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Automated Multicriterial Plan Generation for Prostate Cancer Patients with Metal Hip Prostheses: Comparison of Planning Strategies

M Dirkx

M Dirkx*, P Voet, S Breedveld, B Heijmen, Erasmus MC-Daniel den Hoed Rotterdam

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

Purpose: To compare different IMRT planning strategies for patients with metal hip prostheses and to determine the impact of increasing the number of beam directions.

Methods: All plans were generated fully automated (i.e., no human trial-and-error interaction) using iCycle, our in-house developed algorithm for multicriterial optimization of beam angles and fluence profiles, allowing objective comparisons of planning strategies. For 18 prostate cancer patients (eight with bilateral hip prostheses, ten with a right-sided unilateral prosthesis), two planning strategies were evaluated: i) full exclusion of beams containing beamlets that would deliver dose to the target after passing through a prosthesis (IMRTremove, as recommended by AAPM taskgroup 63), and ii) exclusion of those beamlets plus a 5 mm margin only (IMRTcut). Plans with optimized coplanar and non-coplanar beam arrangements were generated. Differences in PTV coverage and sparing of organs at risk (OARs) were quantified. The impact of beam number on plan quality was evaluated.

Results: Especially for patients with bilateral hip prostheses, IMRTcut significantly improved rectum and bladder sparing compared to IMRTremove, while adequate PTV coverage was maintained. For 9-beam coplanar plans, rectum V60Gy reduced by 17.5% ± 15.0% (maximum 37.4%, p=0.036) and rectum Dmean by 9.4% ± 7.8% (maximum 19.8%, p=0.036). Further improvements in OAR sparing were achievable by using non-coplanar beam set-ups, reducing rectum V60Gy by another 4.6% ± 4.9% (p=0.012) for non-coplanar 9-beam IMRTcut plans. Larger reductions in rectum V60Gy, by on average 13.5% ± 7.9% (p=0.012), were observed when comparing 7- and 15-beam coplanar plans.

Conclusion: Excluding only beamlets that pass through a prosthesis prior to delivering dose to the PTV (IMRTcut strategy) significantly improved OAR sparing as opposed to full exclusion of beams containing such beamlets. Application of non-coplanar beam arrangements and, to a larger extent, an increased number of beams further improved plan quality.

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