Copenhagen, Denmark

ESTRO 2022

Session Item

Implementation of new technology and techniques
Poster (digital)
PTV margin analysis for radiotherapy tumor boosting in rectal cancer using BioXmark liquid fiducials
Yves Willems, The Netherlands


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

Yves Willems1, Richard Canters1, Lisanne Timans1, Thirza Opbroek1, Frank Verhaegen1, Evert Van Limbergen1, Maaike Berbée1

1Maastro, Radiation oncology, Maastricht, The Netherlands

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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.

Material 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.

For GTVboost, PTV margins to ensure a minimum dose to the CTV of 95% for 90% of patients were 0.3cm, 0.8cm, and 0.3cm for the lateral, craniocaudal, and anteroposterior directions, respectively. The PTV margin to cover 90% of all fractions was 1.2cm for CTVelec.

The calculated PTV margins are less than the margins that are generally used in CBCT based EBRT boost treatments for rectal cancer patients. Therefore, implantation of BioXmark® fiducials and CBCT marker matching using these fiducials may allow for significantly increased dose escalation to target volumes and reduced dose to normal tissue compared to CBCT based boosting without fiducial implantation and could be an alternative to MRI-linac based boosting.