The Treatment of Cancer: Photon Therapy Versus Proton Therapy

Abstract

When it comes to treating persistent and chronic illnesses such as cancer, doctors and their patients have many options. The current report provides a literature review of scientific and peer-reviewed publications on photon therapy versus proton therapy as treatment options. The basic research question concerns the relative efficacy of each cancer treatment method. The basic rationale for selecting this topic relates to the potential medical promise of proton therapy. By comparison to photon therapy and modalities like IMRT, proton therapy can, at least in theory, offer significant advantages in cancer treatment due to the inherently more precise radiation delivery mechanism. As such, the purpose of the current literature review is to determine what types of cancer, if any, are most appropriately treated with each respective treatment method. The major conclusions of the study suggest that proton therapy is the most appropriate treatment method for head and neck cancer and, perhaps, other forms of cancer like prostate cancer that require precision in the delivery of radiation. Photon therapy and modalities like IMRT, on the other hand, are more appropriate for lung cancer and other types of cancer that require generalized radiation treatment.

Introduction

Photon therapy in its various modalities – 3DcRT, intensity-modulated radiation therapy (IMRT), or tomotherapy – has been one of the preferred ways of treating cancer for more than 20 years (Shrieve & Loeffler, 2012, p. 117). More recently, proton beam therapy (PBT) has been introduced as an alternative cancer treatment. In some camps, proton beam therapy has generated significant enthusiasm for its potential as a safer and more effective approach in the treatment of some types of cancer. The basic research problem addressed in the current literature concerns the fact that no scientific or medical consensus has emerged regarding the relative pros and cons of photon and proton therapy. Yet, it would appear that both photon therapy and proton therapy offer their respective and proper applications in the clinical setting. It follows that from a clinical perspective, the information reviewed in this report should help establish parameters for understanding the efficacy and appropriateness of Photon therapy and proton therapy for different types of cancer including prostate cancer, head and neck cancer, lung cancer, and CNS malignancies. The scope of the literature review examines both the theoretical pros and cons of photon therapy and proton therapy as well as findings of studies based on clinical application results. Thus, it is expected that the reviewing audience can utilize the literature review as the basis for understanding the state of medical science in the photon therapy versus proton therapy debate. Thereby, a qualified and informed point of view can be established with respect to the current needs for future research and investigation.

The Basic Debate: Photon Therapy versus Proton Therapy

Photon therapies like IMRT use x-rays or gamma rays to deliver radiation beams to tumors and other cancerous tissues and cells (National Cancer Institute, 2014). As one of the more recent photon therapy modalities, IMRT has advanced the capabilities of radiation therapists to conform the radiation to target locations in the body, thus reducing collateral effects and damage to surrounding healthy tissue areas. Despite these advancements, photon therapies like IMRT are anything but a magic bullet in cancer therapy and treatment. With IMRT, a 20% - 60% increase in dose significantly elevates the chances of secondary cancer (Schneider, Lomax, & Palmer, 2006). In fact, morbidity and the risk of secondary malignancies remains a major concern for many cancer treatment patients (Group Health Cooperative, 2014).

Photon therapy, like IMRT, is used to treat cancer by targeting radiation at cancerous tumors and cells. A theoretical advantage of proton therapy concerns the ability to control and direct the proton beam. Photon therapy is a somewhat inexact and messy process whereby radiation spillage (i.e., lateral dispersion) is unavoidable. Regardless of the precision of the vector calibrations, photon therapy will invariably result in radiation damage to non-target areas of the body. With PBT, however, the highest percentage of energy is delivered at the end of the beam path with 100% of the radiation dosage, at least in theory, hitting the targeted tissue (Group Health Cooperative, 2014). Also, the use of protons reduces the incidence of secondary cancer by as much as 50% (Schneider, Lomax, & Palmer, 2006). This means that the proton beam follows a pre-designed path and stops right where it is supposed to stop.

