PTV margins for postoperative pelvic nodal radiotherapy (PNRT) using a dose accumulation workflow
PO-1553
Abstract
PTV margins for postoperative pelvic nodal radiotherapy (PNRT) using a dose accumulation workflow
Authors: Miguel Noy1, Daniel Soliman2, Michael Karp1, Matthew Studensky2, Matthew Abramowitz1, Nesrin Dogan3, Alan Pollack1, Elisabeth Bossart3, Alan Dal Pra4
1University of Miami , Radiation Oncology , Miami , USA; 2University of Miami , Medical Physics, Miami , USA; 3University of Miami , Medical Physics , Miami , USA; 4University of Miami , Radiation Oncology, Miami , USA
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Purpose or Objective
Elective PNRT can improve prostate cancer (PCa) outcomes and is commonly used in the postoperative setting. Although guidelines for PCa pelvic nodal contouring have been recently published, PTV recommendations for nodal volumes are limited. Herein, we sought to determine whether PTV margins for PNRT are necessary in the context of daily CBCT-guided postoperative radiotherapy.
Material and Methods
Six PCa patients treated with daily CBCT-guided intensity-modulated arc therapy with simultaneous integrated boost 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. Pelvic nodal volumes were based on RTOG guidelines except the superior margin that extended to the bifurcation of the iliac vessels. CTV52.7 included the vessels and about 7mm radial margin, “carving out” bowel, bladder, bone, and muscles. 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 coverage and OAR doses.
Results
Forty-two CBCTs were assessed. Though the CTV52.7 coverage slightly dropped compared to the original plan, in 100% of the fractions the CTV52.7 received a median dose >= 5% higher than planned. In 78.5% of the fractions, at least 95% of CTV52.7 was covered by 95% of the prescription while 100% of the prostate bed received the prescription dose. In 81% of the fractions at least 90% of the CTV52.7 received 100% of the planned prescription dose. OAR doses were on average approximately 10% higher than in the plan, but still acceptable.
Conclusion
In the context of daily CBCT-guided postoperative PNRT with strict bladder and rectal preparation protocols, our preliminary quantitative analysis demonstrated that pelvic nodal CTV coverage was acceptable suggesting that PTV margins could be possibly omitted.