Fiducial marker motion relative to the tumor bed has a significant impact on PTV margins in partial breast irradiation

Abstract

Introduction: With the introduction of accelerated partial breast irradiation (APBI) and the trend of reducing the number of fractions, the geometric accuracy of treatment delivery becomes critical. APBI patient setup is often based on fiducials, as the seroma is frequently not visible on pretreatment imaging. We assessed the motion of fiducials relative to the tumor bed between planning CT and treatment, and calculated margins to compensate for this motion. Methods: A cohort of seventy patients treated with APBI on a Cyberknife was included. Planning and inroom pretreatment CT scans were registered on the tumor bed. Residual motion of the centers of mass of surgical clips and interstitial gold markers was calculated. We calculated the margins required per desired percentage of patients with 100% CTV coverage, and the systematic and random errors for fiducial motion. Results: For a single fraction treatment, a margin of 1.8 mm would ensure 100% CTV coverage in 90% of patients when using surgical clips for patient set-up. When using interstitial markers, the margin should be 2.2 mm. The systematic and random errors were 0.46 mm for surgical clip motion and 0.60 mm for interstitial marker motion. No clinical factors were found predictive for fiducial motion. Conclusions: Fiducial motion relative to the tumor bed between planning CT and APBI treatment is nonnegligible and should be included in the PTV margin calculation to prevent geographical miss. Systematic and random errors of fiducial motion were combined with other geometric uncertainties to calculate comprehensive PTV margins for different treatment techniques.

The Factors of Fiducial Marker Motion and Individual Margin Assessment in Postoperative Breast Cancer Radiotherapy

“…Furthermore, in this study, patients were immobilized using a Civco body board combined with the thermoplastic elastomer lm, which was different from the study of Harris. And large variations are shown for setup errors using different patient positioning devices [10] . We found that breast size and ducial position can predict ducial motion to a certain extent, and the corresponding PTV margins also have a statistically signi cant difference, but the individualized PTV margin still needs to be validated with a larger sample size.…”

“…In this study, patients all received radiotherapy 5 months after surgery. Since the length of the time interval between simulation and treatment can affect the ducial motion [10] , in our study, the treatment all happened within two weeks after simulation.…”

“…Zhang Y and others have demonstrated that the position of the ducial mass center relative to the center of the cavity was stable during treatment, and the ducial markers are a suitable surrogate for the lumpectomy cavity [9] . While Hoekstra N [10] thought that the motion of ducials relative to the tumor bed occurring between planning CT and treatment was clinically signi cant and should be included in the PTV margin calculation. But so far, no clinical factors such as tumor bed size, the interval between planning CT and the rst fraction were found signi cantly associated with ducial motion.…”

Comparison of visibility of iodinated hydrogel and gadolinium-modified hyaluronic acid spacer gels on computed tomography and onboard imaging

“…Using the HA-gel mixtures may provide the radiation oncologist with more information on the tumour bed location and extent, resulting in less variability in delineation. Movement of surgical clips between surgery and treatment is a possibility, requiring increased planning target volumes [27] .…”

Five Fraction External Beam Partial Breast Irradiation

“…In recent studies evaluating intrafraction lumpectomy cavity motion, cavity displacement was <0.5 cm, and even less with respiratory motion management, demonstrating that a 0.5 cm PTV margin or even less is reasonable with respiratory motion management. 35,36 The PTV should be cropped 3 to 5 mm inside skin to create a PTV_eval for dose volume histogram analysis. Overall, the CTV plus PTV expansion result in a 1.5 to 2.0 cm target expansion on the tumor bed and clips.…”