Even in the absence of sufficient empirical evidence, cancer researchers and medical practitioners understand the value and limitations of proton therapy in comparison to photon treatments. With its superior target precision, proton therapy promises to be most effective in cases where cancer is localized in irregularly shaped tumors. In cases where cancer is more widespread and/or metastasized, proton treatment would not be the best option. IMRT provides greater flexibility with respect to vectored targeting of cancer (Shrieve & Loeffler, 2012). For this basic reason, many healthcare experts and physicians now consider proton therapy to be the most precise and advanced form of radiation therapy (Loma Linda University Medical Center, 2014). A theoretical advantage, therefore, emerges for proton therapy - namely, the ability to deliver higher and more effective doses of radiation to tumors and other cancerous tissues without increasing the toxicity and damage to healthy, normal tissues (Brada, Johannesma, & DeRuysscher, 2007).

Experts Debate Photon Therapy and Proton Therapy

Critics do not deny that proton therapy yields benefits when dose escalation is needed. In other words, for more aggressive radiation treatments, the lower levels of lateral dispersion associated with proton treatment result in biochemical advantages for the patient. As some critics of proton therapy point out, however, the overall survival rate for photon and proton group patients are the same. The researcher further criticizes proton therapy for its relatively high cost - not to mention the fact that proton cancer treatment centers are not common and patients must, therefore, travel and endure the associated costs of not working and so forth (Rossi & Chodak, 2013). Rossi contends that while critics of proton therapy complain about the costs, they, in turn, often fail to mention the costs of photon IMRT therapy which now exceeds $1 billion per year; yet IMRT was never tested in a prospective, randomized fashion before being thrust into widespread clinical practice.

Summarily, advocates of proton beam therapy view this technology as an evolutionary advancement in radiation oncology. By comparison to photon therapy and modalities like IMRT, proton therapy irradiates less normal tissue. In fact, by virtue of their unique physical characteristics, protons will always spare far more normal tissue than any X-ray-based external-beam treatment method (Rossi & Chodak, 2013). Medically and scientifically, herein, one discovers the most fundamental theoretical advantage of proton therapy – namely, that destroying healthy tissue is never a medical or health benefit for the cancer patient.

Morbidity, Value Relative to Cost, Risk of Secondary Malignancies

Most of the controversy surrounding the photon versus proton debate concerns critical medical and healthcare issues such as morbidity, value relative to cost, and risk of secondary malignancies. In this respect, photon therapy has been used for more years than proton therapy. Therefore, a more adequate data set exists for photon therapy and the IMRT modality such that key variables like morbidity are fairly well understood (Group Health Cooperative, 2014). In the case of proton beam therapy, however, the lack of empirical evidence and the absence of robust data sets forces researchers to rely on theory and principle to support claims about this method of cancer treatment and key issues like morbidity, value relative to cost, and risk of secondary malignancies.

The Case for Proton Therapy (National Association for Proton Therapy)

The National Association for Proton Therapy (NAPT) advocates PBT on the basis of theory, clinical and treatment advantages, and cost. Theoretically, researchers at NAPT are not saying anything new about proton therapy in terms of its superior precision in the delivery of radiation particles. The same holds true with respect to NAPT claims about the reduced side effects of proton therapy. As a somewhat controversial assertion (i.e., one lacking robust empirical evidence), researchers at NAPT not only claim that proton beam therapy is less likely to cause negative side effects than photon therapy, but they go as far as claiming that PBT often has no side effects at all while also providing higher cure rates for cancer patients (Guevara-Castro, 2012). Summarily, if NAPT claims are correct, then important questions remain about the costs and the feasibility of widespread implementation.

Pragmatic Considerations of Proton Therapy Clinics

Many critics of proton beam therapy remain skeptical of this treatment method for lack of empirical evidence supporting the claims of the NAPT. In addition, the technology and equipment required to deliver proton beams are highly complex and expensive. Technologically, machines for producing radioactive x-rays are relatively small - compact enough, in fact, to fit into virtually any modest office space. The machinery and tools for generating proton beams, on the other hand, are immense in both size and weight. A proton beam generating device weighs upwards of 220 tons and can occupy the space equivalent to a football field (Beil, 2011). Thus, decision-makers at medical and healthcare centers considering the implementation of a proton therapy treatment clinic must consider the space and load limitations of their facility.

