Ablative radiation therapy for locally advanced pancreatic cancer: techniques and results

Abstract

Standard doses of conventionally fractionated radiation have had minimal to no impact on the survival duration of patients with locally advanced unresectable pancreatic cancer (LAPC). The use of low-dose stereotactic body radiation (SBRT) in 3- to 5-fractionshas thus far produced a modest improvement in median survival with minimal toxicity and shorter duration of treatment, but failed to produce a meaningful difference at 2 years and beyond. A much higher biologically effective dose (BED) is likely needed to achieve tumor ablation The challenge is the delivery of ablative doses near the very sensitive gastrointestinal tract. Advanced organ motion management, image guidance, and adaptive planning techniques enable delivery of ablative doses of radiation (> = 100Gy BED) when more protracted hypofractionated regimens or advanced image guidance and adaptive planning are used. This approach has resulted in encouraging improvements in survival in several studies. This review will summarize the evolution of the radiation technique over time from conventional to ablative and describe the practical aspects of delivering ablative doses near the GI tract using cone beam CT image (CBCT) guidance and online adaptive MRI guidance.

Keywords: Ablative radiation; CBCT guided radiation therapy; Hypofractionated ablative radiation; IGRT; MRI guided radiation therapy; Pancreatic adenocarcinoma.

Fig. 1

Contouring and plan evaluation. a and c Simulation CTs showing GTV (cyan), PTV high dose (red) and PTV microscopic dose (yellow) as well as stomach (orange) with a carve-out structure (brown) used to ensure exclusion of stomach from PTV high dose as demonstrated by the white arrow. b and d Dose distributions with the lowest displayed dose set to the critical max point dose for stomach (60Gy). White arrow indicates that 60Gy isodose line is away from the surface of the stomach, which was achieved by creating a PRV (not shown). c and d An example that includes an optional PTV ultra-high dose (magenta)

Fig. 2

CBCTs are used verify the target position as well as day-to-day variation in the position of the adjacent luminal GI tract. Simulation CTs of two patients displaying the critical max point dose for stomach (yellow) (a) and small bowel (magenta) (c). Corresponding DIBH CBCT images displaying the same isodose lines (b and d) are shown to the right. Stomach position may be affected by filling with food and air (a and b), while the duodenum is very reproducible (c and d)