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Session Item

Monday

November 30

14:15 - 15:30

Clinical Stream 1

Multidisciplinary treatment of hepatocellular cancer

Overview: Over the past decade we witnessed an enormous evolution in the treatment of hepatocellular cancer. In this session the different treatment options will be discussed. Depending on the stage and the location of the disease these patients can be treated with either stereotactic body radiotherapy (SBRT) with photons or heavy ions or transcatheter arterial chemoembolization (TACE) or selective internal radiation therapy (SIRT). For more advanced stages the different systemic treatment options will also be addressed.

Session Type: Symposium

Track: Clinical

Journey:

10:38 - 10:46

Barium-contrasted bone-cements: impact on dosimetry

Presentation Number: PH-0363

Abstract

Abstract Title:

Barium-contrasted bone-cements: impact on dosimetry

Authors: Crowe|, Scott(1,2)*[sb.crowe@gmail.com];Lathouras|, Marika(1);Lancaster|, Craig(1);Kairn|, Tanya(1,2);

(1)Royal Brisbane and Women's Hospital, Cancer Care Services, Herston, Australia;(2)Queensland University of Technology, Science and Engineering Faculty, Brisbane, Australia;

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Purpose or Objective

Polymethyl methacrylate (PMMA) based bone cements are a commonly used cranioplasty material and are specifically used when tumour involvement prevents the re-insertion of bone flaps. These cements are contrast enhanced (e.g. with barium), to provide a CT number similar to bone in CT images. Where density overrides are not employed, radiotherapy treatment planning systems may treat the bone cement as bone-equivalent, despite it having substantially different radiological properties for MV treatment beams. For treatments of volumes near the bone flap, this may result in disagreement between calculated and delivered dose.

Material and Methods

To assess the effective radiological properties of barium-contrasted PMMA bone cements, measurements were made with a physical sample slab of DePuy CMW 1 bone cement. Specifically, relative electron density (RED) was acquired using CT images with nominal energies of 120 kVp and approximately 3.5 MV, and transmission measurements for therapeutic beams with energies 100 kVp / 6.3 mm Al HVL and 6 MV. Monte Carlo dose calculations were performed, using the chemical composition of the cement and measured mass density. A 6 MV beam was simulated, incident on a simple slab geometry of 6 mm water, 6 mm flap (modelled alternatively as either water, bone cement, cortical bone, or tri-layered cortical and cancellous bone), and 12 cm water.

Results

The measured RED compared to water is shown in table 1. With increasing energy, the relative electron density approaches that of water (less than nominal PMMA density, due to air infiltration during fabrication of the material).

System	Energy	RED
WOmed T-300	100 kVp, 6.3 mm	2.5 ± 0.1
Siemens SOMATOM CT	120 kVp	1.66 ± 0.03
TomoTherapy MVCT	Approx. 3.5 MV	1.04 ± 0.03
Varian Clinac	6 MV	1.02 ± 0.08

Dose calculations assuming a water flap and a bone cement flap agreed with each other within uncertainties, outside the flap. For a 6 MV beam orthogonally incident on the slab geometry, dose differences of 7% were observed at the water-flap interfaces between simulations performed with bone cement and with cortical bone. Specifically, compared to actual bone, PMMA bone cement provides reduced back scatter (which would reduce scalp doses) and reduced attenuation (which would increase intracranial dose).

Conclusion

For treatments of targets near volumes of bone cement, significant dose errors of up to 7% may result where density overrides are not used in the treatment planning system. Since bone cement implants may be mistaken for bone flaps in CT images (and kV IGRT images), care must be taken when planning post-surgical radiotherapy treatments of tumour beds or the scalp, to identify whether bone cement has been used.