Modeling of the metallic port in breast tissue expanders for photon radiotherapy.

The purpose of this study was to model the metallic port in breast tissue expanders and to improve the accuracy of dose calculations in a commercial photon treatment planning system (TPS). The density of the model was determined by comparing TPS calculations and ion chamber (IC) measurements. The model was further validated and compared with two widely used clinical models by using a simplified anthropomorphic phantom and thermoluminescent dosimeters (TLD) measurements. Dose perturbations and target coverage for a single postmastectomy radiotherapy (PMRT) patient were also evaluated. The dimensions of the metallic port model were determined to be 1.75 cm in diameter and 5 mm in thickness. The density of the port was adjusted to be 7.5 g/cm3 which minimized the differences between IC measurements and TPS calculations. Using the simplified anthropomorphic phantom, we found the TPS calculated point doses based on the new model were in agreement with TLD measurements within 5.0% and were more accurate than doses calculated based on the clinical models. Based on the photon treatment plans for a real patient, we found that the metallic port has a negligible dosimetric impact on chest wall, while the port introduced significant dose shadow in skin area. The current clinical port models either overestimate or underestimate the attenuation from the metallic port, and the dose perturbation depends on the plan and the model in a complex way. TPS calculations based on our model of the metallic port showed good agreement with measurements for all cases. This new model could improve the accuracy of dose calculations for PMRT patients who have temporary tissue expanders implanted during radiotherapy and could potentially reduce the risk of complications after the treatment.

Yoon J ，Xie Y ，Heins D ，Zhang R ... -  《Journal of Applied Clinical Medical Physics》

Dosimetry around metallic ports in tissue expanders in patients receiving postmastectomy radiation therapy: an ex vivo evaluation.

Moni J ，Graves-Ditman M ，Cederna P ，Griffith K ，Krueger EA ，Fraass BA ，Pierce LJ ... -  《-》

In vivo dosimetric impact of breast tissue expanders on post-mastectomy radiotherapy.

Temporary tissue expanders with metallic ports for gradual saline injection are increasingly employed to facilitate breast reconstruction after post-mastectomy radiotherapy (PMRT). Treatment beams therefore pass through a high-density rare-earth magnet. Measurements ex vivo suggest attenuation of dose to the skin and chest wall at clinical risk of relapse. The purpose of the study was to quantify the resulting dose reduction in vivo, compared with treatment planning system (TPS). Sixteen patients receiving PMRT had in vivo dosimetry prospectively performed with ethics board approval. Port was located within the expanded chest wall using the planning CT scan. Strips of radiochromic film were laid on the skin surface underneath the bolus. To aid interpretation, ex vivo measurements were also performed, including comparison with TPS predictions. An average 7% reduction in dose to skin surface was measured in 15 of 16 patients. This was reproducibly located in the 'shadow' of the magnet, corresponding to each of the paths of the medial and lateral tangents. The average area was 1.07 cm(2) (range 0.39 cm(2) to 2.36 cm(2)). Ex vivo measurements confirmed attenuation of the beam in the shadow of the port. The surface area of the 'cold-spot' varied with angle of the beam relative to the metallic port. Dose attenuation in vivo differed from that predicted by the TPS. Dose is attenuated in the 'shadow' of the tissue expander port in patients receiving PMRT. This is likely to be clinically insignificant for most, but centres should undertake appropriate measurements before utilising TPS predictions.

Gee HE ，Bignell F ，Odgers D ，Gill S ，Martin D ，Toohey J ，Carroll S ... -  《-》

Technical Note: Clinical modeling and validation of breast tissue expander metallic ports in a commercial treatment planning system for proton therapy.

To validate breast tissue expander metallic port (MP) models in a commercial treatment planning system (TPS) in proton pencil beam scanning (PBS) treatments for breast cancer patients with breast tissue expanders. Three types of MPs taken out of a Mentor CPX4, a Natrelle 133, and a PMT Integra breast tissue expanders and a 650 cc saline filled Mentor CPX4 expander were placed on top of acrylic slabs, and scanned using a Siemens Somatom Definition AS Open RT CT scanner. Structure templates for each of the MPs were designed within Eclipse TPS. The CT numbers for the metallic parts were overridden to reflect measured or calculated relative proton stopping powers (RPSPs). Mock targets were contoured in acrylic to represent postmastectomy chest-wall radiation therapy (PMRT) targets. Plans with different beam incident angles were optimized using the Eclipse TPS to deliver uniform prescription dose to the target using Hitachi Probeat-V PBS beams. Eclipse calculated doses and an in-house Monte Carlo (MC) code calculated doses were compared to the measured Gafchromic EBT3 film doses in acrylic. TPS/MC and film dose comparison results showed that (1) 3%/2 mm/10% threshold Gamma pass rates were better than 90.8% in the acrylic target region for all plans; (2) comparing TPS and film doses for the individual beam plans in the MP dose shadow areas, the area with dose difference above 5% ([ΔA] 5%) ranged from 1.1 to 5.0 cm2 , and the maximum dose difference ([ΔD] 0.01 cm2 ) ranged from 12.5% to 25.0%; (3) comparing MC and film doses for the individual beam plans in the MP dose shadow areas, the (ΔA) 5% varied from 1.1 to 2.9 cm2 and (ΔD) 0.01 cm2 varied from 8.5% to 24.2%; (4) for a plan composed of three individual beams treating through the Mentor CPX4 expander, the TPS (ΔA) 5% was less than 0.13 cm2 , and the (ΔD) 0.01 cm2 was less than 6% in the MP dose shadow areas. It is feasible to treat patients with tissue expanders using multiple PBS beams using a structure template with CT number overridden to represent the measured/calculated RPSP for MPs for PBS treatment planning. MC dose was more accurate than analytical dose in the areas with high dose gradient caused by the density heterogeneity of the breast tissue expander MPs.

Kang Y ，Shen J ，Bues M ，Hu Y ，Liu W ，Ding X ... -  《-》

Effects of the Metallic Port in Tissue Expanders on Dose Distribution in Postmastectomy Radiotherapy: A Tridimensional Experimental Model of Dosimetry in Breast Reconstruction.

da Silva MF ，de Oliveira HF ，Borges LF ，Carrara HHA ，Farina JA Jr ... -  《-》