E Subashi*, E LoCastro, V Brennan, A Apte, M Zelefsky, N Tyagi, Memorial Sloan-Kettering Cancer Center, New York, NY

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  E Subashi

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MO-B-TRACK 6-5 (Monday, 7/26/2021) 11:30 AM - 12:30 PM [Eastern Time (GMT-4)]

Purpose: Treatment of the prostate with an integrated boost to the dominant intra-prostatic lesion (DIL) using ultra-hypofractionated radiotherapy has been implemented in a high-field MR-Linac. We study the feasibility of using quantitative relaxometry for response assessment during the course of this treatment regimen.

Methods: Quantitative maps of the longitudinal (T1) and transverse (T2) relaxation times were acquired on the Elekta Unity MR-Linac using the variable flip-angle (VFA) and multi-echo spin-echo (MESE) method. Imaging parameters for the VFA method were: TR/TE=20/2.3ms, FA={4°,22°}, FOV=320x320x200mm, reconstructed voxel=0.7x0.7x3mm, acquisition time=1.5mins. For MESE: TR/TE/ΔTE=4000/22/11ms, 8 echoes, FOV=320x320x100mm, reconstructed voxel=0.5x0.5x3mm, acquisition time=5.5mins. The bias and uncertainty in the quantitative maps were first determined using a NIST-traceable phantom with known relaxation values. T1/T2 was measured at each fraction in eight patients undergoing ultra-hypofractionated therapy to the prostate with an integrated boost to the DIL. The prescription was 800cGy/900cGy x 5 to the prostate/DIL, respectively. The VFA, MESE, online-planning images, structure set, and dose were imported in CERR (Computational Environment for Radiotherapy Research) where the images from each relaxometry acquisition were rigidly registered to the online-planning dataset. T1 and T2 values were analyzed within the GTV and CTV as a function of fraction number.

Results: In the NIST phantom, the measured bias and coefficient-of-variation for T1=[100ms–2600ms] was [-1.1%–12.3%] and [2.8%–9.6%] respectively. For T2=[30ms–1200ms], the measured bias and coefficient-of-variation was [-13.4%–5.1%] and [3.1%–16.5%] respectively. Across all patients and fractions, T1(CTV)=1775±102ms, T1(DIL)=1525±142ms, T2(CTV)=124±19ms, T2(DIL)=104±11ms. T1 and T2 were significantly longer in the CTV. Linear regression analysis showed that in the DIL there is a significant increase in T1 and a significant decrease in T2. This trend was also observed in the CTV but was not significant.

Conclusion: Quantitative relaxometry may provide useful and complementary guidance as an imaging biomarker for therapy response assessment.

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Keywords

MR, Quantitative Imaging, Image-guided Therapy

Taxonomy

Not Applicable / None Entered.

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