2351 - Compromised Target Coverage with Protracted-Fractionation/Moderately Hypofractionated Radiation Treatment Courses for Pancreatic Cancer

I. C. Liu, W. T. Hrinivich, A. Narang, and J. J. Meyer; Department of Radiation Oncology and Molecular Radiation Sciences, Johns Hopkins University School of Medicine, Baltimore, MD

Purpose/Objective(s): Proximity of organs at risk (OAR) has led to interest in protracted-fractionation (PF: 15 to 25 fractions) courses employing moderate hypofractionation (MHF: 3-4 Gy/fraction) for the treatment of pancreatic adenocarcinoma, as an alternative to the limitations of classical SBRT. However, underdosing of target volumes near their interfaces with OARs is still seen even with PF-MHF courses. The impact of compromised (relative to the prescription dose) target coverage and global target dose heterogeneity on tumor control probability is an active area of investigation. We report our institution’s initial treatment planning experience with PF-MHF in pancreatic cancer. Materials/

Methods: We retrospectively reviewed radiation courses for pancreatic cancer planned with a PF-MHF approach: 45 Gy in 25 fractions (1.8 Gy/fraction) to PTV with 75 Gy (3 Gy/fraction) as an integrated boost to the GTV. We included patients treated with unresected disease as well as patients treated for local recurrence following surgery. We then collected describing dosimetric parameters for the GTV: D99.9%, D0.1cc, Dmean, V75, and V60Gy. To assess global DVH characteristics, we also calculated the generalized equivalent uniform dose (gEUD) value, using two different a values (-5 and -15). For the intact pancreas plans, we evaluated whether the gEUD differed depending on the location of tumor (head/uncinate versus body/tail).
Results: There was a total of 27 plans included in our analysis: 15 and 12 intact and resected pancreas plans, respectively. The median and range of collected dosimetric parameters are as follows: D99.9% 50.1 Gy (35.0 – 77.2 Gy), D0.1cc 81.3 Gy (77.9 – 91.6Gy), Dmean 74.7 Gy (70.8 – 78.6 Gy), V75 Gy 71.2% (50.99 - 100%), and V60 Gy 92.48% (79.28 – 100%). Assuming an a value of -5 and -15, the median (range) gEUD values were 71.0 Gy (61.5 – 78.6 Gy) and 63.2 Gy (49.4 – 78.6 Gy), respectively. For the intact pancreas plans, there were no statistically significant differences in the mean gEUDs between head/uncinate (n=7) and body/tail (n=8) plans even if factoring in the GTV volumes.
Conclusion: Although the dose was escalated within the GTV, the relatively low coverage by the prescription dose, and correspondingly low gEUD values, especially for highly negative a values, illustrate that the PF-MHF approach is associated with numerically significant dosimetric compromise. One consideration is to prioritize gEUD optimization for further refinement. Continued investigation of how dose heterogeneity impacts local control is also warranted.