Optimizing dwell time weight for interstitial needles in intracavitary/interstitial hybrid brachytherapy: balancing tumor coverage with organ sparing using the inverse planning technique.

The recommended dwell time weight of the needle in intracavitary/interstitial hybrid brachytherapy (HBT) has been 10-20%. This study aimed to investigate the correlation between the weight constraint of the needle and normal organ doses in uterine cervical cancer HBT. This study included 30 cervical cancer patients who received HBT with tandem/ovoid applicators. In our clinical practice, treatment plans were generated without the constraint of the dwell time weight of the needle. The cases where this weight exceeded 20% were replanned. An inverse planning technique with locking downscaled needle dwell time was used to reproduce isodose lines of clinical plans. Replanning repeated with downscaling of the dwell time until the weight of the needle fell <20% (Needle-Lock plan). The Needle-Lock plans were rescaled to coincide with the high-risk clinical target volumes D90 of clinical plans. D2cc in normal organs and the overdose area >200% of the prescribed dose were evaluated. In 17 of 30 (56.7%) clinical plans, the weight of the needle exceeded 20%. The rectum, bladder and sigmoid colon D2cc significantly increased with the Needle-Lock plan. The overdosage area also increased significantly (P < 0.01). The correlations between the needle number and the increase of D2cc in the rectum and sigmoid colon (P < 0.01) were statistically significant. Limiting needle dwell time weight by 10-20% increased bladder and rectum doses, especially with multiple needles. These findings suggest that needle dwell time weight recommendations could need to be reconsidered based on individual and institutional situation.