Introduction {#sec1-1}
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Cancer is one of the most feared diseases. Starting from the diagnosis of cancer, its progression (i.e., metastasis to bone and organs), adverse effects of its treatment (chemotherapy, radiation, and surgery), and diagnostic procedures (biopsies and radiological diagnostic scans) can cause physical, psychological, and emotional problems affecting patients\' quality of life (QOL).\[[@ref1][@ref2][@ref3][@ref4][@ref5][@ref6][@ref7][@ref8][@ref9]\]

The statistics of new cancer cases and cancer-related mortality is scary. According to the 2016 report of the American Cancer Society, more than 1.6 million new cancer cases were diagnosed each year,\[[@ref10]\] about 32.6 million people were living with cancer worldwide, and the number has been increasing with time. It is estimated that about 33% of women and 50% of men would develop cancer during their lifetime; about 15% of all deaths worldwide would be attributed to cancer, about 77 million people worldwide would die of cancer, and it would surpass heart diseases.\[[@ref10][@ref11]\]

With advances in diagnostic methods and improved treatment strategies, it is expected that the number of cancer survivors will continue to increase and pose a great challenge to health care system.\[[@ref12][@ref13][@ref14][@ref15][@ref16][@ref17]\]

Despite the availability of powerful technology and strong and targeted medicines, the desired therapeutic success in cancer care and other chronic diseases remains an elusive goal for the modern medicine. In addition, the conventional medical interventions are expensive and associated with undesirable toxicities. The patients, therefore, may turn to nonconventional therapies, e.g., complementary and alternative medicine (CAM).\[[@ref18]\] Increasing interest in CAM and demands from the public, medical professionals, media, and government agencies had led the National Institute of Health in 1998 to establish the National Center for Complementary and Alternative Medicine (NCCAM) to explore those practices that are not currently considered to be a part of conventional (or main stream) medicine practiced, especially by MDs in the USA such as (i) whole medical systems (Ayurveda, Chinese traditional medicine, homeopathy, and naturopathy); (ii) mind--body medicine (yoga, meditation, relaxation, visualization/imagery, cognitive therapy, aromatherapy, dance, healing touch, hypnosis, music, art, prayer, sleep promotion, support groups, etc.); (iii) biologically based practices (dietary supplements, herbal products, shark cartilage, etc.); (iv) manipulative and body-based practices: acupressure, acupuncture, chiropractic, massage, osteopathic manipulation; and (v) energy medicines (Qi gong, Reiki, therapeutic touch, electromagnetic fields, and alternating-current or direct-current fields). In other words, a group of diverse medical and healthcare systems, products, and practices that are "not usually taught in medical schools, not available in most hospitals, clinics, and private practices, and often not reimbursed or otherwise routinely accessible."\[[@ref19]\]

The definition of CAM, however, has been changing over time. Since some of the whole medical systems -- Ayurveda, homeopathy, naturopathy, and Chinese traditional medicine, are used as one of the main medical practices in India and China; only a few patients in the USA (3%--6%) make use of them. These have now been dropped from the list of CAM and the NCCAM has reincarnated with a new name, The National Center for Complementary and Integrative Health "NCCIH."\[[@ref20]\] The term "integrative medicine" is becoming more popular. The commonly used current terminology is complementary medicine (therapies used in conjunction with conventional medicine); alternative medicine (therapies used in place of conventional medicine); and integrative medicine (use of evidence-based CAM practices with conventional medicine).

The use of CAM is significantly increasing over the years. About 38% persons in the USA are using CAM for managing pain, arthritis, cardiovascular diseases, cancer, and psychological and emotional problems such as stress, anxiety, and depression, at the cost of approximately \$40 billion.\[[@ref21][@ref22][@ref23][@ref24]\] The use of integrative medicine, particularly in cancer care, is so popular that a number of medical schools and cancer centers are now offering programs in integrative medicine to their patients. "Integrative oncology" in particular is emerging as a new discipline in cancer centers.

Western scientists are now moving away from the matter-based approach that they had been practicing and realizing the importance of mind--matter relationship. In their quest to understand the subtler dimensions of the universal laws and gain new insights in the mind--matter relationship, they are turning to understand the wisdom and practices of the East.

