change the surgical procedure or require significant additional operating room time, both important concepts to allow for widespread use in all operating theatres [8] . Most importantly FGR appears to be a reliable and reproducible procedure with very little, if any, morbidity attributed to the actual fluorescence or fluorescent probe [8] .
For these, and reasons to follow, FGR is becoming widespread in surgical
oncology. This paper will highlight the fundamental components of fluorescence and Florescent guided resection in oncology. The clinical success and limitations are then highlighted. Finally, means to improve outcomes are offered. FGR Fundamentals: To achieve a successful procedure the following are critical and needed components: 1) Fluorescence: Many natural and synthetic structures have the ability to emit light when activated. When this light is emitted as fluorescence these structures are termed fluorophores [9] . Fluorescence phenomena occur when a photon of incoming light energy interacts with an electron of a fluorophore. The incident light photon may transfer its energy to the resting electron of the fluorophore bringing this electron to an excited higher energy state. Often, when this excited electron returns to its ground state, energy is
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lost. Usually the energy is lost as detectable light of a longer wavelength (fluorescence) (Fig 2). This florescence may be detected. If a naturally occurring fluorophore (such as melanin) is activated this is termed autofluorescence. Very few tumors will naturally autofluoresce. In most cases a fluorophore is applied to the tumor as a fluorescent probe (see below). 2) Fluorescent probes: These structures require numerous qualities to allow for clinical success. They may be introduced orally, intravenously or topically. Clinically an ideal probe would have most of the following characteristics. 1) Specificity: The probe should essentially target the tumor exclusively [10] . This allows the surgeon to feel confident that all fluorescing tissue is malignant. 2) High Quantum Yield: A distinct and reliable difference in fluorescence between tumor and normal tissue is needed. This difference in fluorescence allows the surgeon to differentiate tumor from normal tissue. Conceptually this would be similar to a high signal to noise ratio [11] . 3) Non toxic: The probe should not be toxic to normal tissue. 4) Short half‐life: The probe should clear tissues in hours or days. 5) Amphylicity: The ability to travel via the bloodstream without a carrier is useful.
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6) Activation wavelength: The infrared (600 – 900 nm) wavelength of light travels through tissue and is of clinical benefit (see text) 7) Detection: Commercially available detection devices for the fluorescence is critical. 8) Regulatory approval: This allows for widespread use. 9) Elementary procedure: The key to clinical introduction is a reliable procedure that does not add complexity to the surgery. Currently many fluorescent probes are commercially available, but few have been designed and tested in clinical trials [12] . Most agents were developed for other purposes (i.e. Photodynamic Therapy) and have now been repurposed for FGR [13] . These agents have the benefit of being found clinically safe based on their previous lives. Two major families of probes exist: non targeted and targeted probes. New families based for example on nanoparticles are expected [14,15] . a) Non targeted fluorescent probes: These agents are not tumor specific [5,16] . Rather they appear to concentrate in rapidly proliferating tissues such as wounds. As tumors often are with proliferating regions these probes have found a role in FGR. As these probes have been available for decades (for other uses) they are considered generally safe and importantly, are commercially available. In this category are Toluidine Blue, Indocyanine Green, Fluorescein, Violet Acetate and Lugol’s Iodine among others [4] . While not specifically designed for FGR, these agents have many of the characteristics needed to be successful fluorescent probes. Still enhanced probes with more specificity are more likely to become the standard for this surgery.
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b) Targeted Fluorescent Probes
These agents appear to specifically concentrate in tumor for various reasons
[17] . Most commonly they are poorly cleared from malignancy as compared to surrounding normal tissue. In addition upregulated receptors (such as lipoprotein) found in tumors may allow for these probes to concentrate in the malignancy. The most common and successful targeted probes are generally photosensitizers developed for photodynamic therapy [18] . They have characteristics that allow for both fluorescence and to allow for singlet oxygen production (the photodynamic reaction) which is destructive to the tumor. Therefore these probes may have dual function‐to allow for fluorescent guided surgery and PDT. An underappreciated concept.
