IGRT strategies for pelvic lymph node irradiation in high-risk prostate cancer: motion and margins

Publication date: Available online 8 September 2017
Source:International Journal of Radiation Oncology*Biology*Physics
Author(s): Lucy Kershaw, Laila van Zadelhoff, Wilma Heemsbergen, Floris Pos, Marcel van Herk
PurposeFor optimal irradiation of pelvic lymph nodes (LN) in high-risk prostate cancer, definition of margins to determine the planning target volume(s) is essential. Detailed quantification of the relative motion of the LN, seminal vesicles (SV) and prostate is currently lacking. This work aimed to quantify these motions and define margins for image-guided radiotherapy based on bony anatomy or prostate correction strategies for a 3 or 6 degrees-of-freedom couch.Material and MethodsNineteen patients had a planning CT followed by a mean of 11 repeated CTs during radiotherapy. The prostate, SV, external and internal iliac LN regions on the left and right were outlined on each CT. Systematic and random uncertainties were determined along with correlations between the motion of these regions. CTV-PTV margins required to take only motion into account were calculated for each guidance method.ResultsFor bone guidance, motion of prostate and LNs was largely uncorrelated. Margins to compensate for motion ((LR, SI, AP) in cm) based on a 3 degrees-of-freedom couch were; prostate: (0.2, 0.6, 0.8), SV: (0.4, 0.9, 1.0) and LN: (0.3, 0.4, 0.6). For prostate guidance, margins were calculated for correlated motion; prostate: (0, 0, 0), SV: (0.3, 0.5, 0.4) and LN: (0.3, 0.5, 0.9). For a 6 degrees-of-freedom couch, these margins were; prostate: (0.2, 0.6, 0.8), SV: (0.3, 0.9, 1.0) and LN: (0.3, 0.4, 0.3) for bone guidance. For prostate guidance, margins were; prostate: (0, 0, 0), SV: (0.2, 0.5, 0.4) and LN: (0.3, 0.6, 0.6)ConclusionsImage-guided radiotherapy based on bony anatomy requires larger prostate and SV margins, and guidance on prostate requires larger LN margins. Neither guidance strategy is optimal, and a combination of the two, or treatment adaption after a number of fractions might be preferable. Calculation of the total margin should also include delineation uncertainties.

Teaser

Definition of margins to determine the optimal image-guided radiotherapy (IGRT) strategy for pelvic lymph node irradiation is essential in high-risk prostate cancer. In this work, these margins were derived from systematic and random motions measured using repeated CT scans in 19 patients, based on matching to either bony anatomy or prostate for a 3 or 6 degrees-of-freedom (DOF) couch. When matching to bony anatomy, margins were smaller for lymph nodes than when matching to prostate, but larger for prostate and seminal vesicles. The prostate and seminal vesicle margins were unchanged when using a 3 vs 6 DOF couch, but lymph node margins were smaller in the anterior-posterior direction.


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