Yoga, the mind--body medicine of CAM, is comprised of a wide range of techniques, which gradually harmonizes the body and mind as compiled by Patanjali in his Yoga Sutras.\[[@ref25]\] Because of its health-related benefits, yoga has been an integral part of Ayurveda, the oldest and indigenous medical system practiced in India and described in *Charak Samhita* and *Susruta Samhita*.\[[@ref26][@ref27]\] Yoga (meaning union or to join) has been used by Hindus and Buddhists for thousands of years for maintaining good health as well as a spiritual practice (union of the individual self with the universal self, salvation). During the last five decades, there has been a worldwide interest in yoga practices. In its various forms and for various reasons (physical and mental health-related benefits), yoga is now practiced worldwide by millions of people irrespective of their age, gender, race, religion, and nationality. The surging interest in yoga may be appreciated by the fact that, in the year 2012, more than 20 million Americans were using yoga spending more than 10 billion dollars annually on yoga classes and products, and since 2011, about 200 titles are added each year on the use of yoga in different medical conditions.\[[@ref28][@ref29][@ref30]\] Integration of yoga with conventional cancer care is a "patient-centered approach that nurtures the physical, emotional, and spiritual wellbeing of cancer patients."\[[@ref31]\] Despite a number of reports and reviews supporting efficacy of yoga in health care, the awareness and integration of yoga in conventional healthcare remain limited.\[[@ref24]\] Therefore, the main purpose of this review is to familiarize cancer patients and their caregivers (oncologists, nurses, family members, and patients) with the research evidence of the beneficial effects of yoga and to encourage more scientifically focused research so that yoga therapy is fully recognized and integrated into cancer therapeutic programs.

Methods {#sec1-2}
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A search of English language literature published through December 15, 2016, on the use of yoga in cancer was conducted using the National Library of Medicine electronic database, PubMed. An advanced search using Boolean operators (i.e., "AND," "OR," and "NOT") was performed using the medical subject heading terms and the keywords: yoga, meditation, *pranayama*, breathing exercises, mindfulness-based stress reduction, and cancer. The primary results obtained were further analyzed using filters, i.e., reviews, clinical trials, and others. Following initial independent dual examination of the titles and the abstracts of the clinical trials, the articles that were found irrelevant, e.g., protocol developments, telephone surveys, and using Qi gong (even though it may have some elements of breathing exercises), were excluded from further analysis. Additional secondary references were obtained from the reviews and other publications.

Results {#sec1-3}
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Of a total of 864 articles identified through the electronic database search, 203 articles were clinical trials \[[Figure 1](#F1){ref-type="fig"}\]. After examination of the titles and the abstracts, 138 studies met the inclusion criteria - 28 single armed trials \[[Table 1](#T1){ref-type="table"}\]\[[@ref32][@ref33][@ref34][@ref35][@ref36][@ref37][@ref38][@ref39][@ref40][@ref41][@ref42][@ref43][@ref44][@ref45][@ref46][@ref47][@ref48][@ref49][@ref50][@ref51][@ref52][@ref53][@ref54][@ref55][@ref56][@ref57][@ref58][@ref59]\] and 110 randomized and nonrandomized controlled trials: 18 from 1996 to 2008 \[[Table 2](#T2){ref-type="table"}\],\[[@ref60][@ref61][@ref62][@ref63][@ref64][@ref65][@ref66][@ref67][@ref68][@ref69][@ref70][@ref71][@ref72][@ref73][@ref74][@ref75][@ref76][@ref77]\] 34 from 2009 to 2012 \[[Table 3](#T3){ref-type="table"}\],\[[@ref78][@ref79][@ref80][@ref81][@ref82][@ref83][@ref84][@ref85][@ref86][@ref87][@ref88][@ref89][@ref90][@ref91][@ref92][@ref93][@ref94][@ref95][@ref96][@ref97][@ref98][@ref99][@ref100][@ref101][@ref102][@ref103][@ref104][@ref105][@ref106][@ref107][@ref108][@ref109][@ref110][@ref111]\] and 58 from 2013 to 2016 \[[Table 4](#T4){ref-type="table"}\].\[[@ref112][@ref113][@ref114][@ref115][@ref116][@ref117][@ref118][@ref119][@ref120][@ref121][@ref122][@ref123][@ref124][@ref125][@ref126][@ref127][@ref128][@ref129][@ref130][@ref131][@ref132][@ref133][@ref134][@ref135][@ref136][@ref137][@ref138][@ref139][@ref140][@ref141][@ref142][@ref143][@ref144][@ref145][@ref146][@ref147][@ref148][@ref149][@ref150][@ref151][@ref152][@ref153][@ref154][@ref155][@ref156][@ref157][@ref158][@ref159][@ref160][@ref161][@ref162][@ref163][@ref164][@ref165][@ref166][@ref167][@ref168][@ref169]\]

