ESTRO 2022 - Abstract Book

S1463

Abstract book

ESTRO 2022

were statistically compared between each pair of accelerators using Mann-Whitney non parametric test. The percentage of time saved using Halcyon rather than one of the other machines was also evaluated (comparing median values). Results Using IA software, treatment times of up to 356, 1212, 577 and 190 patients were retrospectively and systematically recorded for H&N, breast, prostate and rectum sites respectively during a period of three months. For all anatomical sites, a statistically significant time reduction (p<<0.001) was obtained for H compared to both TB and DHX linear accelerators. The percentages of saved time using H were 28% (TB) and 59% (DHX) for H&N, 41% (TB) 57% (DHX) for breast, 44% (TB) 60% (DHX) for prostate, 46% (TB) 41% (DHX) for rectum.

Conclusion Halcyon is able to drastically reduce the overall VMAT treatment time for all investigated anatomical sites. Despite of the relatively short observation period, InSightive Analytics software allowed for a systematic, large-population analysis which was characterized by an high level of statistical significance.

PO-1665 PTV margin analysis for radiotherapy tumor boosting in rectal cancer using BioXmark liquid fiducials

Y. Willems 1 , R. Canters 1 , L. Timans 1 , T. Opbroek 1 , F. Verhaegen 1 , E. Van Limbergen 1 , M. Berbée 1

1 Maastro, Radiation oncology, Maastricht, The Netherlands

Purpose or Objective Radiation dose escalation is expected to result in an increased clinical complete response rate in rectal cancer patients. Thus, dose escalation may enable more patients to qualify for an organ sparing approach by omission of surgery. To date, however, the results of dose escalation trials that have performed external beam radiotherapy (EBRT) boosts are inconclusive. These results may be explained by limited visibility of the boost target volume (GTVboost) on conventional pre-treatment imaging leading to large planning target volume (PTV) margins, which limits the extent of potential dose escalation. A promising solution for this problem is the usage of fiducial markers. A novel liquid fiducial marker, BioXmark®, has recently demonstrated good positional stability during treatment and good visibility on CT and conebeam CT (CBCT) imaging without marker related toxicity in rectal cancer patients (Opbroek, to be published). This study aimed to determine the required PTV margins for EBRT boosting in rectal cancer patients when using BioXmark® fiducials. Materials and Methods Nineteen patients treated with neoadjuvant chemoradiotherapy (10MV photons) for locally advanced rectal cancer were included in the analysis. Before treatment, two liquid markers were injected into the rectal wall cranially of the tumor, and two caudally. Alignment before each fraction was based on both bone and anatomical CBCT matching. An additional CBCT was performed after every fraction. For GTVboost, one center of mass of all eligible fiducial markers during treatment was calculated for every fraction. Since the needed PTV margins for an online fiducial match were evaluated, the interfraction displacement for GTVboost was set to 0. The PTV margin needed to correct for intrafraction displacement of the GTV was calculated using PTV margin = 2.5 * Σ + 0.7 * σ ' (Van Herk, IJROBP 2000). For the elective target volume (CTVelec), the clinical target volume (CTV) was isotropically expanded with 8-15mm in steps of 1mm. The PTV margin for CTVelec taking into account intra- and interfraction movement was based on the minimum added isotropic margin needed to cover 90% of CTVelec for all fractions.