In vivo dosimetry with luminescent microdosimeters in 192Ir brachytherapy of breast cancer: development of technology and clinical testing

International organizations - WHO, IAEA, ICRP strongly recommended to consider in vivo dosimetry (UVD) as an important part of a quality assurance program in radiotherapy and to use the technique for monitoring treatment delivery, errors detection, verification of dose delivering. It is particularly relevant for brachytherapy with high dose rate sources, such as 192Ir due to high absorbed dose gradient at small distance from the source. During multifractional adjuvant brachytherapy of breast cancer there is considerable difference between observed absorbed doses distribution in tissues and calculated dose distribution from the dosimetry plan. The difference is due to mobility of the breast gland tissue and change of the mammary gland (MG) shape resulted from radiotherapy. The study presents innovative technology, in vivo dosimetry with thermoluminiscent microdosimeters LiF:Mg,Ti, used for high-dose rate brachytherapy of breast cancer. The technique was applied to verify calculated doses. The method was tested during the performance of multifractional (10 fractions, two fractions a day) adjuvant brachytherapy of the breast cancer with 192Ir. Micro-sized luminescent dosimeters (about 100 μm) were assembled and placed on the points marked on the skin of healthy breast lobes near points of entry and exit of introducers guiding 192Ir sources. We assumed that breast skin areas around the points of entry and exit of the introducer with the 192Ir-source were the mostly affected areas of the breast, the critical area, resulted from exposure to radiation. Local doses were measured in 100 points (total) marked on the breast skin in 12 patients with breast cancers. The results of measurements were the following: in 34% of all measurements, the IVD results and calculated (planned) local doses fitted together within the limits of measurement errors; in 21%, IVD-measured local doses significantly exceeded the calculated doses, the excess ranged from 15 to 58%; in 45% of all measurements, the measured local doses were significantly lower than the calculated doses (the difference between the calculated and measured doses ranged from 26 to 58%. In eight patients, IVD-measured total (10 fractions of brachytherapy) local absorbed doses to the skin of the mammary gland exceeded 10 Gy (ranged from 10.1 to 27.0 Gy). From obtained results it is evident that calculated, planned, radiation doses to be delivered to the breast during brachytherapy procedure with 192Ir should be verified, and the use of in vivo dosimetry, reliable and effective technique, could be the best choice for these purposes. If measured local absorbed doses to the breast skin are equal to or exceed 10 Gy, that is a risk factor of radiation associated complications development after the treatment completion. That is a reason to conduct posttreatment monitoring of health status of patients in order to prevent development of adverse effects.