An Analysis of PTV Margins for Postoperative Pelvic Nodal Radiotherapy (PNRT) Using a Dose Accumulation Workflow

Abstract

<h3>Purpose/Objective(s)</h3> PTV margins are created to ensure coverage of the target volume. There is no uniform consensus on the PTV margins for prostate bed radiotherapy. Our institution's PTV margins for prostate bed are 8mm everywhere but posterior - a size that leads to PTVs on the order of 2.5 times the prostate bed. We hypothesized that the current PTV margins for prostate bed is larger than necessary in the context of daily cone-beam CT (CBCT)-guided intensity-modulated arc therapy. <h3>Materials/Methods</h3> Nineteen PCa patients treated with daily CBCT-guided intensity-modulated arc therapy were studied. Patients received 68 Gy to the prostate bed (PTV68) and 52.7 Gy to the elective pelvic nodes (CTV52.7, no PTV margins), all in 34 fractions. Patients followed strict bladder and rectal preparation protocols and had daily pretreatment CBCT images. Patient data, consisting of planning CTs and 7 representative CBCTs (1/week), were anonymized. Using a dose accumulation workflow, synthetic CTs (sCT) were created from the planning CTs and the representative CBCTs. Volumes were deformed to the sCT and checked for consistency. The sCTs were sent to the planning system, and a single fraction of the treatment plan was recalculated on the sCT. These daily dose distributions were used to assess target coverages and OAR doses on a daily/weekly basis. <h3>Results</h3> One hundred and thirty-three CBCTs were assessed. In 100% of the fractions, at least 97% of the prostate bed was covered by the prescription dose. Only 2 patients had a max, mean, and median dose to the prostate bed less than planned (though the mean, max and median doses were all greater than 2Gy/fx) at approximately 1 cGy below a value planned per fraction; the remaining 17 patients were at or above the aforementioned planned doses. Though the CTV52.7 coverage dropped slightly compared to the original plan, in 100% of the fractions the CTV52.7 received a median dose equal to or higher than planned. In 74% of the fractions, at least 95% of CTV52.7 was covered by 95% of the prescription dose. In 100% of the fractions at least 90% of the CTV52.7 received 90% of the planned prescription dose. On average there was a 2% difference in the volumes of OARs at the V40, V65 and maximum dose points. <h3>Conclusion</h3> Our results suggest that, in the context of daily CBCT-guided postoperative RT with strict bladder and rectal preparation protocols, a PTV margin of 8 mm everywhere and 6 mm posteriorly is sufficient if not slightly larger than needed. Further studies on margin reduction and its clinical implications are warranted.