Direction modulated brachytherapy (DMBT) for treatment of cervical cancer: A planning study with ¹⁹²Ir, ⁶⁰Co, and ¹⁶⁹Yb HDR sources: A

Purpose: To evaluate plan quality of a novel MRI-compatible direction modulated brachytherapy (DMBT) tandem applicator using ¹⁹²Ir, ⁶⁰Co, and ¹⁶⁹Yb HDR brachytherapy sources, for various cervical cancer high-risk clinical target volumes (CTV_HR). Materials and Methods: The novel DMBT tandem applicator has six peripheral grooves of 1.3-mm diameter along a 5.4-mm thick nonmagnetic tungsten alloy rod. Monte Carlo (MC) simulations were used to benchmark the dosimetric parameters of the ¹⁹²Ir, ⁶⁰Co, and ¹⁶⁹Yb HDR sources in a water phantom against the literature data. 45 clinical cases that were treated using conventional tandem-and-ring applicators with ¹⁹²Ir source (¹⁹²Ir-T&R) were selected consecutively from intErnational MRI-guided BRAchytherapy in CErvical cancer (EMBRACE) trial. Then, for each clinical case, 3D dose distribution of each source inside the DMBT and conventional applicators were calculated and imported onto an in-house developed inverse planning optimization code to generate optimal plans. All plans generated by the DMBT tandem-and-ring (DMBT T&R) from all three sources were compared to the respective ¹⁹²Ir-T&R plans. For consistency, all plans were normalized to the same CTV_HR D90 achieved in clinical plans. The D_{2 cm3} for organs at risk (OAR) such as bladder, rectum, and sigmoid, and D90, D98, D10, V100, and V200 for CTV_HR were calculated. Results: In general, plan quality significantly improved when a conventional tandem (Con.T) is replaced with the DMBT tandem. The target coverage metrics were similar across ¹⁹²Ir-T&R and DMBT T&R plans with all three sources (P > 0.093). ⁶⁰Co-DMBT T&R generated greater hot spots and less dose homogeneity in the target volumes compared with the ¹⁹²Ir- and ¹⁶⁹Yb-DMBT T&R plans. Mean OAR doses in the DMBT T&R plans were significantly smaller (P < 0.0084) than the ¹⁹²Ir-T&R plans. Mean bladder D_{2 cm3} was reduced by 4.07%, 4.15%, and 5.13%, for the ¹⁹²Ir-, ⁶⁰Co-, and ¹⁶⁹Yb-DMBT T&R plans respectively. Mean rectum (sigmoid) D_{2 cm3} was reduced by 3.17% (3.63%), 2.57% (3.96%), and 4.65% (4.34%) for the ¹⁹²Ir-, ⁶⁰Co-, and ¹⁶⁹Yb-DMBT T&R plans respectively. The DMBT T&R plans with the ¹⁶⁹Yb source generally resulted in the greatest OAR sparing when the CTV_HR were larger and irregular in shape, while for smaller and regularly shaped CTV_HR (<30 cm³), OAR sparing between the sources were comparable. Conclusions: The DMBT tandem provides a promising alternative to the Con.T design with significant improvement in the plan quality for various target volumes. The DMBT T&R plans generated with the three sources of varying energies generated superior plans compared to the conventional T&R applicators. Plans generated with the ¹⁶⁹Yb-DMBT T&R produced best results for larger and irregularly shaped CTV_HR in terms of OAR sparing. Thus, this study suggests that the combination of the DMBT tandem applicator with varying energy sources can work synergistically to generate improved plans for cervical cancer brachytherapy.