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Quality Study of Beam Angle Optimization Algorithm In IMRT Plan for Cervix Cancer Radiotherapy

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T Pang

T Pang*, B Yang, J Qiu, F Zhang, Peking Union Medical College Hospital, Beijing, Beijing

SU-E-T-643 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose:
To evaluate the benefit of Beam angle optimization (BAO) algorithm for organ sparing in IMRT plans for cervical cancer

Methods:
Twelve cervical cancer patients were planned using Eclipse planning system. The structures of target volume and organs-at-risk (OAR) were delineated by the attending physician. Institutional standard of 9 equally spaced beam IMRT plan, and a 9 field IMRT plan with beam orientation optimized by BAO algorithm were performed for each patient .The BAO seeks patient-specific beam angle optimization based on PTV-OAR geometry and given dose sparing constraints. Therefore, it is expected that BAO-plans would offer additional organ sparing benefit by optimization of the beam angles. All plans are normalized to prescription dose covering 95% of PTV volume.

Results:
There is no obvious quality improvement by using beam angle optimization. Specifically, minimum target coverage that measures the PTV volume covered by 95% of prescription dose (V95%) is lower in BAO plans. As a result, the Conformity Index and Homogeneity Index are worse in BAO-plans. For OAR sparing, there was no significant difference in bladder V30 and D40. No significant difference was observed for V30, V40 in rectum. The small bowel DVH showed that V40 and Dmax increased by 0.6% and 1.4 Gy respectively. The V10,V30, Dmean of bone marrow increased by 1.2%,5.6%,1.3Gy respectively ,the cauda equina D0.1cc was increased by 2 Gy. At last, there was no significant difference in femoral heads D5%.

Conclusion:
These is no significant benefit of the BAO algorithm for either PTV coverage or OAR sparing for cervical cancer IMRT plans. For sites like cervical cancer, the target volumes have very complicated relationships with the OARs, the BAO is not capable of analyzing such complicated PTV-OAR relationship, hence, equally spaced 9 beam configuration maybe more robust in achieving balanced clinical goals.

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