![Flow diagram of included and excluded studies](IJY-11-3-g001){#F1}

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Summary of single-armed clinical trials of yoga in cancer patients from 2001-2016

![](IJY-11-3-g002)

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Summary of randomized controlled trials of yoga in cancer patients from 1996-2008

![](IJY-11-3-g003)

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Summary of randomized controlled trials of yoga in cancer patients from 2009 to 2012

![](IJY-11-3-g004)

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Summary of randomized controlled trials of yoga in cancer from 2013 to 2016

![](IJY-11-3-g005)

A total of 10,660 patients who participated in these studies had had breast cancer, colorectal cancer, leukemia, lymphoma, lung cancer, pediatric cancer (unspecified), prostate cancer, hematopoietic stem cell transplant patients, and others \[Tables [1](#T1){ref-type="table"}--[4](#T4){ref-type="table"}\].

The measures included almost all the symptoms and side effects listed in [Table 5](#T5){ref-type="table"}. In addition, the effect of yoga was reported on the telomere length, telomerase activity, T-cell subsets, signaling pathways of inflammatory transcription genes, cytokines, nuclear factor kappa B, and cAMP response element binding protein.\[[@ref77][@ref131]\] In the vast majority of the studies, the yoga intervention was found to be beneficial and yielded positive effect on all the measures.

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Cancer and its treatment-related adverse symptoms

![](IJY-11-3-g006)

Even though the study protocols used different types and styles of yoga -- *Hatha yoga, Iyengar yoga, Dru, Vini*, restorative yoga, etc., \[Tables [1](#T1){ref-type="table"}--[4](#T4){ref-type="table"}\], all of them were slight variations of Patanjali\'s *Ashtanga Yoga* (*asanas, pranayamas,* and *dhyana*) tailored to the patients\' need and/or investigators\' convenience. The duration and frequency of yoga practices also differed widely in different studies. The interventions were administered either to a group or individually by an experienced yoga instructor at the study site and/or practiced at home with the help of DVDs provided by the investigators. The effect of yoga was examined on almost all the cancer-related symptoms and treatment-related side effects listed in [Table 5](#T5){ref-type="table"} and on the markers of immunity, inflammation, stress, etc.

The studies were conducted in 20 countries from five continents (Australia, Asia, Europe, North America, and South America). Majority of the studies had been done in the USA (*n* = 61), followed by Canada (*n* = 21), India (*n* = 9), UK (*n* = 8), Sweden (*n* = 6), Germany (*n* = 5), Australia (*n* = 4), three each from Denmark, Japan, Slovenia, South Korea, and Turkey, two from the Netherlands, and one each from Brazil, Italy, Iran, Poland, Singapore, Switzerland, and Taiwan.

Discussion {#sec1-4}
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This article presents a systematic review of the efficacy of yoga interventions as adjuvants to conventional cancer care; i.e., cancer and cancer treatment-related symptoms and side effects.

The diagnosis of cancer and its treatment-related toxicity causes a high degree of emotional distress in patients and their families, consequently leading to a number of negative implications.\[[@ref81][@ref104][@ref116][@ref170][@ref171][@ref172]\] Family members of cancer patients, the parents of children with cancer in particular, and the spouses of cancer survivors experience increased anxiety, depression, and feeling of helplessness.\[[@ref173][@ref174][@ref175][@ref176]\] Furthermore, pharmacological interventions used to alleviate the adverse symptoms are also associated with side effects, toxicity, and addictions, posing a great challenge for the cancer care providers. Therefore, the patients tend to use nonpharmacological therapies.

The literature review presented here shows that the yoga interventions are beneficial in improving the adverse symptoms in cancer patients -- caused either by the disease or its treatment. Due to the immensity of its beneficial effects, the integrations of yoga in cancer care are gradually increasing,\[[@ref177]\] and a number of major cancer centers are adding integrative oncology in their programs. The prevalence, demographics, and trend of the use of yoga practices in various disease conditions have been reviewed by Field.\[[@ref29]\]

The salient features of yoga as therapeutic tool are that it is simple and easy to administer; it could be administered at any time, at any place, and by any person, irrespective of age or gender; is cost-effective; and could be administered to one person or to a group in the clinical settings or at home. The patients are involved in their own therapy. In contrast to pharmacological interventions, it is nontoxic, nonpervasive and therefore could be used during pregnancy and lactation. It may be used by patients, medical professionals, and caregivers for their own benefit and for others. An important point to note is that yoga also enriches the spiritual needs of patients that are not met by conventional therapy (surgery, radiation, and chemotherapy). It could be used as an adjuvant with other treatments.

