ESTRO 2022 - Abstract Book

S1669

Abstract book

ESTRO 2022

Conclusion In SCRT for rectal cancer, smaller PTV margins result in reduced dose to OAR. This is observed at the objectives and across the dose volume histogram for this population.

PO-1883 Imaged-guided brachytherapy in cervical cancer: treatment planning before each fraction

B.M. Clara 1 , R.A. José Pascual 2 , H.M. Antonio 1 , A.D. Pablo 1 , R.C. Àngels 2

1 Hospital Clínic de Barcelona, Radiation Oncology, Barcelona, Spain; 2 Hospital Clínic de Barcelona, Radiation Oncology , Barcelona, Spain Purpose or Objective Cervical Cancer (CC) is one of the most frequent tumours in women worldwide. In CC organs at risk (OAR) can receive a significant brachytherapy dose due to their proximity to the cervical tumour depending on its position in relation to the cervix and the external radiotherapy dose distribution. To determine whether image-guided brachytherapy (IGBT) planning of each treatment session allows dosimetric benefits in the OARs (bladder, rectum, intestine and sigma) compared to planning only in the first fraction and considering the same D2cm 3 obtained in the following sessions. Materials and Methods The doses to OAR were retrospectively compared in 42 patients with CC treated from September 2017 to August 2021. The patients received 4 sessions with the same application using the Utrecht applicator +/- parametrial interstitial implant. CTV-HR EQD2 > 85 Gy were administered on 3 consecutive days (1st day 1 fraction, 2nd day 2 fractions separated by 6h and 3 rd day the last 1 fraction). In the 1st fraction, planning was performed by magnetic resonance (MR) and the following with computerized tomography (CT). The doses to OAR were compared considering two strategies (S1 and S2). In S1 we performed treatment planning on the first day and the D2cm 3 OAR dose values obtained were applied in the remaining fractions. In S2 treatment planning was performed in the first session based on MR findings, and in the following fractions treatment planning was performed using the dwell times of the 1st fraction and, if D2cm 3 of any OAR exceeded tolerance, the plan was recalculated, optimized or the dose decreased based on the dose received by OAR. Statistics: Shapiro's test, Student's t-test and Wilcoxon's test.

Results

Bladder Average

Rectum Average

value

daily

Mean

value

1

value

daily

Mean

value

1

EQD2 a/b = 3 Gy

calculation

calculation

calculation

calculation

75.11 -1.87

76.55

63.16 -0.32

63.36

Difference (%)

p-valor

Student,

0.04

0.68

Wilcoxon

Bowel

Sigmoid Average

Average

value

daily

Mean

value

1

value

daily

Mean

value

1

EQD2 a/b = 3 Gy

calculation

calculation

calculation

calculation

55.85 -0.97

56.4

62.63 -0.78

63.12

Difference (%)

p-valor

Student,

0.46

0.74

Wilcoxon

There were significant differences in D2cm 3 bladder between S1 and S2 (p = 0.042), with the mean dose in the bladder being 1.4 Gy higher in S1. No differences were found in the final dose to the D2cm 3 for rectum (p = 0.68), intestine (p = 0.46) and sigma (p = 0.74). In 27/42 patients (64%), variations in the filling of the OAR or movements relative to the applicator required treatment be re-planned in one, two or three of the remaining fractions to obtain doses below the tolerance values of each OAR. Conclusion In CC IGBT daily planning allows a mean dose reduction, specifically to the bladder in which significant dosimetric differences were observed between the two planning strategies, demonstrating that treatment planning in each fraction allows fulfilling the tolerance doses in OAR.