To evaluate the dosimetric impacts of tissue heterogeneities and presence of air gap for the low energy X-ray intraoperative radiation therapy (IORT). A treatment unit of INTRABEAM system with various applicators was modeled in Radiance treatment planning system and the dose calculation with tissue heterogeneities correction was achieved utilizing Hybrid Monte Carlo algorithm. The dosimetric characteristics of INTRABEAM system with spherical and flat applicators in the presence of 2- and 4-mm-thick air gaps between applicator surface and target tissue were investigated. The discrepancies of deposited doses in the media of water, adipose tissue, muscle and cortical bone were quantitatively evaluated for all the spherical, flat and needle applicators. IORT treatment plans of breast cancer and vertebral metastasis were simulated using the Radiance system, the dose distributions were calculated and compared to evaluate the dosimetric impact of tissue heterogeneities in clinical scenarios. Due to the presence of air gap, actual dose delivered to target tissue was significantly lower than the planned dose with clinical dosimetry. The maximum relative dose differences with the presence of 2- and 4-mm air gaps reached 31.0% and 48.9% for the spherical applicator with 15 mm diameter. Comparing with the deposited dose in water, deposited doses in muscle and adipose tissue were lower, whereas dose deposition in cortical bone increased significantly. The relative doses at reference point in cortical bone ranged from 252.7% to 340.4% and from 191.8% to 259.2% for the spherical and flat applicators in different sizes, respectively. For the simulated IORT treatment plans, apparent differences of the doses to target volume and organs at risk were found between the dose distributions with and without tissue heterogeneities corrections. The presence of air gap between applicator surface and target tissue leads to obvious dose reduction to target volume, any air gap should be avoided in clinical practice. The tissue heterogeneities result in significant dosimetric deviation from the deposited dose in water. The clinical dosimetry protocol considering patient as water-equivalent is not appropriate for all the treatment sites, actual dose deposition in the region with tissue heterogeneities presence should be carefully evaluated.
Zhang et al. (Sun,) studied this question.