The Impact of Photon and Proton Treatment Modalities on the Dose Distribution in Field and Out-of-field

In an attempt to settle the theoretical debate surrounding photon therapy and proton therapy, Palm and Johannson provide a review focusing on studies about the variations in photon (IMRT) and proton dose distribution as related to lateral dispersion and radiation-related health risks for cancer patients. As the researchers discuss, in comparison to proton therapy, IMRT increases the out-of-field dose distribution and irradiated non-target volume, resulting in a potentially increased probability of secondary malignancies (Palm & Johansson, 2007). Therefore, proton therapy may provide a safer and more effective treatment option for some cancer patients.

Photon Therapy versus Proton Therapy: Secondary Cancer Incidence

An article by Schneider, et al. investigates IMRT and proton therapy in relation to secondary cancer incidence. Specifically, the researchers analyze the effects of radiation dose distribution to assess potential increases in the probability of secondary malignancies. Overall, the researchers show that proton therapy is a significantly safer method of cancer treatment in relation to the critical issue of secondary malignancies. With IMRT, more specifically, a 20% - 60% increase in dose significantly elevates the chances of secondary cancer, whereas the use of protons, to the contrary, reduces the incidence of secondary cancer by as much as 50% (Schneider, Lomax, & Palmer, 2006).

State of Science of Proton Beam Therapy

In a recent article, Allen, et al., evaluate the state of the medical science for proton beam therapy. The research is predicated on the observation that a dearth of empirical evidence exists with respect to the efficacy of PBT for treating a wide range of cancers and malignancies, most notably CNS malignancies, lung cancer, head cancer, and neck cancer (Allen, Pawlicki, & Dong, 2012, p. 9). Overall, the researchers find that proton therapy appears to be more effective than photon therapy in treating these various cancer and malignancy types. Yet, the researchers caution that the existing data is insufficient to support definitive conclusions about the efficacy of proton therapy. Conclusively, they recommend that more studies and clinical trials be carried out in order to determine the most appropriate and effective clinical settings for PBT (Allen, Pawlicki, & Dong, 2012, p. 9).

Treating Prostate Cancer

As one of the leading causes of cancer deaths for men in the United States (Center for Disease Control, 2013), the potential of proton beam therapy for improving the success rate of prostate cancer treatment has become the subject of much discussion and investigation in the healthcare industry. Therefore, in a recent publication, researchers for the Group Health Clinical Review Criteria review the state of science with respect to PBT and prostate cancer. Standard treatment approaches and options for prostate cancer include photon radiotherapy and surgery. As the researchers note, with either approach, the patient will invariably endure serious negative side effects including potentially, loss virility and/or sexual function (Group Health Cooperative, 2014). For most men, these types of long term impacts of cancer treatment are life-altering and psychologically unacceptable.

With respect to traditional radiation therapy, researchers for Group Health Clinical Review Criteria note that lower doses delivered to the tumor combined with the accuracy of delivery reduce unwanted genitourinary and gastrointestinal side effects (Group Health Cooperative, 2014). Also, with the development of new techniques for reducing lateral dispersion, many patients are experiencing reduced negative side effects. Summarily, these findings are encouraging for proponents of proton beam therapy as this cancer treatment method provides the benefits of more precise targeting of tumors and cancerous tissue. Nonetheless, it becomes evident from the research that PBT is still far from an exact or precise science.

