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Current Intraoperative Radiotherapy (IORT) Technology

Earlier IORT techniques were technically cumbersome and often relied on transporting the patient from the operating theatre to the radiotherapy unit during surgery. Alternatively, purpose-built operating theatres were constructed with built-in IORT systems and operating personnel shielding systems. These technical and financial limitations to IORT delivery placed a substantial constraint to the widespread adoption of IORT in a variety of hospital settings. Recently, technological advances in miniature radiotherapeutics have enabled the development of mobile IORT devices, which present fewer technical and logistical limitations. Several mobile devices are currently available:

·	Intrabeam™ System (Photoelectron Corporation, Lexington MA, USA) ·	Mobetron System (Oncology Care Systems Group of Siemens Medical Systems, Intraop Medical Inc, Santa Clara CA, USA) ·	Novac 7 System (NRT SpA (New Radiant Technology), Italy).(This company has taken over the know-how of Novac7)

Whether IORT has an identical biological effect on tissue compared with external beam radiotherapy is being investigated but because it is more conformational, the same physical dose of IORT is equivalent to a much higher dose of external beam radiotherapy. With regards to the specific mobile IORT systems currently being used in early breast cancer, Vaidya et al. report that the Intrabeam™ IORT system can deliver a physical dose of 5.0 Gy administered at a distance of 1.0 cm from the breast tumour cavity for a period of 21 minutes that is equivalent to a biologically effective dose of 21.7 Gy.

Using their Novac 7 IORT technology, Veronesi et al. have estimated that an external beam dose of 60Gy delivered in 30 fractions at 2 Gy/fraction is equivalent to a single IORT fraction of 20-22 Gy, (using an a /ß ratio at 10 Gy, typical for tumours and acute reacting tissues). Using this same equation, but calculating the tolerance of late responding tissues (a /ß ratio at 3 Gy) this equivalent value rises to at least 110 Gy.

As yet, there is no firmly established standardised IORT dose or dose rate for use in early breast cancer. IORT doses investigated for use in early breast cancer have ranged from 5 Gy to 22 Gy using a variety of different IORT systems.

IORT is typically administered during breast conserving surgery after every attempt has been made to ensure that there are clear surgical margins, even in cases where a re-excision must be undertaken. This is in contrast to its use at other tumour sites where it can be used where there are positive surgical margins but further surgical excision is not possible.