The clinically successful photosensitizers have been shown to concentrate in
tumor and clear normal tissue. Given their fluorescent capability many photosensitizers are now used for FGR. This list includes among others Photofrin, Foscan, Verteporfin and Texaphyrin.
Of note, one particular photosensitizer, aminolevulinic acid (ALA), has moved
to the forefront of FGR [19,20] . ALA is a pro drug and part of the heme synthetic pathway. Excess (applied) ALA is converted to Protoporphyrin IX (PPIX) both a potent fluorescent agent and photosensitizer. The conversion to PPIX is favored in malignant and premalignant cells making ALA an excellent targeted probe. By hexalating ALA (HAL), a stronger fluorescent agent is created [21] . c) Other Probe Concepts
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Nanoparticles may be created to target specific lesions and may well serve as the probes of the future [5] . Probes designed to fluoresce when attached to specific receptors also have been created [22] . A particularly interesting group includes VEG‐F and folate receptor based probes, as these receptors are commonly over expressed in malignancy [23] . 3) Fluorescence detection: The photon of light energy emitted as fluorescence can be detected, measured and localized [24] . If intense enough the fluorescence may be directly visible. Often the florescence is weak requiring amplification. This may be easily done by a CCD‐camera which readily detects even very weak levels of fluorescing light. Clinical Concepts for Successful Fluorescence Guided Surgery
A successful FGR would allow the surgeon a clear identification of the
fluorescing tumor to allow for complete resection. The clinical reality of this is clouded by a number of limitations. This requires an understanding of the interaction of light with tissue.
Ideally the tumor would autofluoresce and not require any applied probes.
While autofluorescence is a possibility in some tumors of the endo‐bronchus most tumors do not autofluoresce. Further autofluorescence phenomena in tumor is difficult to clinically detect [25] . The inherent fluorophores in the body (melanin for example) do not appear to be concentrated to a great degree in most tumors. Further, inherent fluorophores are found in most normal cells. Therefore weak auto fluorescence of normal tissue and tumor is expected, making differentiation of the
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two difficult. Yet subtle differences in inherent fluorophore concentration between tumor and normal tissue do exist. The difference often is slight and results in a low signal to noise ratio, meaning it is difficult by direct visualization to differentiate normal from neoplastic tissue. A number of physics based techniques and algorithms are in the developmental stage to better enhance auto‐fluorescence detection [4,8,11,25] . Despite these enhancements, it remains unclear if auto‐ fluorescent techniques are yet reliable enough on their own to successfully guide surgery [8] . Currently almost all FGR starts with an applied florescent probe that has been allowed to accumulate to a far greater degree in the tumor than in surrounding normal tissue [25] . This high signal (tumor) to noise (normal tissue) ratio should then allow visual detection of the florescent signal from the tumor to guide the surgeon’s resection. As normal tissue should not significantly fluoresce this may translate into a better ability to spare nonmalignant tissue from being resected. Clinically, removing less normal tissue should translate into fewer side effects.
To achieve this clinical outcome the fluorescent probe must be in the target
and the fluorescence from the probe must be detected. As will be shown this is much harder than it sounds.
When a photon of light interacts with tissue many pathways may be possible
[26] . Essentially the light itself may be absorbed or scattered, usually multiple combinations occur (Fig 3). For simplicity we will separate these events (absorption and scattering) with the understanding that both occur.
