ESTRO 2021 Abstract Book

S1278

ESTRO 2021

Conclusion

Overall, the reconstructed NTCP treatment

was on average around 0.5 % lower than pre-treatment calculations

indicated by ∆NTCP planning

. All studied patients would still qualify for proton treatment despite these

differences. Tracking of NTCP in thoracic proton indications could provide additional quantitative measures to aid in clinical decision-making regarding plan adaptation during the course of treatment. Further work on large patient cohorts is needed to validate these findings based on clinical outcome data.

PO-1553 PTV margins for postoperative pelvic nodal radiotherapy (PNRT) using a dose accumulation workflow M. Noy 1 , D. Soliman 2 , M. Karp 1 , M. Studensky 2 , M. Abramowitz 1 , N. Dogan 3 , A. Pollack 1 , E. Bossart 3 , A. Dal Pra 4 1 University of Miami , Radiation Oncology , Miami , USA; 2 University of Miami , Medical Physics, Miami , USA; 3 University of Miami , Medical Physics , Miami , USA; 4 University of Miami , Radiation Oncology, Miami , USA 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. Materials 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.

PO-1554 Is Dose Accumulation in online MR-guided treatment of Liver metastases Reproducible? I. Wahlstedt 1 , J.D. Tascon-Vidarte 2 , A.G. Smith 3 , S. Darkner 3 , S. Risum 4 , M. Pøhl 4 , K. Boye 4 , M. Josipovic 4 , C.