Impact of Prostate Focused Alignment on Planned Pelvic Lymph Node Dose in an RTOG Framework
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Purpose: Prostate patients with positive lymph node margins receive an initial course of 45Gy to the PTV comprised of prostate, seminal vesicles, and lymph nodes with a 1cm margin. The prostate is localized via implanted fiducial markers before each fraction is delivered using portal-imaging. However, the pelvic lymph nodes are affixed to the bony anatomy and are not mobile in concert with the prostate. The aim of this study is to determine whether a significant difference in pelvic lymph node coverage exists between planned and delivered external beam therapy treatments for these patients. Methods: The recorded prostate motions were gathered for twenty patients; conjointly the pelvic lymph node motions were determined by manual registration of the bony anatomy in the Kv-images. The difference between the prostate and the bony anatomy coordinates were input into Eclipse as field shifts to represent the deviation in planned vs delivered pelvic lymph node coverage. Results: Structure volume at D(100) was recorded for each patient for two structures: summed pelvic lymph nodes and pelvic lymph nodes +1cm margin to express their contribution to the PTV. For the pelvic lymph nodes +1cm, the average difference between the planned coverage and calculated delivered coverage was 4%, with a paired p-value of p=0.0006. Based upon bony anatomy registration, only 25% of patients received the intended dose coverage based upon D(100) dose criteria for pelvic lymph node +1cm. Dose value differences between the two plans at minimum were 2.48±3.29Gy, at mean were 0.51±0.47Gy, and at maximum were 0.26±0.33Gy. For the pelvic lymph nodes, the average difference between the planned coverage and calculated delivered coverage was 0.8%, with a paired p-value of p=0.58. Conclusions: The results indicate a significant difference exists between the planned coverage and calculated delivered coverage for the pelvic lymph nodes +1cm, with no significant difference for pelvic lymph nodes. We conclude that lymph node motion must be considered with the prostate motion when aligning patients before each fraction.