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Absorption: The introduced light photon may be partially or completely absorbed by the tissue it interacts with. Since the target (tumor) is only a small portion of these tissues most of the light photons are absorbed by non target tissue. The most abundant tissue component is water which preferentially absorbs virtually all applied light above 1100 nm in wavelength [27] . Hemoglobin and melanin, also quite common in tissue, will absorb most light in the 400 nm (blue) to 500 nm (green) wavelengths [28] . This leaves the red to infrared wavelength to not be absorbed and penetrate tissue (and also allows the red tinge to lighter colored skin). As red light can penetrate tissue to perhaps 1 cm this is currently the limitation in depth for clinical fluorescent detection and FGR. Scattering: If a photon is not completely absorbed by tissue it may scatter [26] . The most extreme form of scatter is reflection where the photon does not even penetrate tissue. Usually multiple scatterings occur, with loss of light energy at each event. Another important concept with scattering is refraction. Here the light photon changes direction at the interface of different tissues including air. With all the scattering and absorption in normal tissue only a relatively tiny number of introduced light photons actually interact with the fluorescent probe. Fluorescent Imaging: The small portion of photons absorbed by the probe may be energetic enough to create fluorescence. The fluorescent photon will now also either be absorbed by tissue or scatter further. It is these scattered light photons that may eventually be detected and imaged by a fluorescent detector [29,30] . The incident light and scattered light may also create autofluorescence. Essentially naturally occurring fluorophore [5] exist in the body including hemoglobin which may under
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appropriate circumstance also fluoresce. Of particular interest is that scar tissue may autofluoresce and be mistaken for tumor. Tumor that has undergone radiation and chemotherapy often forms scar like tissue which may autofluoresce making for a difficult clinical situation to determine scar or viable tumor. This is a not uncommon clinical situation. These are potentially serious limitations of FGR. Fluorescent Detection Tools
To achieve clinical acceptance the tools for detection should fit well within
the usual parameter of the surgery [31,32] . Introducing unique, time‐consuming and expensive tools will make FGR difficult and hinder widespread introduction. Fortunately, many of the commonly used operating room and procedure room equipment can be designed or retrofitted for FGR. Currently white operating room light is a critical component to allow the surgeon to see the tumor bed. So light based surgery including therapy using fluorescence is not a radical change. During FGR the surgical tools for resection remain the same. What is added is the fluorescent probe and an ability to detect fluorescence. This may be as simple as a pair of polarized goggles to assist in visualizing the fluorescing field [31] . The surgical field is now illuminated by two light sources. The usual white light allows for detection of normal anatomy. A second light source, filtered to the wavelength that allows for fluorescence for the particular fluorescent probe is also employed. Detectors can then be used to overlay the fluorescent region on the white light image. Generally these detectors are CCD‐ cameras. In addition high resolution spectroscopy tools and fiberoptic probes can be
12
hand guided over the anatomy in question to give even more specific information on the concentration of the fluorescent probe and it's fluorescence. This allows for very precise localization of abnormal tissue. The tools to detect this fluorescence are often off the shelf in origin. More recently, commercially available and regulatory agency approved devices have become widespread. Neurosurgical operating microscopes, commonly employed for tumor resection, now come with fluorescent imaging and detection capability. The surgeon can change this illumination source on or off with a switch. The ease of use has made FGR a straightforward part of many neurological procedures [33] . Similarly the cystoscope used by urologists to assess the lower urinary tract can often illuminate with fluorescence detection [34] . Again, as this same tool is used for standard white light procedures and for fluorescent guided procedures the use of FGR in urology continues to grow. The commonly used endoscopes for pulmonary and GI procedures can also be equipped for fluorescence [35] . As the operator can easily switch between white and filtered light FGR endoscopic procedures are quite amendable with these tools. Clinical Outcomes and Limitations: Currently most surgery and medical procedures are undertaken with intense white light (room light) which allows the operator good visualization of the operating field with minimal shadowing. Unfortunately most tumors and lesions appear visually the same as surrounding normal tissue [4,36] . The mass of the tumor may be visualized but where the tumor ends (margins) and “normal” tissue
13
starts is generally anything but clear [37] . The surgeon through judgment and experience will attempt to achieve tumor free margins of resection. As no one wishes to intentionally leave tumor behind the margins can be generous. This still does not guarantee tumor free resection but may, in fact, remove too much normal tissue and result in functional morbidity. This is not the intention of the procedure.
