Session Item

Online MR-adaptive RT enables daily replanning to adapt the changes seen in cervical RT, such as uterus-cervix position and rectum and bladder filling variability affording more accurate targeting of disease and reducing toxicity. This workflow requires considerable effort from the interdisciplinary team. The frequency of required adaptations on a daily basis has been reviewed.

Four women with stage FIGO stage IIB node-negative cervical cancer were included who received 45Gy/25 fractions. Treatment was according to the EMBRACE II guidelines planning on Monaco (v5.40.01) (Elekta, Stockholm) and using an adapt-to-shape (ATS) “MR-adapted” protocol. We compared the cumulative daily re-plans MR-adapt plan versus MR-guided plan. Doses to target structures (D99% to CTV_T_LR and CTV_E), and organs at risk (D0.1% to the bladder, rectum, bowel and sigmoid) were extracted for every fraction. MR-adapted plans were retrospectively analysed. MR-guided plans were created by transferring the dose from the reference plan to the daily MR image and then recalculating without adapting to the shape of the day. We then compared the dose for MR-adapted and MR-guided plans for each structure on a daily basis using paired two-tailed t-test; p<0 .05 was considered significant. For MR-guided plan, any structure exceeding the dose criteria >2% was considered an adaptation.

92 MR-adapted and 92 MR-guided plans were reviewed. Available fractions (fx) were (22, 24, 22, 24) for patients (Pt) 1 to 4 respectively. The remaining fx were delivered on a conventional Linac. Dose comparison and statistical results are shown in Table1. All MR-adapted plans had less than 2% variation in all targets and OAR. Differences in CTV_T_LR dose coverage were significant for 3/4 Pts (figure1). Number of fx exceeding the <2% dose variance with MR-guided plans: CTV_T_LR (Pt1= 8, Pt2 =4, Pt4 = 1). There was significant underdosing to the CTV-E with MR-guided plan for all Pts. Number of fx exceeding the <2% dose variance CTV_E MR-guided plans: (Pt1=13, Pt2 =15, Pt3=13, Pt4= 11). MR-adapted plan reduction D0.1% to bowel (2Pt), sigmoid (3Pt) and bladder (2Pt). Number of fx exceeding the <2% dose variance with MR-guided plans: bladder (Pt2=2, Pt4 =1), bowel (Pt2=5, Pt4 =3), sigmoid (Pt1= 2, Pt3 =3, Pt4=2). Overall, the number of fx which required adaptation was: {Pt1= 16\22 (72%)}, {Pt2=20\22 (83%)}, {Pt3=13\22 (59%)}, {Pt4=11\24(46%)}.

MR-adapted improved the CTV_T_LR and CTV_E D99% coverage in 72%, 83%, 59% and 46% of total fraction for Pt 1 to 4 when compared to MR-guided. For OAR, patients benefited from a small decreased dose to the bladder, sigmoid and bowel. Number of fx exceeding the <2% dose variance with MR-guided plans was > 40% in all patients, suggesting that frequent adaptation is beneficial. This warrants extra investigation in a larger population to determine the optimal frequency of the adaptation.