Evaluation of the radiotherapy treatment planning in the presence of a magnetic valve tissue expander

Robustness of three external beam treatment techniques against inter-fractional positional variations of the metal port in breast tissue expanders

INTRODUCTION

Temporary breast tissue expanders contain a metal port that varies in position throughout the course of radiation treatments. The purpose of this study was to quantify the robustness of the three most common external beam treatment techniques (tangential three-dimensional conformal radiation therapy [3DCRT], volumetric modulated arc therapy [VMAT], and helical tomotherapy) against our measured inter-fractional positional variations of the port.

METHODS

For eight breast cases, a clinical plan was created for each of the three techniques. The dosimetric effect of our previously measured inter-fractional port errors was evaluated for two classes of error: internal port errors (IPEs) and patient registration errors (PREs). For both classes of error, daily variable and systematic errors were modeled, and their cumulative effects were compared against the originally planned doses.

RESULTS

For systematic IPE, the 1%-99% range in point dose differences inside a 5-mm target abutting the implant was the highest for tangential 3DCRT, and it was within 6% and 9% when calculated with Monte Carlo and collapsed cone calculation engines, respectively. Daily variable PRE resulted in mean changes of -3.0% and -3.5% to V_100%Rx of the target for VMAT and tomotherapy, respectively. For nearby organs, daily variable PRE resulted in changes to V_20Gy of the ipsilateral lung of less than 2% in all three techniques, while V_5Gy of the heart increased by as much as 6% in VMAT and 10% in tomotherapy.

CONCLUSIONS

When IPEs were modeled, dose variability was the largest in tangential 3DCRT, leading to areas of underdosage in the shadow of the port. When PREs were modeled, the target coverage and nearby organs were affected the most in VMAT and helical tomotherapy. In reality, port positional errors result from a combination of IPE and PRE, suggesting that VMAT and tomotherapy are more robust when patient registration errors are minimized, despite the presence of IPE.

Effects of metal implants and a metal artifact reduction tool on calculation accuracy of AAA and Acuros XB algorithms in small fields

PURPOSE

In this study, the dosimetric accuracy of analytical anisotropic algorithm (AAA) and Acuros XB (AXB) dose calculation algorithms (Varian Medical Systems, Palo Alto, CA) was investigated for small radiation fields incident on phantoms of various metals that include stainless steel grade 316L (SS316L) and titanium alloy grade 5 (Ti5) implants. In addition, the effects of using metal artifact reduction for orthopedic implants (O-MAR, Philips Healthcare, Cleveland, OH) were evaluated.

METHODS

The evaluations of AAA and AXB were performed by comparing the crossline profiles calculated by AAA and AXB with GafChromic_TM EBT3 film measurements at the phantom-implant interfaces and in close vicinity of implant materials for small field sizes (1 × 1 cm² , 2 × 2 cm² , 3 × 3 cm² , and 4 × 4 cm² ) of a 6 MV flattening filter free photon beam. O-MAR corrected and uncorrected (UC) computed tomography (CT) images were used for dose calculations. The values of average and standard deviations (SD) of Hounsfield unit (HU) for selected regions of each case were evaluated. The differences in average dose percentages in defined regions were calculated to quantify the relative dosimetric changes between doses calculated on UC and O-MAR corrected CT images.

RESULTS

Compared to UC images, the values of SD were reduced, and the average HU became closer to its reference value in the O-MAR images. There was some discrepancy in average dose percentage differences between calculations using UC and O-MAR images at 1 cm above the SS316L implant (average dose percentage differences were AXB/UC = 5.9% and AXB/O-MAR = -1.2%; AAA/UC = 2.2%, and AAA/O-MAR = -0.8%). Neither AAA nor AXB algorithms predict increase in dose at upper phantom-implant interface (4.9%, 9.9%. 13.5%, and 13.8% for the fields from 1 × 1 cm² to 4 × 4 cm² , respectively). At the side of the SS316L implant (where dark streak artifacts exist), dose difference averages were estimated as - 1.1% and 22.3% when AXB/O-MAR and AXB/UC calculations are compared with EBT3 measurements, respectively. Dose predictions at 1 cm below the SS316L implant were underestimated by AXB/O-MAR (average -0.5%) and AXB/UC (average 2.0%).

CONCLUSIONS

The O-MAR tool was shown to have a favorable dosimetric effect or no effect on the calculations in the upper proximity of the implant materials. The dose differences between EBT3 film measurements and calculations at upper phantom-implant interfaces were smaller when they were calculated using O-MAR images. However, the dose differences increased when O-MAR corrected images were used for AAA calculations at lower phantom-implant interfaces. Use of O-MAR enabled closer agreement for the AXB algorithm, especially in the dark streak artifact regions. The O-MAR algorithm should be used when the dose is calculated with the AXB algorithm in cases of patients with the metal implants. The estimations using AAA and AXB algorithms, in phantom setups, with Ti5 implant material were found to be closer to the EBT3 film measurements, when compared with the same estimations using SS316L implant material.

