ESTRO 2024 - Abstract Book

S2454

Clinical - Urology

ESTRO 2024

Pre-treatment images were rigidly registered to the images from the time of recurrence and verified using the implanted fiducial markers. An overlap of the primary GTV in the pre-treatment scans with a high metabolic activity on PSMA-PET images and/or MRI conspicuity in images from the moment of recurrence was identified as a recurrence at the location of the primary GTV. MRI images were used for tumor identification in case PSMA-PET images were not conclusive, or not available. The near-minimum dose to the GTV (D98%) was obtained for each patient.

Results:

There were 28 patients with an intraprostatic recurrence; 21 in the standard arm (77 Gy in 35 fractions to the whole prostate gland) and 7 in the focal boost arm (standard treatment + isotoxic focal boost to the GTV of up to 95 Gy). The median failure time was 4.8 years (range: 1.7 – 7.4 years). The location of intraprostatic recurrence was analyzed for 24 patients whose imaging data was available. Twenty three patients (96%) had recurred at the location of the primary GTV. Examples of pre-treatment and recurrence images can be seen in Figure 1. For one patient, the recurrence appeared outside of the primary GTV (Fig.1e) at the location of the anterior fibromuscular stroma. At this location it is difficult to distinguish between tumor and healthy tissue on MRI as they both have a low intensity signal in both T2-weighted MRI and ADC scan. Upon review, this location was identified on pre-treatment MRI as potentially malignant by a uroradiologist. There was a discordance in GTV identification in the pre-treatment images for one of the patients. This patient recurred at two locations, at the location of the primary GTV and at a location which was described as potentially malignant on the pre-treatment imaging report. However, this location was not found suspicious upon review by a uroradiologist.

Figure 1 Examples of pre-treatment T2 weighted MRI and PSMA PET images from the moment of recurrence. GTV delineations in the pre-treatment images are in yellow and the location of high metabolic activity on PSMA-PET is highlighted in blue. Presented are four examples of recurrence at the location of primary GTV (a-d) and one example where patient recurred at a location outside of the primary GTV (e). The median D98% was 75.78 Gy (range:73.28 – 86.54 Gy). Only one patient who recurred locally received a substantial focal boost of 86.54 Gy. The remaining 27 patients, although some also assigned to the focal boost arm, all received D98% < 81 Gy, which reflects the concessions that were made to the boost dose due to the prioritization of the organs at risk constraints.

Conclusion: