Influence of the Presence of Tissue Expanders on Energy Deposition for Post-Mastectomy Radiotherapy

Abstract

An increasing number of studies have shown that post-mastectomy radiotherapy presents benefits associated with the patients survival and a significant fraction of the treated patients makes use of tissue expanders for breast reconstruction. Some models of tissue expanders have a magnetic disk on their surface that constitutes heterogeneity in the radiation field, which can affect the dose distribution during the radiotherapy treatment. In this study, the influence of a metallic heterogeneity positioned in a breast tissue expander was evaluated by means of Monte Carlo simulations using the MCNPX code and using Eclipse treatment planning system. Deposited energy values were calculated in structures which have clinical importance for the treatment. Additionally, the effect in the absorbed energy due to backscattering and attenuation of the incident beam caused by the heterogeneity, as well as due to the expansion of the prosthesis, was evaluated in target structures for a 6 MV photon beam by simulations. The dose distributions for a breast treatment were calculated using a convolution/superposition algorithm from the Eclipse treatment planning system. When compared with the smallest breast expander volume, underdosage of 7% was found for the largest volume of breast implant, in the case of frontal irradiation of the chest wall, by Monte Carlo simulations. No significant changes were found in dose distributions for the presence of the heterogeneity during the treatment planning of irradiation with an opposed pair of beams. Even considering the limitation of the treatment planning system, the results obtained with its use confirm those ones found by Monte Carlo simulations for a tangent beam irradiation. The presence of a heterogeneity didńt alters the dose distributions on treatment structures. The underdosage of 7% observed with Monte Carlo simulations were found for irradiation at 0°, not used frequently in a clinical routine.

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

“…One of the limitations of this study is that we only investigated one type of the tissue expander (McGhan Style 133, ALLERGAN, Santa Barbara, CA, USA), while other brands of tissue expanders are also used for breast reconstruction,20 and only modeled the metallic port in one TPS (Pinnacle, Philips Healthcare, Fitchburg, WI, USA). However, it seems that the tissue expander used in this study was the most popular type used in the US and other countries according to literature14, 15, 16, 17, 24, 25, 31 and Pinnacle is one of the most widely used TPS worldwide, which means our model is applicable to most clinics.…”

“…Thompson and Morgan15 manually assigned a bulk density of 7.9 g/cm 3 to the implant in TMS TPS (Nucletron, Veenendaal, the Netherlands) and concluded this kind of modeling was ineffective; Chen et al18 used a series of phantom and film measurements to find an electron density relative to water of 11.8 for the metallic port in an open field photon beam in Eclipse TPS (Varian Medical Systems, Palo Alto, CA, USA). However, this value did not yield good agreement between measurements and TPS calculations for both 6 and 18 MV photon beams; Trombetta et al20 assigned the density of the metallic port in the Eclipse TPS to be 5 g/cm 3 which is the highest available value in the system. Except for one abstract19 in which the details and accuracy of the model were not provided, none of the previous studies created and validated a model of the metallic port with generic dimensions and density that can be applied to different beam energies, neither did they investigate the impact of the metallic port on any advanced techniques like intensity‐modulated radiotherapy (IMRT) or volumetric‐modulated arc therapy (VMAT), while these techniques have been used to treat postmastectomy patients 26…”

“…The in vivo film measurement one side at a time for one PMRT patient treated with 15 MV beams concurred with ex vivo results, while TLD measurements for six patients treated with 15 MV beams showed smaller dose variation (86%~101% of prescription dose). They recommended using 15 MV photons with compensating bolus to treat patients; Chatzigiannis et al17 used Monte Carlo (MC) simulation and found 7% to 13% dose reduction with the expander in place for two 6 MV tangential photon beams, and around 6% dose reduction for 18 MV beams; Chen et al18 reported that tissue expanders with metal ports will increase dose heterogeneity and reduce dose coverage significantly for patients treated with 6 MV or 18 MV opposed tangent photon fields; Sharabi et al reported in an abstract19 5%~20% dose attenuation due to the metallic port and claimed a nondeformable implant model of the port was created based on manufacture specifications, but they did not show the details of the model or validate the dosimetric accuracy of the model; Trombetta et al20 initially reported that no significant change in dose distributions was found for an opposed pair of 6 MV photon beams delivered to a breast phantom containing a metallic port, but later drew a conflicting conclusion in a separate paper21 that the metallic port must be taken into account in the dose calculations; Strang et al22 used TLD measurements and concluded that radiation doses around the tissue expander were unaltered; Srivastava et al23 conducted measurements in a water phantom using a small ion chamber (IC) and concluded that dose perturbation caused by metallic port in photon beams was 5%~20% and this perturbation could not be predicted by TPS; Zabihzadeh et al24 used MC simulation and found a dose enhancement about 15% in front of the port and a dose reduction of about 10% at 5 cm under the port; Gee et al25 used radiochromic films as in vivo dosimeter and found an average 7% dose reduction to skin surface in a sample of PMRT patients with the tissue expander present during radiotherapy.…”

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

“…13,16 However, a recent simulation using Eclipse planning software (Varian Medical Systems, Palo Alto, CA, USA) predicted no significant change in dose. 17 Furthermore, current treatment planning systems (TPS) are known to have limitations in calculating attenuation and heterogeneity because of the presence of very high-density objects in the path of the beam. 11 This is especially significant for surface or skin dose, at high risk of clinical relapse in the context of PMRT, so in vivo measurements are required.…”

Immediate breast reconstruction with anatomical implants following mastectomy: The radiation perspective

“…6,7 Furthermore, the presence of a breast prosthesis during PMRT with photon beam is not associated with significant changes in dose distribution. 8,9 However, immediate implant-based breast reconstruction in patients requiring PMRT remains controversial because of concerns for higher risk of unfavorable reconstructive outcomes, [10][11][12] although some found that the risk of reconstructive failure after PMRT was acceptable. 13 For radiation oncologists delivering PMRT, a key question is whether specific approaches of immediate reconstruction, including implant-based reconstruction, allow for optimal coverage of the chest wall and internal mammary nodes (IMN) with acceptable doses to the heart and lungs.…”