ESTRO 2024 - Abstract Book

S5533

RTT - Patient care, preparation, immobilisation and IGRT verification protocols

ESTRO 2024

For radiation treatment of patients with cervical carcinoma a reproducible bladder filling is necessary. Less variation in bladder contents results in fewer changes in the position of the uterus and Organs at Risk (OARs). Using treatment plans based on a fixed bladder volume with small margins may cause much inconvenience in the clinical workflow, as inadequate target coverage may easily result from inadequate bladder volume. It may be necessary to have the patient drink extra and wait sometime, then reposition, sometimes repeatedly, until CBCT target coverage is adequate. This will result in a disrupted clinical workflow and inconvenience for the patient and RTT’s and possibly inefficient use of scarce resources. Enlarging PTV margins to more that 2 cm has been suggested in the past but results in undesirable large PTV volumes en high OAR doses. In practice, a predictable bladder filling was not achievable with the implementation of a standardized drinking protocol. Therefore, in our institution, bladder volume is assessed using an ultrasound bladder scan (UBS). The measured volume should match the bladder volume on the planning CT. When the UBS was first implemented, the radiation therapists (RTT’s) assessed adequacy of the bladder filling on an intuitive basis, without a clear decision rule. After assessing the bladder by UBS, a CBCT is performed every fraction to ensure that the target is adequately covered. From this practice, we derived decision rules guiding de RTT ‘s about the minimum bladder volume they can accept, before the patient is positioned on the treatment couch. In the clinical treatment workflow of cervical cancer patients, a planning-PET CT is used for treatment planning. The procedure starts with a blanco CT with an empty bladder, followed by PET acquisition and a full bladder scan with contrast in the same session. This procedure results in a planning CT with a comfortably filled bladder, which serves as the reference bladder volume for each treatment fraction. If the bladder volume on UBS is clearly divergent from the reference volume, patients will be asked to drink extra and wait until the bladder has filled more. This procedure is repeated until the bladder volume is acceptable, judged by adequate target coverage on CBCT. UBS volumes assessed before each fraction, as well as the acceptability of the following CBCT were recorded for 285 fractions in 12 patients. These data were analyzed to find a threshold bladder volume that could serve as a reasonable predictor for adequate target coverage on the CBCT. Material/Methods:

Results:

We collected data of 285 fractions from 12 patients. Of all fractions 64 % could proceed with treatment after one UBS. The remaining fractions needed one additional UBS in 29 % en 2 or more assessments in 7 % to achieve acceptable target coverage on de CBCT. By analyzing the UBS volumes as a percentage of the reference volume, and correlating this proportion with the occurrence of an acceptable target coverage on CBCT, we found that in patients with a bladder volume of 70 % or less only 50 % of CBCT's were acceptable, while in patients with a bladder volume of > 70 % of the reference volume, the occurrence of an unacceptable CBCT was reduced to 23%. By using this cutoff value the proportion of patients needing 2 or more UBS was reduced from 14 % in patients with < 70 % bladder volume versus 4 % with > 70 % bladder volume.

Conclusion:

The implementation of a UBS helps to smoothen the patient workflow in cervix radiotherapy, as a very divergent bladder filling will be signaled before the patient is positioned on the treatment couch. Based on our initial experiences with UBS, we defined a bladderfilling of at least 70 % of the reference volume as sufficient to proceed