Dosimetric Impact of Large Displacement of the Metal Port of the Tissue Expander in Intensity Modulated Proton Therapy

Purpose/Objective(s)

Breast cancer patients with tissue expander(s) have been treated with external beam radiotherapy. We report a population distribution regarding the magnitude of the displacement of the metal port of the expander, and provide the simulated dosimetric impact of an observed large displacement (5 cm), specifically for intensity modulated proton therapy (IMPT).

Materials/Methods

This study included 38 patients with saline tissue expanders who had received tomotherapy for breast cancer from 1996 to 2017 at our institution, with daily MV CT imaging. The displacements of the metal port were evaluated by comparing the daily MV CT for patient setup to the planning CT. For each patient the displacements were evaluated over the whole course of 25 to 28 fractions. For the whole patient group, the population distribution of the average displacement of each patient was analyzed. For one specific identified case with major port displacement of approximately 5 cm, an IMPT plan was created to calculate the dosimetric impact of such a large shift. The nominal plan was robustly optimized with 3 fields, with a setup uncertainty of 5 mm, and a range uncertainty of 3.5 %. The proton spots were restricted to be placed within a 5 mm expansion of the metal port. Multiple field optimization was applied for the portion of the CTV containing the port, and single field optimization was used for the remaining volume of the CTV.

Results

For the whole patient group, the first -, second - and third - quantile of port displacements over the course of radiotherapy were 0.17, 0.40, and 0.54 cm; the STD, mean, and range were 0.77, 0.50, 0 and 4.89 cm; the upper and lower 95% confidence intervals were 0.25 and 0.75 cm. The one outlier patient (2.6% of the studied cohort) had an average port displacement of 4.89 cm (+/- 0.78 cm). For dosimetric evaluation, the port was systematically offset over the whole course, resulting in CTV D99 of 49 Gy for the nominal dose but degrading to 30 Gy when the dose was perturbed by the large port displacement. Significant cold spots (19 Gy, 38% prescribed dose) were identified in the CTV chest wall downstream of the shifted port. Hot spots (75 Gy, 150% prescribed dose) were also created, but they were within the breast implant.

Conclusion

Large displacement of the metal port of a breast implant can occur during the course of radiotherapy for some patients. For IMPT treatment, this can cause significant underdose to the CTV downstream of the shifted port. Preventative measures for quality assurance such as verification CT prior to the first fraction, daily kV imaging evaluation of port locations, weekly verification CT, and adaptive re-planning in case of large displacement are recommended for patients with expanders during the IMPT treatment.