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Is There Really a Favorable Image Data Set Between Free-Breathing, Average Or Maximum Intensity Projections for Peripheral Lung SBRT?

D Mihailidis*, L Whaley, M Harmon, P Raja, L Farinash, P Tomara, Charleston Radiation Therapy Cons, Charleston, WV

SU-E-T-671 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: This work investigates the dosimetric impact of 4D-CT treatment planning for lung SBRT. We compare treatment planning and dosimetric characteristic between plans optimized on free-breathing (FB) scans then, computed on maximum and average intensity projections (MIP and AIP). Which data set is more favorable for planning and dose calculations between fully-optimized plans on MIP or AIP versus FB?

Methods: A small number of patients with peripheral lung lesions treated with SBRT in our center with an Oncor-160MLC system, were selected for this study. All patients received a FB helical scan to the entire chest and a 4D-CT scan to a smaller volume that includes the lesion. Internal target volume (ITV) was first delineated on the MIP and a 5mm margin to it, defined the PTV. Multi-field (7-11) inversely optimized planning was done first on the FB set, and then it was transferred to registered MIP and AIP for re-calculation (with heterogeneities), after having propagated and segmented onto MIP and AIP sets all organs-at-risk (OARs), respectively. Additonal plans were created with re-optimization of the beam fluences on AIP and MIP sets. All plans were compared based on dosimetric parameters, distributions and dose-volume histograms (DVHs).

Results: Dosimetric characteristics of FB with re-calculated AIP and MIP plans were similar overall with those of MIP being slightly higher of the three. Interestingly, the re-optimized plans give lower target and involved lung doses most likely due to different average lung CT-numbers between FB, MIP and AIP sets.

Conclusion: For peripheral lung lesions, planning for SBRT with intensity modulation is challenges as different imaging data sets have similar dosimetric characteristics. Re-optimization of the plans on MIP and AIP give a better indication as to which data set is appropriate to be utilized. A more robust protocol is needed for lung 4D-CT treatment planning.

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