Radiation measurement physics
In vivo dosimetry with radiochromic films in low-voltage intraoperative radiotherapy of the breast
EBT2 radiochromic films were studied and used forin vivo dosimetry in targeted intraoperative radiotherapy (TARGIT), a technique in which the Intrabeam system (Carl Zeiss, Oberkochen, Germany) is used to perform intraoperative partial breast irradiation with x-rays of 50 kVp.
The energy of the radiation emitted by the Intrabeam with the different spherical applicators, under 1 and 2 cm of solid water, and under the tungsten impregnated rubber used for shielding of the heart in TARGIT of the breast, was characterized with measurements of half-value layer (HVL). The stability of response of EBT2 was verified inside this range of energies. EBT2 films were calibrated using the red and green channels of the absorption spectrum in the 0–20 Gy dose range delivered by the Intrabeam x-rays. The dependence of film response on temperature during irradiation was measured. Forin vivo dosimetry, pieces of radiochromic films wrapped in sterile envelopes were inserted after breast conserving surgery and before TARGIT into the excision cavity, on the skin and on the shielded pectoralis fascia for treatments of the left breast.
HVLs of the Intrabeam in TARGIT of the breast correspond to effective energies of 20.7–36.3 keV. The response of EBT2 was constant inside this range of energies. We measured the dose to the target tissue and to organs at risk in 23 patients and obtained an average dose of 13.52 ± 1.21 Gy to the target tissue. Dose to the skin in close proximity to the applicator was 2.22 ± 0.97 Gy, 0.29 ± 0.17 Gy at 5–10 cm from the applicator, and 0.08 ± 0.07 Gy at more than 10 cm from the applicator. Dose to the pectoral muscle for left breast treatment was 0.57 ± 0.23 Gy.
Our results show that EBT2 films are accurate at the beam energies, dose range, and irradiation temperature found in TARGIT and thatin vivo dosimetry in TARGIT with EBT2 films wrapped in sterile envelopes is a feasible procedure. Measured dose to the organs at risk indicates that the technique is safe from side effects to the skin and the heart.