Dose and volume parameters for MRI-based treatment planning in intracavitary brachytherapy for cervical cancer

Purpose: Magnetic resonance imaging (MRI)-based treatment planning in intracavitary brachytherapy allows optimization of the dose distribution on a patient-by-patient basis. In addition to traditionally used point dose and volume parameters, dose-volume histogram (DVH) analysis enables further possibilities for prescribing and reporting. This study reports the systematic development of our concept applied in clinical routine. Methods and Materials: A group of 22 patients treated with 93 fractions using a tandem-ring applicator and MRI-based individual treatment planning for each application was analyzed in detail. High-risk clinical target volumes and gross tumor volumes were contoured. The dose to bladder, rectum, and sigma was analyzed according to International Commission of Radiation Units and Measurements (ICRU) Report 38 and DVH parameters (e.g., D 2cc represents the minimal dose for the most irradiated 2 cm 3). Total doses, including external beam radiotherapy and the values for each individual brachytherapy fraction, were biologically normalized to conventional 2-Gy fractions (α/β 10 Gy for target, 3 Gy for organs at risk). Results: The total prescribed dose was about 85 Gy αβ10, which was mainly achieved by 45 Gy external beam radiotherapy plus 4 × 7 Gy brachytherapy (total 84 Gy αβ10). The mean value was 82 Gy αβ10 for the point A dose (left, right) and 84 cm 3 for the volume of the prescribed dose. The average dose to the clinical target volume was 66 Gy αβ10 for the minimum target dose, 87 Gy αβ10 for the dose received by at least 90% of the volume, with a mean volume treated with at least the prescribed dose of 89%. The mean D 2cc for the bladder was 83 Gy αβ3, the ICRU point dose was 75 Gy αβ3, and the dose at the ICRU point plus 1.5 cm cranially was 100 Gy αβ3. The average dose to the rectum was 64 Gy αβ3 for D 2cc and at ICRU point 69 Gy αβ3. The sigma D 2cc was 63 Gy αβ3. Conclusion: A standard loading pattern should be used as the starting point for MRI-based optimization. Individual changes of active dwell positions and dwell weights are guided by a concept of DVH constraints for target and organs at risk. In our clinical routine, the dose to point A and dose received by at least 90% of the volume for the clinical target volume are both comparable to the prescribed dose. The DVH constraints for organs at risk allow reproducible treatment plans, helping to detect and avoid severe overdosage.