Even when removing gross tumor (gross total resection) microscopic tumor
too small to visualize may remain. More often small satellite tumors near the primary exist but cannot be visualized by the surgeon. Of as much consequence are small regional metastasis that may exist but are too small to visualize under the routine white light surgery, which is our current standard of care. In all instances the patient’s outcome will not be as optimal as wished for due to the consequences of residual tumor or undue normal tissue resection. It is in these instances that FGR can truly shine. As the following clinical highlights attest FGR may improve tumor resection rates while minimizing normal tissue resection [4‐6] . This can translate into clinically improved outcomes. Brain/Spinal Cord
Among the original clinical sites tested for FGR include the brain and spinal
cord [38] . As these tumors rarely spread, local control of disease often directly translates into improved survival rates. This is also why treatment is so aggressive involving surgery, radiation and chemotherapy as every attempt for local control is usually employed. This is also likely why morbidity from treatment can be so high too. Any intervention that could improve the chance of tumor control and/or
14
morbidity reduction would likely be welcomed. This is why FGR has spread quickly into the world of neurosurgery as it can increase gross resection rates while simultaneously decreasing resection of normal brain tissue, an important combination [39] . Important too is that the FGR procedure can be readily accommodated in the neurosurgical operating room suite [40] . Currently once the brain or spinal cord is exposed an operating room microscope is brought into place so the surgeon can magnify and visualize these critical structures. During FGR the operating room lights (microscope lights) are switched from white light to blue light (literally via a simple on off switch). The blue light allows for the fluorescence of the applied probe to literally light up the tumor (Fig 1). The extent of the tumor and its spread are now much more discernible to the surgeon’s eye. Resection then follows. Non fluorescing tissue is considered normal and is spared. Clinically this is a much more precise surgery.
Multiple papers have shown that the extent of fluorescence correlates well
with the extent of tumor [41] . Further the fluorescence much more accurately shows disease then even the most state of the art imaging (MRI, PET/ CT) [42] . This allows for a more accurate surgery. What is also critical is the FGR is real time, simply switching light sources allows the surgeon to see the extent of disease in real time during surgery. This is a very time efficient process. For example intraoperative MRI, done during surgery, is not real time as imaging takes many minutes and may add hours to an already long procedure. Further intraoperative MRI scanners are rare around the world, while fluorescent capable operating microscopes are common.
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Some of the earliest work started as part of photodynamic therapy [43] .
Often prior to PDT the photosensitizer (generally Photofrin) was fluoresced confirming location of residual disease which was then followed by PDT as part of planned treatment. Subsequently, the photosensitizer ALA was also employed to enhance fluorescence detection of tumor [44] . Currently the use of ALA as the fluorophore for FGR as a stand alone treatment has moved to the forefront [45] .
The pro drug ALA is given orally or intravenously several hours before
surgery. ALA is enzymatically converted to PPIX a potent fluorescing agent (and photosensitizer). ALA crosses the blood brain barrier and concentrates up to 10x greater in malignant tissue compared to normal tissues. This means a very bright differentiation between normal brain and tumor. With training the surgeon can easily visualize this difference under blue light. The tumor essentially appears red while normal tissue appears blue.
FGR has been undertaken in various primary brain and spinal tumors.
Overall morbidity with the procedure appears low [46] . The actual surgical procedure does not appear more morbid then white light surgery. As more normal tissue can be spared, the surgery may be less morbid in that regard. The probe itself may have some morbidity. Though ALA is well tolerated, [47] when given orally a small percent of patients experience mild nausea. A small percentage may also have temporary elevation of liver functions. Enough ALA systematically circulates that a 72 hour window of sunlight photosensitivity is expected. Room light is of minimum consequence but sunlight may lead to burn. During these 72 hours daylight should be avoided.
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As Glioblastoma (GBM) is a common and extremely aggressive primary brain
tumor multiple studies of FGR have been conducted for this diagnosis. In particular as survival even with resection, chemotherapy and radiation produces relatively short survival (months) with a high morbidity expected this is a diagnosis needing innovative therapy.