International multidisciplinary expert panel consensus on breast reconstruction and radiotherapy

BACKGROUND

Conflicting evidence challenges clinical decision-making when breast reconstruction is considered in the context of radiotherapy. Current literature was evaluated and key statements on topical issues were generated and discussed by an expert panel at the International Oncoplastic Breast Surgery Meeting in Milan 2017.

METHODS

Studies on radiotherapy and breast reconstruction (1985 to September 2017) were screened using MEDLINE, Embase and CENTRAL. The literature review yielded 30 controversial key questions. A set of key statements was derived and the highest levels of clinical evidence (LoE) for each of these were summarized. Nineteen panellists convened for dedicated discussions at the International Oncoplastic Breast Surgery Meeting to express agreement, disagreement or abstention for the generated key statements.

RESULTS

The literature review identified 1522 peer-reviewed publications. A list of 22 key statements was produced, with the highest LoE recorded for each statement. These ranged from II to IV, with most statements (11 of 22, 50 per cent) supported by LoE III. There was full consensus for nine (41 per cent) of the 22 key statements, and more than 75 per cent agreement was reached for half (11 of 22).

CONCLUSION

Poor evidence exists on which to base patient-informed consent. Low-quality studies are conflicting with wide-ranging treatment options, precluding expert consensus regarding optimal type and timing of breast reconstruction in the context of radiotherapy. There is a need for high-quality evidence from prospective registries and randomized trials in this field.

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

The purpose of this research was to develop an experimental model of dosimetry using a breast phantom and evaluate the effects of the metallic port in tissue expanders on dose distribution in postmastectomy radiotherapy. Dose distribution was assessed using an experimental acrylic dosimetry simulator in the absence and presence of a metallic disc (MD), which is similar to that used in tissue expanders containing a magnetic port, by collecting coronal and sagittal radiochromic films. Dosimetry film analysis did not show any changes in dose distribution, except for a MD shadow in the sagittal film where the dose distribution rate was on average 14% lower than in other areas. This model very closely resembled actual breast reconstruction with a tissue expander containing a magnetic port. Scattering or attenuation of the irradiation dose due to interference of the MD was not evident in areas that could jeopardize the effectiveness of radiation therapy. Therefore, the presence of the MD inside the tissue expander does not likely have an impact on radiotherapy effectiveness during immediate breast reconstruction.

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.

Postmastectomy Radiation Therapy and Two-Stage Implant-Based Breast Reconstruction: Is There a Better Time to Irradiate?

BACKGROUND

The ideal timing of postmastectomy radiation therapy (PMRT) in the setting of two-stage implant-based breast reconstruction remains unclear. In this cohort study, the authors sought to determine whether complication rates differed between patients who received PMRT following tissue expander placement (TE-XRT) and those who received PMRT after exchange for permanent implants (Implant-XRT) utilizing using prospective, multicenter data.

METHODS

Eligible patients in the Mastectomy Reconstruction Outcomes Consortium study from 11 institutions across North America were included in the analysis. All patients had at least 6-month follow-up after their last intervention (i.e., implant exchange for TE-XRT patients, and radiation for Implant-XRT patients). Complications including seroma, hematoma, infection, wound dehiscence, capsular contracture, and implant loss were recorded.

RESULTS

The authors identified a total of 150 patients who underwent immediate, two-stage implant-based breast reconstruction and received PMRT. Of these, there were TE-XRT 104 patients (69.3 percent) and 46 (30.7 percent) Implant-XRT patients. There were no differences in the incidence of any complications or complications leading to reconstructive failure between the two cohorts. After adjusting for patient characteristics and site effect, the timing of PMRT (i.e., TE-XRT versus Implant-XRT) was not a significant predictor in the development of any complication, a major complication, or reconstructive failure.

CONCLUSION

In the setting of PMRT and two-stage implant-based reconstruction, patients who received PMRT after expander placement (TE-XRT) did not have a higher incidence or increased odds of developing complications than those who received PMRT after exchange for a permanent implant (Implant-XRT).

CLINICAL QUESTION/LEVEL OF EVIDENCE

Therapeutic, III.

A Nationwide Survey of UK Oncologists' Views on the Choice of Radiotherapy Regime for the Reconstructed Chest Wall in Breast Cancer Patients

Aims. This paper describes a UK survey of the choice of radiotherapy regime for the reconstructed chest wall in breast cancer patients. Questions focused on which fractionation regime consultants choose, their reasons for this, whether the type of reconstruction influences their choice, and whether bolus is used in patients who have undergone immediate reconstructive surgery. Materials and Methods. Between July 2014 and July 2015 a survey was sent by email to UK consultant radiation oncologists treating breast cancer. Results. The response rate was 73%. 67% of respondents use 40 Gray (Gy) in 15 fractions, with 22% using 50 Gy in 25 fractions and 7% using other regimes. For 90% of consultants the type of reconstruction did not influence their decision regarding choice of fractionation. 83% of respondents do not usually use a bolus for chest wall radiotherapy in patients who have had immediate reconstructive surgery. Conclusions. This survey illustrates there is variation in practice in the management of patients with breast cancer who have undergone immediate reconstructive surgery in the UK. There is a need for further research to determine which fractionation regime is optimal, whether the type of surgery is relevant, and whether bolus should be added.