Prostate Cancer Patients Report on Benefits of Proton Therapy

Ideally, medical science and cancer treatment are advanced by means of findings based on well-designed empirical studies that provide the foundation for clinically proven interventions and treatments. Yet, medical science also has a long history involving the introduction of treatments outpacing formal inquiry and investigation. In many cases, anecdotal evidence acquired from patient reports can provide a strong and even accurate indication of the efficacy of a new treatment method. This would appear to be the case with PBT. In a recent article, researchers for the National Association for Proton Therapy present their findings based on an examination of more than 2,000 cases of proton therapy treatment for prostate cancer from 1991 to 2010. Specifically, the researchers discuss the following variables: satisfaction with care, sexual function, bladder and bowel function, overall quality of life, and emotional and physical health limitations. The researchers report extremely high rates of patient satisfaction with 96% of patients reporting that they were satisfied or extremely satisfied with proton therapy (DaVanzo, Reuter, & Pick, 2014, p. 4). With respect to the other variables, the findings of the study are most promising and optimistic. Whereas the biggest downside to photon and surgical prostate treatments concerns the impact on sexual function as well as urinary and bowel function, proton therapy patients consistently report positive results in more than 90% of cases (DaVanzo, Reuter, & Pick, 2014, p. 4). Summarily, the researchers report similarly high scores for the remainder of the variables.

Prostate Cancer and Photon Therapy (IMRT)

In recent decades, intensity-modulated radiation therapy (IMRT) has been used significantly for the treatment of prostate cancer. Yet, the utilization of high-energy photons in the treatment of prostate cancer continues to raise widespread concerns for medical researchers and clinicians about lateral dispersion and secondary neutron dose for patients. Therefore, in a recent article medical researchers discuss the use of 10MV photons and 6MV photons. Based on clinical tests, the researchers report that the difference in efficacy was statistically insignificant between 10MV photon therapy treatments and 6MV photon therapy treatments (Solaiappan, Singaravelu, & Prakasarao, 2009). Ultimately, the researchers conclude that 10MV photons do not yield significant treatment advantages over 6MV photons (Solaiappan, Singaravelu, & Prakasarao, 2009). 6MV photons should, therefore, be used in order to help reduce the deleterious side effects caused by higher photon energy levels (Solaiappan, Singaravelu, & Prakasarao, 2009).

Head and Neck Cancer

Head and neck cancer refers to different types of carcinomas from multiple subsites in the upper aerodigestive tract from the nasopharynx through the hypopharynx (University of Texas Anderson Cancer Center, 2014). Due to the location of the cancer tumors, the intricacy of anatomical structures in the head and neck, and the unavoidable later dispersion of radiation, cancer treatments like photon IMRT often result in health complications and morbidities (University of Texas Anderson Cancer Center, 2014. Another complicating issue in head and neck cancer concerns the common magnified effect of interfraction or inter-field variation as sinus filling occurs (University of Texas Anderson Cancer Center, 2014). With proton therapy’s superior precision in targeting radiation beams, however, health complications and morbidities can be reduced and/or controlled to significantly greater degrees.

In another recent publication, researchers at the MD Anderson Cancer Center report on case studies involving the use of proton therapy for patients with cancer in critical structures of the head and neck. The lateral dispersion and collateral damage from IMRT treatments make this approach dangerous for patients with head and neck cancer. IMRT treatment to the neck and head can readily result in secondary malignancies to the brain and/or brain damage. The first patient ever treated at the MD Anderson Cancer Center had a cancerous growth mass wrapped around the base of the brain stem (University of Texas Anderson Cancer Center, 2014). The proton treatment began in 2010 and was completed by January 2011. The patient responded positively to the proton treatment and chemotherapy regimen for a full recovery, all the while the patient was able to work full-time and carry on normal family life (University of Texas Anderson Cancer Center, 2014).

PBT versus Photon Therapy for Lung Cancer

Lung cancer is by far the most deadly form of cancer (Rofeh Cholim Cancer Society, 2014). Even more, lung cancer is one of the most commonly treated forms of cancer (Rofeh Cholim Cancer Society, 2014). Given the presenting features and characteristics of lung cancer, radiation treatment is the only modality to treat stage I NSCLC in cases of medical inoperability when a patient is not admitted in surgery due to the fact that the patient's condition would not benefit from a surgical procedure (Allen, Pawlicki, & Dong, 2012, p. 8-9). While the efficacy of photon radiation lung cancer treatment continues to be somewhat disappointing, medical researchers are finding some appropriate uses for PBT. Specifically, PBT has been shown to result in an 80–90% local control rate with hypofractionated radiation in early-stage lung cancer (Allen, Pawlicki, & Dong, 2012, p. 8-9). Overall, nonetheless, researchers report no significant advantages of PBT in comparison to photon IMRT treatment for lung cancer.

