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


METHODS
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.


RESULTS
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.


CONCLUSIONS
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.
Department of Radiation Oncology, Chris O'Brien Lifehouse, Sydney, New South Wales, Australia.|Central Clinical School, School of Medicine, University of Sydney, Sydney, New South Wales, Australia.
Harriet E
HE
Gee
Radiation Oncology Department, Auckland City Hospital, Auckland, New Zealand.
Fiona
F
Bignell
Department of Radiation Oncology, Chris O'Brien Lifehouse, Sydney, New South Wales, Australia.
David
D
Odgers
Department of Radiation Oncology, Chris O'Brien Lifehouse, Sydney, New South Wales, Australia.
Simran
S
Gill
Department of Radiation Oncology, Chris O'Brien Lifehouse, Sydney, New South Wales, Australia.
Darren
D
Martin
Department of Radiation Oncology, Chris O'Brien Lifehouse, Sydney, New South Wales, Australia.
Joanne
J
Toohey
Department of Radiation Oncology, Chris O'Brien Lifehouse, Sydney, New South Wales, Australia.
Susan
S
Carroll
D000328:Adult; D018427:Breast Implants; D001943:Breast Neoplasms / Q000532:radiotherapy*; D005260:Female; D005373:Film Dosimetry; D006801:Humans; D008408:Mastectomy / Q000534:rehabilitation*; D008875:Middle Aged; D000069079:Radiation Exposure / Q000032:analysis*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D018714:Radiotherapy, Adjuvant / Q000379:methods*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D015625:Tissue Expansion Devices; D016896:Treatment Outcome
breast neoplasm/radiotherapy; breast neoplasm/surgery; radiometry; radiotherapy dosage; tissue expansion device