INTRODUCTION — Pain due to cancer is a complex symptom that affects most aspects of a person’s life, including physical functioning, the performance of activities of daily living, psychological and emotional status, and social interactions.

The prevalence of pain among cancer patients varies widely and is influenced by numerous factors, including the population evaluated, the specific type of cancer and its extent, and the treatment setting [1,2]. The estimated prevalence of chronic pain in populations undergoing cancer treatment ranges from 33 to 59 percent, and it is considerably higher (64 to 74 percent) in patients with advanced disease [1,2].

Given the high prevalence of cancer pain and its potential for profound adverse consequences, all patients with active malignancy should be routinely screened for pain. Patients who report pain require a more comprehensive assessment and specific treatment.

This topic review will cover cancer pain assessment. Classification of the different syndromes of pain in cancer patients and treatment of cancer-related pain are addressed elsewhere. (See "Overview of cancer pain syndromes".)

CANCER PAIN ASSESSMENT — Cancer pain assessment is a complex undertaking. The evaluation begins with a thorough history of both the pain and the underlying malignancy as well as their treatment. Because of the potential impact of pain on quality of life, it is also essential to determine the adverse effects of pain on physical and psychosocial wellbeing, as well as the spiritual impact of the pain. Cancer pain may linger after the cancer is removed (as examples, postmastectomy, postamputation, or post-thoracotomy syndrome), and this may have an important psychological and spiritual impact [3]. (See "Overview of cancer pain syndromes", section on 'Postsurgical pain syndromes'.)

In addition, other physical symptoms and both medical and psychiatric comorbidities should be characterized, as they may be independent targets for therapy. The painful site should be examined in detail, and other physical findings relevant to the patient’s medical status (both in terms of the cancer and medical comorbidities) should be noted.

The patient’s description of the pain, findings on physical examination, objective data from imaging and other tests, and information about the extent of the malignant disease and its treatment are all utilized to assess the likely etiology and underlying pathophysiology of the pain and, if possible, to identify a specific cancer pain syndrome. (See "Overview of cancer pain syndromes".)

Pain characteristics — Pain is a subjective and multidimensional experience (table 1). To optimize care, the pain report should be characterized in terms of multiple domains, beginning with clinically relevant descriptors: intensity, temporal features, location and patterns of radiation, quality, and factors that provoke or relieve the pain. This information may be sufficient to identify a specific pain syndrome (ie, a constellation of clinically meaningful signs and symptoms), which may help to elucidate the etiology, direct the diagnostic evaluation, clarify the prognosis, and suggest the appropriate therapeutic interventions. While the disease process may not be reversible, particularly for metastatic disease, the pain is often reversible with specific treatment. (See "Overview of cancer pain syndromes".)

This information may also allow for inferences regarding the underlying pathophysiology, or it may suggest and guide the need for more information.

Intensity — In the clinical setting, pain intensity is often measured simply by using a verbal rating scale (eg, “mild,” “moderate,” or “severe”) or a numeric scale (eg, “How severe has your pain been, on average, during the past week, on a scale of 0 to 10, where 0 is no pain and 10 is the worst pain you can imagine?” (form 1)). A visual analogue scale (VAS), which is a continuous line, usually 10 cm long and anchored on the ends by “no pain” and “worst possible pain,” also may be used. Pain intensity should be tracked over time using the same rating scale and specific time frame (eg, “pain right now,” “during the past day,” or “during the past week”).

Formal instruments also have been developed that describe pain in multiple dimensions. An example is the Brief Pain Inventory (BPI), a patient self-rating scale that assesses pain intensity and pain interference in various areas of function [4]. A short-form version of the BPI is available (BPI-SF). An older tool is the McGill Pain Questionnaire [5], which assesses the sensory, affective, and evaluative components of pain. A shortened version is available [6]. Both the McGill Pain Questionnaire and the BPI have been translated into several languages. Yet another is the Memorial Pain Assessment Card, which measures pain intensity, pain relief, and mood on simple VAS and verbal scales [7]. (See "Approach to symptom assessment in palliative care", section on 'History'.)

Based on the association between numeric pain ratings and the adverse impact of the pain, efforts have been made to translate the numeric score on a 0 to 10 pain intensity scale into a verbal rating scale (ie, “mild,” “moderate,” and “severe”). The rationale for this work is to attach a linguistically meaningful descriptor to a number that, by itself, may be more difficult to interpret. Although these analyses have yielded slightly different results, they have generally found that the term “moderate” best characterizes a limited midrange of the numeric scale. For example, by associating the numeric pain scores with scores on quality of life measures, one group proposed that pain intensity from bone metastases be considered “mild” if the pain score is 1 to 5, “moderate” if the score is 6, and “severe” if the score is 7 to 10 [8]. Most importantly, these analyses generally support the view that a patient with cancer who describes his or her usual pain as a 7 or greater is likely to be experiencing substantial adverse pain-related consequences and disability. (See "Epidemiology, clinical presentation, and diagnosis of bone metastasis in adults", section on 'Clinical presentation'.)

Temporal assessment — Temporal assessment of the pain may include information about type of onset, duration, course, and daily fluctuation. Acute pain syndromes usually have a well-defined onset and a readily identifiable cause (eg, surgery). The pain is expected to run its course in a short time frame, and management typically focuses on symptomatic relief until this happens.

In contrast, chronic pain is often characterized by an ill-defined onset and a prolonged, fluctuating course. Overt pain behaviors (eg, grimacing) and sympathetic hyperactivity are typically absent, but vegetative signs, including lassitude, sleep disturbance, and anorexia, may be present. The patient usually does not appear to be in pain, and the only definitive way to determine the presence of pain is to obtain a verbal report from the patient.

Most patients with chronic cancer pain also experience periodic flares of pain, often referred to as “breakthrough pain” [9-12]. One important subtype of breakthrough pain is “incident pain,” which is a pain flare that is precipitated by voluntary activity. The recognition of breakthrough pain as a significant problem in cancer patients led to the concept of so-called “rescue” dosing (ie, doses of a short-acting opioid administered on an as-needed basis to manage pain flares during long-term opioid therapy). (See "Cancer pain management with opioids: Optimizing analgesia", section on 'Management of breakthrough pain'.)

Other aspects — Other pain characteristics are important to assess. The quality of the pain (eg, aching, sharp, stabbing, burning, etc) provides information that may help distinguish between nociceptive and neuropathic pain syndromes (see 'Nociceptive pain' below and 'Neuropathic pain' below). The assessment of pain location and referral patterns, and identification of specific factors that can provoke or reduce pain may help to clarify the etiology and elucidate a treatment approach that includes management of precipitants.

Pain etiology — Chronic pain in a cancer patient usually has a cause related to the disease itself or its treatment (ie, chemotherapy, radiation therapy, surgery). Identifying the etiology can clarify the status of the disease, potentially altering the prognosis or informing the plan of care for treatment. Furthermore, clarifying the etiology of the pain can sometimes reveal an opportunity for disease-modifying therapy that may provide some analgesic benefit. As an example, radiation therapy is often performed with pain relief as the primary goal. (See "Overview of cancer pain syndromes".)

When the etiology of the pain is related to the disease, it usually involves direct invasion of pain-sensitive structures by the neoplasm [13]. Bone is the most common pain-sensitive structure affected by the tumor. Other etiologies include injury to or invasion of a component of the nervous system, obstruction of a hollow viscus, distention of organ capsules, distortion or occlusion of blood vessels, or infiltration of adjacent soft tissues [13].

Inferred pathophysiology (types of cancer pain) — Finding an etiology for the pain may influence prognostication or decisions about evaluation or treatment. Similarly, drawing an inference about the type of pathophysiologic process sustaining the pain may offer opportunities to positively impact the care plan. Although the broad pathophysiologic process underlying the pain cannot be objectively verified, clinicians are able to integrate findings from the history, examination, and objective tests to draw inferences and apply broad labels, such as “nociceptive” or “neuropathic” pain. These labels certainly represent a gross simplification of very complex neurophysiologic and neurochemical processes, but they are conventionally accepted because clinical experience and a large body of clinical research indicate that they can be useful in suggesting specific treatment approaches.

Cancer pain may be designated as nociceptive, neuropathic, or psychogenic; mixed disorders are extremely common [14]. Disorders that cannot be categorized based on the available information should be called idiopathic.

Nociceptive pain — Nociceptive pain is related to ongoing tissue injury, which presumably activates the somatosensory systems that alert the organism to noxious events (the nociceptive system) and ultimately lead to the perception of pain. Inflammation is commonly present when tissue injury occurs, and some classifications use the term “inflammatory pain” instead of nociceptive pain; others suggest that inflammatory pain is a subtype of nociceptive pain characterized by activation of the nociceptive system by inflammation of peripheral tissues. Whether or not the inflammatory label is used, nociceptive pains can be divided into those that are sustained by injury to somatic tissues and those that are sustained by injury to visceral tissues:

●Somatic nociceptive pain involves injury to somatic structures, such as bone, joints, or muscles. It is often described by patients as “aching,” “stabbing,” “throbbing,” or “pressure-like” in quality.

●Visceral nociceptive pain involves injury to viscera. It is usually characterized as “gnawing” or “crampy” when arising from the obstruction of a hollow viscus (eg, the bowel lumen), and as “aching” or “stabbing” when arising from other visceral structures, such as organ capsules, myocardium, or pleura.