It is clear that high‐grade tumors such as GBM intensely fluoresce and this
has allowed FGR to improve gross total resection rates [48,49] . As gross resection has direct impact on enhancing survival for GBM, FGR has shown this benefit [50] . An early study by Stummer randomized ALA FGR versus white light surgery [51] . A 20% increase in progression free survival was seen with FGR. The ability to increase complete resections by FGR is routinely 20% higher than under white light surgery [50] . When combined with surgical mapping gross resection rates improved by 80% or more can be achieved [52] . A doubling of complete resection rates may be achievable by FGR vs. white light. Similarly in the meta‐analysis reported by Eljamel, FGR allowed for a 75% gross total resection with an extraordinary six months average survival advantage over white light procedures [50] . These results include elderly patients, who make up a majority of this diagnosis [53] .
Attempts to enhance the ability of FGR by combining it with intraoperative
ultrasound as well as intra op MRI have shown great potential [54,55] .
Fluorescent guided resection has also found a niche in spinal tumors. Here
extraordinary precision in treatment, particularly resection, is required to prevent further damage to the spinal cord. Several studies show an enhanced ability to achieve tumor resection without additional spinal cord morbidity [56,57] . This
17
enhanced ability to accurately resect while better sparing normal, uninvolved tissue, is now also spreading in application in the pediatric brain tumor population [58] . It is important to note that high grade tumors appear to routinely fluoresce, where low grade tumors may not [59] . For high grade lesions fluorescence may be .87 sensitive and 0.87 specific, both impressive figures [59] . However, tumors that have been radiated may not routinely fluoresce [60] . So still much work needs to be accomplished to best define the patients who FGR will benefit. The use of FGR followed by PDT as a combination therapy makes great oncologic sense and should be revisited [38,61] . GU
Another anatomic region with mature clinical outcome for FGR is in urology
[62] . The lower urinary tract is prone to development of malignancies. Due to the aggressive nature of these lesions, and a general lack of other successful treatment options, complete organ resection is common. In particular, when the bladder is removed this leads to life‐changing issues so any therapy that can maintain an intact and functioning bladder would be welcome [63] .
Currently the majority of bladder cancers, even insitu and noninvasive
lesions, eventually require cystectomy. This is due to the very aggressive and life‐ threatening behavior of these cancers. Local resection of tumor often fails as these lesions are multifocal and also difficult to discern from normal bladder urothelium [64] . FGR has found a growing application for these individuals. Simply, ALA based
18
FGR has shown a track record of allowing for better identification of bladder cancers as they fluoresce intensely [65] . Further the florescence of the tumor, as compared to the non fluorescing urothelium has allowed for enhanced rates of complete resection [62,66] . A more intensely fluorescing version of ALA, Hexalated‐ALA, has become even more commonly employed, where available [21] .
Allowing for the widespread use of FGR in urology is that the white light
cytoscope employed to visualize and resect can easily be equipped with fluorescent generating light sources [67] . Essentially the urologic surgeons can easily switch between white light and FGR as needed without much additional training or equipment. This is a growth engine in FGR for urology.
FGR has allowed for increased detection and local control of early bladder
cancers. In a seminal trial FGR showed a 50% residual tumor rate following white light cystoscopy [68] . By adding FGR local tumor control may double. Similarly in another large trial FGR improved detection of insitu bladder cancer by over 30% as compared to white light [69] .
A meta‐analysis of randomized trials has shown that FGR increases local
tumor control, decreases local reoccurrence and improves the chance for clear or negative resection margin [62] . Additional work has shown that FGR may also allow for minimally invasive treatment of the ureter and the pelvic region of the kidney with retention of function [70] .
A very important consideration is that FGR in the GU track may translate into
significant cost savings [71,72] . Fewer patients need additional surgery due to
19
increased tumor control rates of the initial FGR procedure. The additional added survival benefit is also a paramount and critical consideration [73] . Other Anatomic Regions
FGR has been explored clinically in select anatomic regions. As local control
also has direct impact on survival in head and neck cancer this seems a promising group of patients for clinical trial [74] . A phase one clinical trial explored the ability of FGR to improve the rate of clear resection margins in head and neck cancer in oral cancers [75] . Similarly, FGR improved the rate of clear margins as reported by Atallah [76] .
