ESTRO 2024 - Abstract Book

S1078

Clinical - Gynaecology

ESTRO 2024

Material/Methods:

Biological treatment evaluation was performed for 58 locally advanced cervical cancer patients treated with external beam radiotherapy (EBRT; 23-28 fractions of 1.8-2 Gy) plus 4-5 weekly hyperthermia sessions, followed by a 24 Gy PDR brachytherapy boost. Hyperthermia was applied using the 70 MHz AMC-4/ALBA-4D phased array system. Tumour temperatures were measured every 30 sec using three 14-sensor thermocouple probes (0.5 cm spacing) mounted on a silicone-elastomer mould applicator that fixates the thermometry probes on reproducible positions in the vaginal cavity. The time interval between an EBRT fraction and a hyperthermia session was defined as the time between the end of the radiotherapy fraction (beam off) and the start steady-state hyperthermia period. LQ-parameters α and β as a function of temperature and time interval were based on extensive experimental data. The measured median temperatures (T50) and registered time intervals per session were used to estimate the EQD RT in these patients using the extended LQ model. Additionally, we evaluated the possible benefit (in terms of EQD RT ) of optimizing logistics such that the time interval is at most 30 min for each session, as well as the consequence of long time intervals up to 4h in case of sub-optimal logistics, e.g. when patients receive radiotherapy and hyperthermia in different hospitals. Biological treatment evaluation of individual treatment sessions allows to determine patient-specific equivalent dose volume histograms and to evaluate dose enhancement in normal tissue. Figure 1 shows an example of registered average temperatures per thermometry probe and EQD RT for an individual treatment session of a patient receiving 28x1.8 Gy plus hyperthermia with a 54 min time interval. Depending on the local temperature, an increase from 1.8 Gy to 3.03-3.63 Gy in tumor dose was observed. A 54 min time interval is already quite effective, but some further improvement could be realized (3.23-3.89 Gy) when optimizing logistics to realize a 30 min time interval. For suboptimal logistics with a 4h time interval, the EQD RT would substantially reduce to 2.47 2.93 Gy, which is about half the enhancement realized with optimal logistics. Normal tissue enhancement was minimal, even for a 30 min time interval (<0.3 Gy). Figure 2 shows the translation of a tumour temperature volume histogram into an equivalent dose volume histogram, with an EQD RT 50 of 3.44 Gy, a fraction dose enhancement of 1.66 Gy. For time intervals of 30 min and 4h, this enhancement was 1.88 and 0.98 Gy, respectively. Results:

For the entire patient cohort, the median dose enhancement (D50) was calculated per session and summated over the treatment course. Characteristics and results were:

• Median average measured T50: 41.2°C (range 39.7-42.5°C). • Median average recorded time interval: 79 min (range 34-125 min). • Median total dose enhancement for the full treatment course (D50): o

Recorded time intervals: 5.5 Gy (interquartile range (IQR) 4.0-6.6 Gy); o 30 min time intervals: 7.1 Gy (IQR 5.5-8.1 Gy; p<0.001; ~+30%). o 4h time intervals: 3.2 Gy (IQR 2.3-3.8 Gy; p<0.001; ~ 40%)