Neuropathic pain — Pain is labeled as neuropathic if the results of the evaluation suggest that it is sustained by abnormal somatosensory processing caused by a lesion or disease affecting the peripheral or central somatosensory system [15]. Neuropathic mechanisms are involved in approximately 40 percent of cancer pain syndromes, and they can be caused by either the disease or its treatment [13].

Dysesthesias, uncomfortable sensations that are perceived as abnormal and described using terms such as “burning,” “shock-like,” or “electrical,” suggest an underlying neuropathic mechanism for the pain. A neuropathic mechanism can be inferred, however, in the absence of dysesthesia since some patients with nerve injury will describe the pain as “aching” or “throbbing.” Physical examination of a patient with neuropathic pain may identify findings of allodynia (ie, pain induced by non-painful stimuli), hyperalgesia (ie, increased perception of painful stimuli), or other sensory findings. Patients may have other concomitant neurologic findings, such as weakness or changes in reflexes, and some patients have autonomic dysfunction within the anatomic distribution of the pain.

Psychogenic pain — The term “psychogenic pain” is used to describe pain that is believed to be sustained predominantly by psychological factors. It does not refer to the common observation that pain experienced by some patients is exacerbated by psychological factors, or the finding of high pain-related distress or comorbid psychiatric disease. Rather, psychogenic pain implies that the pain is best understood as resulting from psychological processes. This pain is related to other disorders characterized by prominent somatic symptoms associated with significant distress and impairment. The Diagnostic and Statistical Manual of Mental Disorders, 5^th edition (DSM-5), by the American Psychiatric Association, classifies a disorder such as this as a “somatic symptom disorder with prominent pain,” which is diagnosed on the basis of excessive thoughts, feelings, or behaviors related to pain that are distressing, impair functioning, and appear out of proportion to the physical findings [16].

Psychogenic pain appears to be rare in the cancer population. The label should not be applied unless the assessment reveals evidence of psychopathology that is believed to be specifically related to the experience of the pain. Pain of this type is assumed to be truly experienced; it is not a deception. This distinguishes these pain disorders from factitious disorders, which reflect a serious mental disorder in which pain reports may not indicate a true experience of pain (unless the manifestation of the disorder is self-injury, which may be painful), and malingering. The latter disorders appear to be very rare in the cancer population.

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Neuropathic pain" and "Society guideline links: Cancer pain".)

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, “The Basics” and “Beyond the Basics.” The Basics patient education pieces are written in plain language, at the 5^th to 6^th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10^th to 12^th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on “patient info” and the keyword(s) of interest.)

●Basics topics (see "Patient education: Managing pain when you have cancer (The Basics)")

SUMMARY — Pain in the patient with a history of cancer is a complex symptom that affects most aspects of a person’s life, including physical functioning, the performance of activities of daily living, psychological and emotional status, and social interactions.

Assessment of acute (eg, postsurgical) pain is usually straightforward. If the cause is apparent, no further assessment may be needed. On the other hand, cancer patients with chronic pain are more challenging because of the heterogeneity in specific cancer diagnoses, disease activity, past and present history of disease-modifying therapies, prior evaluation and efforts at pain control, impact of pain on quality of life, and existence of relevant medical and psychiatric comorbidities.

This complexity calls for a systematic approach to assessment. The major goals of the assessment are as follows:

●Characterize the pain and its impact on physical functioning and quality of life. (See 'Pain characteristics' above.)

●Become aware of prior testing and treatments performed specifically for the pain.

●Understand the pain in terms of etiology (disease related, treatment related, or neither) and inferred pathophysiology (nociceptive, neuropathic, psychogenic, or mixed), and identify a specific pain syndrome if possible. (See 'Inferred pathophysiology (types of cancer pain)' above and "Overview of cancer pain syndromes".)

●Clarify the extent of neoplastic disease and the opportunities for antineoplastic treatment.

●Note comorbidities, including other symptoms, medical disorders, and psychiatric/psychosocial problems that may be targets for treatment.

The information obtained from the history, physical examination, and imaging studies can lead to changes in the plan of care, such as the following:

●Recommendations for additional evaluation to clarify the etiology or pathophysiology of the pain, or the disease extent.

●Recommendations concerning a disease-modifying therapy (eg, radiation therapy) that may address the etiology of the pain. Recognition that new pain might indicate a serious new morbidity (eg, back pain related to epidural spinal cord compression) and may also suggest the need for urgent treatment.

●Developing a short-term and longer-term plan for symptomatic analgesic therapy.

●Addressing other symptoms or medical or psychiatric/psychosocial comorbidities that may be contributing to suffering.