Indocyanine green was the probe employed by Yokoyama [77,78] . In a series
of reports this FGR allow for far greater sparing of normal functioning tissue when used in para‐pharyngeal tumor resection. Morbidity was reduced and functional outcome was improved. Also the surgeons noted that FGR did not significantly prolong surgery.
In addition to morbidity associated with primary tumor resection, neck
dissection can add significantly in that regard. As most advanced head and neck tumor spread to regional nodes neck dissection is often also employed. For these patients FGR may modify this process. In an interesting clinical trial FGR was used to fluoresce the primary (sentinel) node during resection of oral cavity tumors [79] . For patients with negative sentinel nodes, extensive neck dissection was avoided as was the associated significant functional morbidity of the procedure. The use of FGR to the primary tumor and sentinel node may become a doorway to a very innovative function sparing and far less morbid surgery for head and neck cancer patients.
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FGR has also been assessed in primary lung tumors. Several studies have
shown an ability to spare normal lung by tighter resection essentially of the fluorescing tumor with sparing of nonfluorescent lung [80,81] . This is critical as most individuals with lung cancer have poor pulmonary reserve, and sparing normal lung translates into improved pulmonary status. Further in these studies, unrecognized satellite metastatic lesions seen only under fluorescence would have been left behind in nearly 1 of 10 patients. These satellite tumors, not seen on preoperative imaging studies, fluoresced during surgery. A small group of patients have undergone FGR for liver metastasis [82,83] . Again results showed an increased ability to achieve clear resection margins and also an unexpectedly high number of metastatic but resectable lesions not visualized without fluorescence. Summary and Future Directions
Currently, many cancer patients will have improved local tumor control,
improved progression free survival and improved overall survival when the primary tumor is completely removed. As surgery is the mainstay of this process, techniques that improve complete resection have moved to the forefront of clinical surgery research. FGR has an emerging track record of achieving both enhanced local control by improving the chance of complete resection (clear margins) and also decreased morbidity as these margins are achieved without removal of undue volumes of functioning normal tissue.
The procedure itself is well tolerated and may not significantly prolong the
surgery or anesthesia, both critical factors. Importantly, FGR is a natural extension
21
of the current white light surgical procedure, so rarely, are unique or expensive tools required, both of which are hindrances to worldwide introduction, particularly in the settings of limited or cost contained medical systems.
Ultimately though the question must be answered as to the true value of FGR
both in outcome and functional costs. These will be answered by a combination of well conducted clinical trials and continued innovation by those with hands on experience.
Much work is required in identifying and optimizing fluorescent probes.
Ones currently clinically available have shortcomings, but are with the advantage of being available and also allow for good clinical results. In reality an ideal probe may never pass financing for clinical trial, regulatory agency approval and then commercialization. Therefore to wait for the ideal probe (if ever available) would not benefit patients so continued efforts to maximize yields of current fluorophores is of great worth. Additional enhancements of fluorescent detection tools are also of great need. Conceivably one might create a simple to use goggle or face shield with heads up display. This could not only enhance visualization of the fluorescing tumor but also through virtual reality provide other anatomic or physiologic data to enhance the surgeon’s resection capabilities. An underappreciated aspect of FGR is that when Photosensitizers (PS) are employed as the fluorophore, they can also be activated to cyto‐destructive agents by the Type 2 Photodynamic reaction (Fig 2). This would mainly require more intense light of the appropriate wavelengths for the particular PS [18] . Therefore
22
the surgeon could use FGR as an enhanced means to improve resection, followed immediately by Photodynamic Therapy to wipe up any residual tumor. This duality of the PS should be explored further.
FGR has great potential for both patient and operator. Only time and effort
will tell what it will truly achieve.
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Figure Captions
Figure 1: The fluorophore (ALA) concentrates in the tumor. It fluoresces Red when illuminated. The remaining brain tissue, not having ALA, remains blue. This allows the surgeon an enhanced ability to define and resect tumor while sparing normal surrounding tissue.
Photo courtesy of Sam Eljamel.
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Figure 2: Jablonski Diagram
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Figure 3: Incident light may reflect or be absorbed. If the incident light activates the fluorophore, fluorescence may then occur.
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