Cancer pain terminology: Time to develop a taxonomy that promotes good clinical practice and allows research to progress

The IASP Global Year on Cancer Pain that concluded in October 2009 focused attention on how well cancer pain is managed and highlighted the future research challenges. It has been over 20 years since the publication of the WHO Method for Cancer Pain Relief [9] put oral opioids on the clinical map and showed that the oral route is an effective intervention for around 75% of cancer patients [1]. Yet even in developed countries cancer pain remains prevalent, is of moderate to severe intensity for many, and is undertreated for almost half of all patients [2], [4] and [5]. The emerging picture is sobering for clinical and research communities alike. While there is clearly a need for more basic science research into the understanding of cancer pain and its treatment, waiting for these new discoveries takes the spot-light away from a more uncomfortable reality: the need to address and improve our clinical practice. This edition of PAIN contains a systematic review by Haugen and colleagues of the assessment and classification of cancer break-through pain [6]. Depending on how it is defined, break-through pain occurs in about two thirds of the patients with cancer pain and is associated with higher pain intensity and poorer quality of life [3]. Although patients report a median of four episodes of break-through pain each day, many of these last for less than 30 min and only about half are associated with movement or weight bearing [8] and [10]. Treatment of this type of pain is therefore understandably challenging. In their review, Haugen and colleagues found no clear consensus on the definition or classification of break-through pain in cancer, and so not surprisingly they found no agreed method of clinical assessment [6]. Indeed, some definitions and classifications that they identified were so broad that almost all pain from cancer might be included under the term break-through pain. Although the review was able to suggest the features of an ideal break-through pain assessment tool, the authors stopped short of proposing a definition of this pain. This detailed review is significant for a number of reasons. First, the review originates from the European Palliative Care Research Collaborative and represents an organised multicentre research effort to address important clinical issues in palliative care. This level of organisation is required to impact on clinical and research activity across Europe and beyond and had been lacking in palliative care. Second, the review highlights the poorly developed classification of cancer pain that exists, which continues to hamper good clinical practice and the progress of research. Does this state of affairs arise because break-through pain lacks cohesion as a clinical construct (i.e., it lacks a clear aetiology and patho-physiological mechanism, and lacks distinct and recognisable clinical features) or is it because clinicians and researchers in palliative care lack rigor in their assessment of pain? Both of these are plausible explanations, but either way, this situation is in marked contrast to the practice of oncology where better clinical assessment has contributed to the rapid progress in therapy. Looking to the future, if we acknowledge that rigorous classification and assessment of break-through pain allows for more efficient diagnosis, more timely access to appropriate treatment and more detailed study of prognosis, then every effort should be made in this direction to produce a meaningful system of classification and assessment. However, there is sometimes a danger in trying to over-simplify an area of practice that lacks a cohesive aetiology or phenotype, especially if the pharmaceutical industry has a strong interest in directing management of all break-through pain to one therapy option. The dog must wag the tail and not vice versa. Clinicians and researchers in palliative care should consider what aspects of cancer pain in general, and break-through cancer pain in particular, are amenable to different therapeutic approaches (drug and non-drug) and therefore what features of the pain are discriminatory or what needs to be assessed. It may not be possible to achieve this but perhaps there is a lesson to be learned from the TNM (tumour, nodes, metastases) classification system used in oncology. With TNM, important domains are assessed and then an overall grade is allocated, based on severity or prognosis of the cancer. Sometimes a large tumour, with no metastases, has a similar prognosis to a smaller tumour with many metastases. The analogy for cancer break-through pain might be an overall grade based on how much it interferes with daily activity. For example high frequency, movement related pain of moderate intensity may interfere just as much as low frequency but spontaneous, severe pain. The Edmonton Classification System for cancer pain is a step in this direction [7]. An internationally agreed definition of break-through cancer pain(s), as suggested by the authors, would greatly facilitate the progress of clinical and research activity. Developing such a definition is an obvious role for the IASP as a recognised and respected body that has a longstanding interest in the taxonomy of pain. Surprisingly, there is no special interest group (SIG) concerned with cancer pain. Perhaps formation of such a group would be a fitting output of the IASP campaign in 2009. There is certainly plenty of work to do and building the taxonomy for cancer pain would be an important first step.

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