3576 - Dosimetric Effects of Intrafraction Changes in High Dose Rate (HDR) Cervical Brachytherapy - Can We Avoid Replanning for Every Fraction in MR Guided Brachytherapy Era?

J. Venkatasai¹, V. R. Sim¹, R. Gonzalez-Vaz², E. L. Jones², B. Taylor¹, and I. White¹; ¹Guys Cancer Centre, Guys and St. Thomas NHS Foundation Trust, London, United Kingdom, ²Medical Physics, Guys Cancer Centre, Guys and St. Thomas NHS Foundation Trust, London, United Kingdom

Purpose/Objective(s): 1. Evaluate target and organs-at-risk (OAR) dosimetric changes with replan at each treatment fraction in MRI guided adaptive brachytherapy (MRIGABT) for cervix cancer. 2. Evaluate factors predicting the benefit in replanning. Materials/

Methods: Radiotherapy treatment planning MRI (RTPMRI) were acquired before each high dose rate (HDR) brachytherapy fraction, in 19 patients treated with intrauterine brachytherapy (IUB) using the Venezia^TM applicator with or without needles. Each patient had two IUB applicator insertions and received 28 Gy in 4 fractions following external beam radiotherapy (EBRT) 45 Gy in 25 fractions as EMBRACE II protocol. A replan based on anatomy on MRI acquired on day 2 (D2) of each IUB applicator insertion was planned and delivered (D2_Replan). Total target and OAR doses were compared to conventional treatment where the treatment plan on day 1 (D1) of IUB applicator insertion is also delivered on day 2 (D2_NoReplan). Total biologically equivalent doses for EBRT and brachytherapy were calculated in 2 Gy equivalents using the EQD2 equation and reported as total dose (Gy). Potential factors impacting replanning such as applicator size, use of needles, maximum tumour size pre-brachytherapy, day 1 bladder volume and rectal diameter at the level of target were assessed.
Results: Seventy-six RTPMRI and IUB HDR treatment plans (including 38 replans) were analysed in 19 patients. There was no significant change in the high-risk clinical target volume (HRCTV) across fractions Mean (±SD) D1, D2 are 25.95cc (±17.2),26.76cc (±18.5). The mean (±SD) total dose (EBRT + HRCTV 90) received for D2-replan and D2-no replan was 89.8 Gy (±4.22) and 87.4 Gy (±4.94). There was a clinically significant improvement in target and OAR dose with a replan, but this was not statistically significant as described in Table 1. No OAR dose constraints were exceeded with D2-replan. Replanning improved OAR doses in 21 (55%) out of 38 plans. Without a replan unacceptable doses to rectum, bladder, sigmoid and bowel would have been delivered in 10 (26%), 6 (15%), 8 (21%), 10 (26%) plans respectively. Patients in whom 4 or more needles were used and those with higher OAR dose which exceeds the per fraction constraint on D1 correlated with improvement of dose to OAR with replanning.
Conclusion: Replanning at each treatment in MRIGABT helps consistently achieve HRCTV D90 >87Gy and improved OAR doses in most fractions but is more resource intensive and needs pragmatic selection of cases.

Abstract 3576 – Table 1

	Planning aim (Gy)	D2 Replan Mean (SD)	D2 No replan Mean (SD)	p value
Total dose (EBRT + HRCTV90)	>87Gy	89.8 (±4.22)	87.4 Gy (±4.94)	0.08
	Constraint per fraction (Gy)	D2 Replan Mean (Range), Gy	D2 No replan Mean (Range), Gy
Rectum (D2cc)	<3.93Gy	2.7 (0.8-4.9)	2.6 (0.6-6.07)	0.32
Bladder (D2cc)	<5.45Gy	4.2 (1.3-5.4)	4.2 (1.3-6.75)	0.68
Sigmoid (D2cc)	<4.48Gy	3.31 (1.05-4.9)	3.2 (0.94-5.6)	0.22
Bowel (D2cc)	<4.48Gy	2.57 (0 - 4.3)	2.71 (0 - 5.53)	0.14