Frida Dohlmar1, Åsa Carlsson Tedgren1, Michael Sandborg1, Björn Morén2, Torbjörn Larsson2
1Linköping University, Medical Radiation Physics, Department of Health, Medicine and Caring Sciences, Linköping, Sweden; 2Linköping University, Department of Mathematics, Linköping, Sweden
The treatment planning of high dose-rate (HDR) prostate brachytherapy can be performed by manual or automated methods. Automated treatment planning is prone to yield uneven distribution of dwelling times, giving rise to regions with high dose, often resolved by manual fine-tuning. Here, an earlier developed, in-house adjustment tool is used to improve clinical treatment plans upon spatial properties. A paired observer study was performed to validate the performance of the adjustment tool against the original clinical approved treatment plans.
Dose distributions from 10 anonymized and previously treated patients were adjusted, using the in-house developed tool, and re-imported to the clinical treatment planning system (TPS). A paired blinded observer study was performed with two oncologists set to choose which of the two treatment plans, they would prefer, see Figure 1 for an example where A1 is the adjusted treatment plan and B1 the clinical. The oncologists were also asked to do graded comparisons of the treatment plans with regards to homogeneity, planning target volume (PTV) coverage and organ-at-risk (OAR)/normal tissue sparing on a five step Likert scale. The treatment plans were compared on their dosimetric indices, the dose that 90% of the PTV receives (D90%); the volume in the PTV that receive 100% (V100%) and 200% (V200%) of the prescription dose and the dose that the hottest 0.1 cm3 volume of urethra is receiving (D0.1cc). The homogeneity of the treatment plans was compared using the size of the four largest contiguous areas of dose higher than 200% of the prescription dose.
The adjusted treatment plan was preferred in 15 of 20 cases. The observer study showed that PTV coverage and OAR/normal tissue sparing was considered equal for the clinical treatment plan compared to the adjusted. For homogeneity, there was a tendency in the observer study that the adjusted treatment plan was considered better. The dosimetric indices showed small differences in PTV coverage (D90% and V100%) and urethra sparing (D0.1cc). The adjusted treatment plan had significantly smaller V200% than the clinical treatment plan 9,5 cm3 compared to 10,4 (p=0.002). The four largest contiguous areas of dose higher than 200% of the prescription dose was on average at least 69% smaller for the adjusted treatment plan compared to the clinical.
The in-house adjustment tool improves the clinical treatment plans upon homogeneity. The study will be expanded to include more observers and more prostate cases during 2021.