PTV margin analysis for radiotherapy tumor boosting in rectal cancer using BioXmark liquid fiducials
Yves Willems,
The Netherlands
PO-1665
Abstract
PTV margin analysis for radiotherapy tumor boosting in rectal cancer using BioXmark liquid fiducials
Authors: Yves Willems1, Richard Canters1, Lisanne Timans1, Thirza Opbroek1, Frank Verhaegen1, Evert Van Limbergen1, Maaike Berbée1
1Maastro, Radiation oncology, Maastricht, The Netherlands
Show Affiliations
Hide Affiliations
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.
Results
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.
Conclusion
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.