Real-time in vivo dosimetry using micro-MOSFET detectors during intraoperative electron beam radiation therapy in early-stage breast cancer.
OBJECTIVE
In a previous paper we reported the results of off-line in vivo measurements using radiochromic films in IOERT. In the present study, a further step was made, aiming at the improvement of the effectiveness of in vivo dosimetry, based on a real-time check of the dose.


METHODS
Entrance dose was determined using micro-MOSFET detectors placed inside a thin, sterile, transparent catheter. The epoxy side of the detector was faced towards the beam to minimize the anisotropy. Each detector was plugged into a bias supply (standard sensitivity) and calibrated at 5 Gy using 6 MeV electrons produced by a conventional linac. Detectors were characterized in terms of linearity, precision and dose per pulse dependence. No energy and temperature dependence was found. The sensitivity change of detectors was about 1% per 20 Gy accumulated dose. Correction factors to convert surface to entrance dose were determined for each combination of energy and applicator. From November 2004 to May 2005, in vivo dosimetry was performed on 45 patients affected by early-stage breast cancer, who underwent IOERT to the tumour bed. IOERT was delivered using electrons (4-10 MeV) at high dose per pulse, produced by either a Novac7 or a Liac mobile linac.


RESULTS
The mean ratio between measured and expected dose was 1.006+/-0.035 (1 SD), in the range 0.92-1.1. The procedure uncertainty was 3.6%. Micro-MOSFETs appeared suitable for in vivo dosimetry in IOERT, although some unfavourable aspects, like the limited lifetime and the anisotropy with no build-up, were found. Prospectively, a real-time action level (+/-6%) on dose discrepancy was defined.


CONCLUSIONS
Excellent agreement between measured and expected doses was found. Real-time in vivo dosimetry appeared feasible, reliable and more effective than the method previously published.
Ciocca
Mario
M
Department of Medical Physics, European Institute of Oncology, Milano, Italy. mario.ciocca@ieo.it
Piazzi
Valeria
V
Lazzari
Roberta
R
Vavassori
Andrea
A
Luini
Alberto
A
Veronesi
Paolo
P
Galimberti
Viviana
V
Intra
Mattia
M
Guido
Andrea
A
Tosi
Giampiero
G
Veronesi
Umberto
U
Orecchia
Roberto
R
D001943:Breast Neoplasms / Q000532:radiotherapy* / Q000601:surgery; D002138:Calibration; D002404:Catheterization / Q000295:instrumentation; D005240:Feasibility Studies; D005260:Female; D005373:Film Dosimetry / Q000295:instrumentation*; D006801:Humans; D007430:Intraoperative Care; D015412:Mastectomy, Segmental; D009367:Neoplasm Staging; D009862:Online Systems; D010315:Particle Accelerators; D011446:Prospective Studies; D015642:Radiology, Interventional; D011874:Radiometry / Q000295:instrumentation; D011879:Radiotherapy Dosage; D011882:Radiotherapy, High-Energy; D013997:Time Factors; D014173:Transistors, Electronic