The greatest challenges for cancer care providers are (1) how to prevent onset of cancer (transformation of a cell into a cancer cell), (2) how to arrest its progression, (3) how to cure cancer, and (4) how to maintain the survivors\' QOL. Could the integration of yoga in conventional therapy meet these challenges?

The studies reviewed here provide ample evidence of the beneficial effects of yoga on the psychological, physical, and emotional health and QOL of cancer patients. While direct studies of the effect of yoga on the prevention, progression, and cure of cancer are almost lacking, following indirect observations suggest that yoga may meet the above-listed challenges.

Among other factors, it is well known that the body\'s immune system plays an important role in the development of cancer. During normal cell division, some cells acquire mutations. Aided by the risk factors (tobacco smoke, chemicals, viral infections, etc.), the mutated cell may acquire additional mutations and continues to proliferate and develops into cancer. Normally, an efficient immune system destroys the mutated cell as soon as it is formed; however, if the immune system is impaired or weakened, the mutated cell escapes destruction and continues to pathological proliferation. A number of studies have shown that the sustained stress (commonly observed in cancer patients) negatively affects the cellular immunity.\[[@ref178][@ref179][@ref180]\] An impaired immune system may facilitate tumor development.\[[@ref178][@ref181][@ref182][@ref183][@ref184][@ref185]\] Several studies have shown that yoga reduces stress, depression, and anxiety, changes cellular milieu by genomic alteration, and enhances cellular immunity.\[[@ref186][@ref187]\] It is therefore very likely that yoga could prevent tumorigenesis and progression and possibly help cure cancer. Interestingly, a number of case reports from Meares demonstrated that the practice of yoga/meditation was able to regress the growth of tumors.\[[@ref188][@ref189][@ref190][@ref191]\] The above findings suggest that yoga may help prevent tumorigenesis and progression and ultimately cure cancer. Of course, well-designed studies are needed to examine this hypothesis.

This review revealed a number of gaps in the reported studies. As stated in our earlier review,\[[@ref192]\] one of the most significant methodological problems was the heterogeneity of yoga techniques and the assessment of protocols used in different studies \[Tables [1](#T1){ref-type="table"}--[4](#T4){ref-type="table"}\], thus lacking a standardized approach. Other deficiencies were uncertainty of an effective dose; lack of studies on side effects/risks; small sample sizes; lack of follow-up studies; most of the outcomes were self-reported; lack of recruitment methods and randomized process, cointerventions, and compliance. Furthermore, except for breast cancer, studies on other cancer types remain limited. Other limitations noted were that, in spite of increasing interest in integrating holistic approaches in cancer care, the awareness in public, medical professionals, and caregivers remains limited. It should be noted that, unlike objective pharmacological interventions, yoga is a subjective intervention, and therefore, the same metrics may not be applicable to compare the two approaches, and the problems will persist with this kind of intervention.\[[@ref29][@ref192]\] However, more well-designed randomized control trials using larger sample sizes, longer duration, follow-up studies, well-specified outcome measures, and different cancer types are necessary to establish benefits of yoga in the management of cancer.\[[@ref192]\]

Greater efforts must be made to integrate yoga as a mainstream therapeutic program. This includes scientific mindset and acceptability of yoga as an important therapeutic tool. It is therefore necessary to introduce courses on yoga therapy in medical schools, seek accreditation by authentic agencies, provide insurance coverage for yoga therapies, and educate the public about the benefits of yoga so that it is integrated in the current cancer therapeutic programs. It is also recommended that cancer caregivers are trained and certified as the "yoga therapists" who have knowledge of the particular disease and understand the needs of the patients.

Conclusion {#sec1-5}
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This article reviews the evidence-based research on the effects of yoga in cancer care when integrated as an adjuvant with conventional therapy. Despite a wide range of methodological gaps and limitations, yoga interventions were shown to be beneficial and yielded positive results without any adverse outcomes. While work must continue using well-designed clinical trials, the findings reported here strongly support the integration of yoga in the conventional cancer care.

Financial support and sponsorship {#sec2-1}
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Nil.

Conflicts of interest {#sec2-2}
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There are no conflicts of interest.
