Comparison of the dose escalation potential for two hypofractionated radiotherapy regimens for locally advanced pancreatic cancer.

Objectives: To determine the potential for dose escalation to a biological equivalent dose BED 100 Gy in hypofractionated radiotherapy for locally advanced pancreatic cancer (LAPC).

Materials And Methods: Ten unselected LAPC patients were retrospectively included in the study. Two fractionation regimens were compared (5 and 15 fractions). The aim was to cover 95% of the Planning Target Volume (PTV) with a BED = 54 Gy (base dose = 33 Gy in 5 fractions, 42.5 Gy in 15 fractions) whilst respecting organs-at-risk (OAR) constraints. Once the highest PTV coverage was achieved dose escalation to a BED 100 Gy (escalated dose = 50 Gy in 5 fractions, 67.5 Gy in 15 fractions) was attempted, limiting the PTV maximum dose to 130% of the escalated dose.

Results: In 5 fractions, 95% PTV coverage by both base and escalated doses could be achieved for one patient with PTV more than 1 cm away from OAR. 95% and 90% PTV coverage by the base dose was achieved in one and two patients respectively. In all other patients, coverage even by the base dose had to be compromised to comply with OAR constraints. In 15 fractions, 95% PTV coverage by the base dose was feasible for all patients except one. Dose escalation allowed improvement in target coverage by the base dose in both fractionation regimen whilst covering a sub-volume of the PTV with a BED 100 Gy. Both fractionation schemes were equivalent in terms of dose escalation potential.

Conclusion: LAPC patients with OAR close to the PTV are generally not eligible for hypofractionation with dose escalation. However, this planning study shows that it is possible to cover PTV sub-volumes with a BED 100 Gy in addition to delivering a BED = 54 Gy to 90-95% of the PTV as commonly prescribed to this population. Combined with an adaptive approach, this may maximize PTV coverage by a high BED on days with favourable anatomy.