Geometric Reproducibility of Fiducial Markers and Efficacy of a Patient-Specific Margin Design Using Deep Inspiration Breath Hold for Stereotactic Body Radiation Therapy for Pancreatic Cancer

Abstract

Purpose: In patients undergoing stereotactic body radiation therapy (SBRT) for pancreatic adenocarcinoma, the reproducibility of tumor positioning between deep-inspiration breath holds is unclear. We characterized this variation with fiducials at simulation and treatment and investigated whether a patient-specific breath-hold (PSBH) margin would help account for intrafraction variation at treatment.

Methods and materials: We analyzed 20 consecutive patients with pancreatic cancer who underwent SBRT with deep-inspiration breath holds. At simulation, 3 additional breath-hold scans were acquired immediately after the contrast-enhanced planning computed tomography (CT) scan and used to quantify the mean and maximum variations in the simulation fiducial position (Sim_Var _avg and Sim_Var _max ), as well as to design the internal target volume (ITV) incorporating a PSBH margin.

Results: At treatment, a mean of 5 breath-hold cone beam CT (CBCT) scans were acquired per fraction for each patient to quantify the mean and maximum variations in the treatment fiducial position (Tx_Var _avg and Tx_Var _max ). Various planning target volume (PTV) margins on the gross tumor volume (GTV) versus ITV were evaluated using CBCT scans, with the goal of >95% of fiducials being covered at treatment. The Sim_Var _avg and Sim_Var _max were 0.9 ± 0.5 mm and 1.5 ± 0.8 mm in the left-right (LR) direction, 0.9 ± 0.4 mm and 1.4 ± 0.4 mm in the anteroposterior (AP) direction, and 1.5 ± 0.9 mm and 2.1 ± 1.0 mm in the superoinferior (SI) direction, respectively. The Tx_Var _avg and Tx_Var _max were 1.2 ± 0.4 mm and 2.0 ± 0.7 mm in the LR direction, 1.1 ± 0.4 mm and 1.8 ± 0.6 mm in the AP direction, and 1.9 ± 1.0 mm and 3.1 ± 1.4 mm in the SI direction, respectively. The ITV was increased by 21.0% ± 8.6% compared with the GTV alone. The PTV margin necessary to encompass >95% of the fiducial locations was 2 mm versus 4 mm in both LR and AP and 4 mm versus 6 mm in SI for the ITV and the GTV, respectively.

Conclusions: The interbreath-hold variation is not insignificant, especially in the SI direction. Acquiring multiple breath-hold CT scans at simulation can help quantify the reproducibility of the interbreath hold and design a PSBH margin for treatment.

Figure 1

Sagittal view of interbreath-hold variations of patient 17 at simulation and 5 treatment fractions: A, simulation; B, fraction 1; C, fraction 2; D, fraction 3; E, fraction 4; and F, fraction 5. The red, green, and blue colors are fiducial locations overlaid from all breath holds for each fraction. The interbreath-hold variation is clearly shown with a varying amount for each fraction.

Figure 2

Interbreath-hold variation from 4 simulation computed tomography sets for individual patients. The square represents average variation; the error bar shows the range of the data (minimum to maximum).

Figure 3

Interbreath-hold variation measured from multiple breath-hold cone beam computed tomography sets at treatment. The diamond represents the average variation; the red line represents the median value; the box represents the range from the 25th to 75th quartile; and the whisker represents the range (minimum to maximum). Patients 1 and 8 showed reproducible interbreath-hold variation at both computed tomography simulation and treatment, whereas patient 11 showed increased variation at treatment compared with at computed tomography simulation.