J Niedzielski*, A Koong, A Sobremonte, P Castillo, B Lee, J Yang, UT MD Anderson Cancer Center, Houston, TX

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  J Niedzielski

PO-GePV-M-175 (Sunday, 7/25/2021)   [Eastern Time (GMT-4)]

Purpose: Elekta Unity MR-Linac provides two online plan adaptation approaches: daily plan re-optimization with isocenter adaptation (i.e, adapt-to-position, ATP) and daily re-planning considering anatomical changes (adapt-to-shape, ATS). We aimed to analyze the dosimetric differences between these two approaches for a patient treated with ATS in order to shed a light for optimizing online plan adaptation strategy.

Methods: A 77 year-old female, whom was diagnosed with recurrent adrenocortical carcinoma, was treated with hypofractionated radiotherapy for a metastatic pelvic sidewall tumor that was located proximal to the large bowel. The patient was treated with the Elekta Unity system, with a prescription of 60 Gy delivered in 10 fractions. The ATS workflow was applied to each fraction for online plan adaptation, including re-contouring of the GTV and normal tissues (large bowel, rectum, bladder, and small bowel). Retrospectively, the ATP workflow was applied by re-planning based on the isocenter shift on each daily MR scan. Accumulated dose for each of the two plan adaptation methods was calculated by summation of the absolute DVH of each structure.

Results: The tumor volume did not show significant changes during treatment, but the proximity of large bowel to tumor varied from day to day. The clinical goal of GTV D90%>60Gy was achieved using both methods (60.5Gy vs. 60.1Gy, ATP vs. ATS). However, there were significantly higher doses using the ATP method for large bowel, bladder, rectum, and small bowel (p=0.0001, 0.0001, 0.02, 0.0002, respectively), with the large bowel violated the Dmax<40Gy constraint using the ATP approach (46.1Gy vs. 39.4Gy, ATP vs. ATS). Additionally, the maximum doses of large bowel had a larger variance on the ATP workflow, as compared to the ATS (stdev=3.6Gy vs. 0.9Gy).

Conclusion: ATS approach may offer higher precision than ATP approach in treating tumors proximal to radiosensitive normal tissues, without sacrificing target coverage.

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