Encumbrances to Proper Cancer Treatment in the Healthcare Industry

The debate over the effectiveness of proton and photon cancer treatment brings to light some of the problems in the healthcare industry itself. Perhaps most saliently, cancer treatment, in general, remains a subject for controversy and debate. In the absence of clear cut methods and practices for treating cancer, many physicians base their cancer treatment recommendations on factors other than what might be best for the patient. As the managed care environment in the healthcare industry continues to proliferate, insurance companies often have a bigger say than physicians when it comes to what type of cancer treatment patients will receive. Even more problematically, as a matter of trying to reduce costs and expenses, physicians and insurance companies often recommend active surveillance (i.e., watchful waiting) whereby cancer patients receive treatment only when their cancer shows signs of growing (Beil, 2011). Thus, even as the support for proton beam therapy in treating cancer grows, common practices in the healthcare industry are limiting the rate at which empirical evidence is being compiled. This is not the most effective or objective way to advance medical science.

How to Advance the State of Science for Proton

As the medical science of proton beam therapy continues to be somewhat encumbered by various systemic variables, proponents of this intervention method are forced to ask how the science of proton beam therapy can be furthered. The call for more formal studies and investigations is, of course, the most obvious answer to the question. However, Rossi warns that the optimism surrounding proton beam therapy represents a potential encumbrance to the advancement of objective scientific inquiry. History bears out the researcher’s claim. In the 1980s, for example, an unproven treatment for breast cancer became wildly popular with women. With breast cancer representing a leading cause of cancer-related death, thousands of women flocked for treatment using aggressive chemotherapy with extremely high doses of radiation, annihilating tumors and bone marrow as well (Beil, 2011; Haffty, 2009, p. 1). The treatment strategy, therefore, required the replacement of the lost bone marrow following the end of chemotherapy.

As for relevance to proton therapy research, the basic problem in the breast cancer therapy during the 1980s was that it is virtually impossible to assemble a sample population of women who had chosen other breast cancer treatment options. Thus, the efficacy of the aggressive chemotherapy strategy could not be compared or contrasted to other cancer treatment methods. Eventually, nonetheless, it was found that the aggressive chemotherapy strategy not only did not live longer or enjoy higher survival rates, but they also suffered far more severe and adverse side effects than women who had chosen other breast cancer treatments. Researchers fear that the same type of climate energized by emotional optimism now exists with respect to proton beam therapy.

Literature Review Summary

As a matter of summation, proton therapy, at least in theory, offers significant advantages in cancer treatment due to its inherently more precise radiation delivery mechanism. In fact, photon therapy is a somewhat inexact and messy process whereby radiation spillage into healthy tissue areas is unavoidable. In other words, regardless of the precision of the vector calibrations, photon therapy will invariably result in radiation damage to non-targeted areas of the body. Proton theory greatly reduces the lateral dispersion of radiation. Proton therapy, however, is not a panacea for cancer treatment. It has its practical and theoretical limitations. Ultimately, proton therapy does not negate the utility of photon therapy. Each cancer treatment method offers its own benefits and appropriate applications.

Discussion

It would appear that both photon therapy and proton therapy offer their respective and proper applications in the clinical setting. Photon therapies and IMRT, techniques have been developed to provide improved directional channeling of radiation to target areas. Despite the advancements, however, lateral dispersion of radiation continues to represent a limitation of photon therapy and the IMRT modality. The studies suggest that IMRT increases the out-of-field dose distribution and irradiated non-target volume, resulting in a potentially increased probability of secondary malignancies.

In contrast, a theoretical and even demonstrated advantage of proton therapy over photon therapy concerns the ability to control and direct the proton beam. This makes it possible to use proton therapy to deliver higher and more effective doses of radiation to tumors and other cancerous tissues without increasing the toxicity and damage to healthy, normal tissue. For certain types of cancer, especially head and neck cancer, proton therapy is the superior method as it allows clinicians to treat tumors with the required radiation without damaging the brain, eyes, and other susceptible areas. And what is more, whereas a 20% - 60% increase in dose with IMRT significantly elevates the chances of secondary cancer, proton treatment reduces the incidence of secondary cancer by as much as 50%. Perhaps most importantly, proton treatment would appear to provide men with a viable option for prostate cancer treatment that may not yield unwanted side effects related to sexual dysfunction.

