Interfractional uncertainty in the treatment of pancreatic cancer with radiation.
OBJECTIVE
To compare the interfractional variation in pancreatic tumor position using bony anatomy and implanted fiducial markers.


METHODS
Five consecutively treated patients with pancreatic adenocarcinoma who received definitive intensity-modulated radiation therapy at Stanford University (Stanford, CA) underwent fiducial seed placement and treatment on the Varian Trilogy system (Varian, Palo Alto, CA) with respiratory gating. Daily orthogonal kilovoltage imaging was performed to verify patient positioning, and isocenter shifts were made initially to match bony anatomy. Next, a final shift to the fiducial seeds was made under fluoroscopic guidance to confirm the location of the pancreatic tumor during the respiratory gated phase. All shifts were measured along three axes, left (+)-right (-), anterior (-)-posterior (+), and superior (+)-inferior (-), and the overall interfractional tumor movement was calculated based on these values.


RESULTS
A total of 140 fractions were analyzed. The mean absolute shift to fiducial markers after shifting to bony anatomy was 1.6 mm (95th percentile, 7 mm; range, 0-9 mm), 1.8 mm (95th percentile, 7 mm; range, 0-13 mm), and 4.1 mm (95th percentile, 12 mm; range, 0-19 mm) in the anterior-posterior, left-right, and superior-inferior directions, respectively. The mean interfractional vector shift distance was 5.5 mm (95th percentile, 14.5 mm; range, 0-19.3 mm). In 28 of 140 fractions (20%) no fiducial shift was required after alignment to bony anatomy.


CONCLUSIONS
There is substantial residual uncertainty after alignment to bony anatomy when radiating pancreatic tumors using respiratory gating. Bony anatomy matched tumor position in only 20% of the radiation treatments. If bony alignment is used in conjunction with respiratory gating without implanted fiducials, treatment margins need to account for this uncertainty.
Jayachandran
Priya
P
Department of Radiation Oncology, Stanford University, Stanford, CA, USA.
Minn
A Yuriko
AY
Van Dam
Jacques
J
Norton
Jeffrey A
JA
Koong
Albert C
AC
Chang
Daniel T
DT
D000230:Adenocarcinoma / Q000000981:diagnostic imaging / Q000532:radiotherapy; D019583:Dose Fractionation, Radiation; D006046:Gold; D006801:Humans; D009068:Movement; D015588:Observer Variation; D010190:Pancreatic Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy*; D019736:Prostheses and Implants; D011859:Radiography; D050397:Radiotherapy, Intensity-Modulated; D012119:Respiration; D035501:Uncertainty
D006046:Gold