Cancer-Related Pain

Evidence-based assessment and management of acute and chronic pain in cancer patients, from the WHO analgesic ladder to interventional strategies

Key Points

• Cancer pain is present in 30–50% of patients receiving active treatment and in 70–90% of those with advanced disease — it remains undertreated in a significant proportion of patients worldwide.
• Pain assessment must characterize type (nociceptive somatic, nociceptive visceral, neuropathic), severity (NRS 0–10), temporal pattern, and functional impact; reassessment after every intervention is mandatory.
• The WHO three-step analgesic ladder remains the foundation of cancer pain management: non-opioids for mild pain, weak opioids for moderate pain, and strong opioids for severe pain — with adjuvants added at every step.
• Morphine, oxycodone, and hydromorphone are first-line strong opioids; methadone is highly effective for neuropathic pain and opioid rotation but requires expert management due to its variable and prolonged half-life and QTc risk.
• Neuropathic cancer pain (burning, shooting, allodynia) requires adjuvant co-analgesia: gabapentinoids, SNRIs (duloxetine), TCAs, and corticosteroids — opioids alone are insufficient for neuropathic components.
• Bone metastasis pain should prompt DEXA/imaging to assess fracture risk; bisphosphonates (zoledronic acid) and denosumab reduce skeletal-related events; radiation (single-fraction 8 Gy) achieves pain response in 60–70% of cases.

Pathophysiology and Pain Classification

Cancer pain is not a single entity — it arises through distinct mechanisms that require different therapeutic approaches. Accurate classification is the essential first step. **Nociceptive somatic pain:** Arises from activation of nociceptors in skin, bone, muscle, and connective tissue. Characteristics: well-localized, aching, stabbing, or throbbing. Examples: bone metastasis pain, post-surgical pain, chest wall invasion, soft tissue tumor. Responds well to opioids and NSAIDs. **Nociceptive visceral pain:** Arises from nociceptors in hollow or solid visceral organs. Characteristics: poorly…

WHO Analgesic Ladder and Step Therapy

The WHO three-step analgesic ladder (1986, revised 2019 to a four-step model by some authorities) provides the organizing framework for cancer pain pharmacotherapy. **Step 1 — Mild pain (NRS 1–3): Non-opioid analgesics** - **Acetaminophen (paracetamol)**: 325–650 mg every 4–6 hours (max 4 g/day in healthy patients; 2 g/day with hepatic impairment or heavy alcohol use); effective as monotherapy for mild pain and as an opioid-sparing adjunct at all steps - **NSAIDs**: particularly effective for bone pain and inflammatory pain. Ibuprofen 400–600 mg every 6–8 hours; naproxen 250–500 mg twice…

Opioid Selection and Dose Titration

**First-line strong opioids:** *Morphine:* - Reference standard opioid; broad availability and extensive clinical experience - Oral: immediate-release (MSIR) 5–15 mg every 3–4 hours; extended-release (MS Contin) every 8–12 hours - IV/SC: one-third of oral morphine dose (oral:parenteral ratio 3:1) - Caution in renal impairment: active metabolite morphine-6-glucuronide (M6G) accumulates → sedation and respiratory depression; use hydromorphone or fentanyl instead in CKD *Oxycodone:* - Oral only (no IV formulation in the US); oral:morphine ratio approximately 1:1.5 (oxycodone 10 mg ≈ morphine 15…

Managing Opioid Side Effects

Side effect management is as important as analgesic titration — preventable side effects are the most common reason patients reduce or stop effective opioids. **Constipation:** Universal — tolerance does NOT develop to opioid-induced constipation. Must be prophylactically treated from day 1 of opioid therapy. - First-line: stimulant laxative — senna (sennosides) 2 tabs twice daily, titrate to effect; add osmotic laxative (polyethylene glycol, lactulose) if needed - Avoid bulk-forming laxatives (psyllium) as primary treatment in cancer patients — insufficient fluid intake leads to impaction -…

Adjuvant Analgesics for Neuropathic and Bone Pain

Adjuvants should be added at every step of the ladder when indicated by pain type — they are not reserved for refractory cases. **Neuropathic pain adjuvants:** *Gabapentinoids:* - Gabapentin: start 100–300 mg at bedtime, titrate to 300–1,200 mg three times daily; analgesic onset 1–2 weeks; dose-reduce in renal impairment; side effects: sedation, dizziness, peripheral edema - Pregabalin: start 75 mg twice daily, titrate to 150–300 mg twice daily; linear pharmacokinetics (more predictable than gabapentin); similar side effect profile *SNRIs:* - Duloxetine (Cymbalta): 30 mg daily × 1 week, then…

Interventional Pain Strategies and Special Syndromes

When systemic pharmacotherapy is insufficient, dose-limiting, or poorly tolerated, interventional strategies offer targeted analgesia with reduced systemic burden. **Interventional options:** *Celiac plexus block / neurolysis:* - Indicated for upper abdominal visceral pain from pancreatic cancer, gastric cancer, or hepatic metastases - Mechanism: interrupts afferent visceral nociceptive signals from the celiac plexus - Neurolytic (alcohol) block provides sustained relief (weeks to months) in 70–90% of pancreatic cancer patients; can reduce opioid requirements significantly - Performed by…