Proton therapy is not, however, recommended for lung cancer and other types of cancer where cancerous cells are dispersed. These types of cancer conditions require a more generalized delivery of radiation. In such cases, photon radiation with IMRT and other delivery methods represent the more appropriate method. All considered the data suggests that the potential applications of proton therapy are far-reaching with untold benefits to patients. Yet, a dearth of research and empirical evidence exists to robustly support such claims. It is recommended, therefore, that researchers invest more time and resources in investigating the efficacy of proton therapy in treating the different types of cancer discussed in the current literature review and analysis: lung cancer, head and neck cancer, prostate cancer, and more.

Conclusion

The current report has provided a literature review of scientific and peer-reviewed publications on photon therapy versus proton therapy. The basic research question has addressed the relative efficacy of each cancer treatment method. The basic rationale for examining photon therapy and proton therapy concerns the potential medical promise of proton therapy. As findings have shown, by comparison to photon therapy and modalities like IMRT, proton therapy can provide significant advantages in cancer treatment due to the more precise radiation delivery technology. Protons are projected along a magnetic field such that the high-level energy is directed exactly onto the designated area and tissue. With this advantage, proton therapy becomes the treatment method of choice for certain cancers. In this respect, the major conclusion of the current study is that proton therapy is the most appropriate treatment method for head and neck cancer and, perhaps, other forms of cancer like prostate cancer that require precision in the delivery of radiation. Photon therapy and modalities like IMRT, on the other hand, are more appropriate for lung cancer and other types of cancer that require generalized radiation treatment.

As for a direction for that future research may take, it must be recognized that proton therapy is prohibitive for many medical institutions. It is inherently complex, but even more, the equipment takes up massive amounts of space. In fact, as reported in this study, the proton therapy machinery weighs in excess of 200 tons and requires the space equivalent to a football field. Future research will almost certainly need to be focused on finding ways to reduce the weight and size of proton equipment and machinery in order to make it more practical for medical institutions.

In addition to addressing the physical encumbrances of proton therapy technology, the ability of this technology in providing a prevision-based delivery mechanism represents the most appealing aspect of this technology. Future research should not only focus on providing robust empirical support regarding the treatment and health benefits of this cancer treatment method but even further, investigations should focus on trying to discover ways of improving the precision of proton therapy. By doing so, proton therapy may become a plausible treatment option for currently untreatable forms of cancer – certain types of tumorous brain cancers, for example.

Finally, future research should be based on a cooperative, complementary paradigm for photon and proton therapy. Too much of the research reviewed in the current study reveals an adversarial aspect. Photon therapy would appear to be construed, in some cases, as a threat to the status quo. This does not, however, need to be the case. Obviously, there are cases in which photon therapy may be the superior method of treatment and other instances where proton therapy is the proper choice. Future research should focus on clearly defining and delineating the proper application and treatment domains of each treatment method.

All considered the importance of the current literature review is a matter of the necessity of providing the most effective cancer treatments possible. Cancer is one of the most pervasive and deadly diseases in the world today. For decades, researchers have made stepwise progress in the treatment of cancer. Sometimes, advancements in the treatment of cancer have been accompanied by egregious mistakes engendered by a climate of heightened optimism and emotion. As the unwarranted craze over aggressive breast cancer treatment in the 1980s demonstrates, this type of phenomenon can result in false hope and the unnecessary suffering of innocent patients. Although the potential medical benefits of proton treatment are beginning to become evident, the same type of emotionalism seems to characterize this alternative cancer treatment therapy. In this respect, the current literature should serve to temper emotions and provide the basis for rational and objective thinking about proton therapy. Scientific and medical inquiry, in other words, must properly proceed in a manner whereby the evidence yields conclusions, not false hope and emotions.

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