Intensity-Modulated Radiotherapy (IMRT)

Overview

Intensity-modulated radiotherapy (IMRT) is an advanced radiation planning and delivery technique that modulates the intensity of radiation beams across many angles to conform high doses tightly to the tumour target while sparing adjacent normal tissues. In SBRT for pancreatic cancer it is used as the planning algorithm to achieve conformal dose distributions respecting critical OAR constraints (duodenum, stomach, small bowel, liver, kidneys, spinal cord).

Used by

  • Used as the planning modality in linac-based SBRT for locally advanced pancreatic cancer; IMRT-SBRT with explicit duodenal constraints (≤5% of duodenum receiving ≥22.5 Gy) achieved 94% 1-year local control with reduced late GI toxicity compared to unconstrained single-fraction regimens. A Stanford Phase II (n=16) paired 45 Gy conventional IMRT with a 25 Gy/1 fx SBRT boost, achieving 94% freedom from local recurrence at 1 year but 13% grade 3+ acute toxicity. PMID:27826200

Notes

  • IMRT enables the tight conformal dose distributions required for SBRT to pancreatic targets adjacent to duodenum, stomach, and small bowel.
  • Planning constraints for the pancreatic SBRT reference protocol (Herman et al.): V15Gy <9 cc, V20Gy <3 cc, V33Gy <1 cc for proximal duodenum/stomach/small bowel; spinal cord V8Gy <1 cc.
  • Volumetric modulated arc therapy (VMAT) is a rotational variant of IMRT also used in this setting.
  • Larger PTV margins with unconstrained IMRT-SBRT planning are associated with worse local control and higher toxicity (Danish series: 57% 1-year local control, 79% grade 2+ acute toxicity with 5 mm transverse / 1 cm craniocaudal margins).

Sources

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