Comparative Dosimetric Analysis of Combined Intracavitary and Interstitial Needles Placement vs. Intracavitary Alone for Cervical Cancer

Abstract

To compare the target volume coverage and normal tissue avoidance for high-dose rate (HDR) brachytherapy boost for cervical cancer between intracavitary (IC) tandem and ovoid (T&O) with additional free-hand interstitial (IS) needles (IC-IS) vs. IC brachytherapy alone under ultrasound guidance. We reviewed IC T&O cases with free-hand IS needles for cervical cancer treatment from Nov 2018 to Oct 2019. Ultrasound guidance was used for applicator placement. Treatment planning was CT-based; high-risk clinical target volumes (HR-CTVs) and organs at risk (OARs) were delineated before each fraction. Minimum acceptable target coverage was 90% of the prescription dose delivered to the HR-CTV (D90). If these objectives were unmet with the standard IC applicator, IS needles were added at the discretion of the physician. For this analysis, each IC-IS case was re-planned (r) with IC alone and again with IC-IS, matching the dose-limiting normal tissue structure for that treatment. HR-CTV with D90 along with OARs D2cc means and 95% confidence intervals were evaluated between rIC-IS and rIC plans using a paired t-tests, p = 0.05. Acute side effects were evaluated using CTCAE v5.0 criteria. We identified 6 patients with 20 IC-IS treatments after whole pelvis radiation. Between two to six IS needles were placed for each treatment. All fractions were planned for 7 Gy. Mean D90 per fraction to the HR-CTV for iIC-IS, rIC, and rIC-IS needles was 7.6 Gy (6.3–9.4) vs. 7.1 Gy (4.9–9.3) vs. 7.8 Gy (6.3–9.8) (p = 0.0000008), respectively, with a relative percent-dose increase of 10.6% from rIC alone to rIC-IS. Regarding OARs, mean D2cc per fraction for iIC-IS, rIC alone, and rIC-IS were: Bladder - 5.3 Gy (3.7–6.4) vs. 5.5 (4.1–6.2) vs. 5.5 (3.93 – 6.19) (p = 0.78); Rectum - 3.1 Gy (2.2–4.2) vs. 3.2 (2.1–4.1) vs. 3.3 Gy (2.2–4.1) (p = 0.14); Sigmoid - 3.4 Gy (2.3–4.2) vs. 3.4 (2.5–4.2) vs. 3.4 (2.5– 4.2) (p = 0.49). Dosimetric results demonstrated in the table. All patients tolerated the procedure well without any CTCAE v5.0 Grade 3 to 5 adverse events. For appropriately selected patients, adding IS needless to IC HDR brachytherapy boost can improve dose coverage of HR-CTV for cervical cancer while not increasing dose to OARs. Long term follow-up is warranted for evaluating differences in clinical outcomes and associated toxicity.Tabled 1Abstract 2856; Table; Comparison of initial (i) IC + IS cases vs. re-planned (r) IC alone vs. rIC-IS (Gy)Patient Number (# fractions)iIC-IS D90 HRCTVrIC alone D90 HRCTVrIC-IS D90 HRCTViIC-IS D2cc BladderrIC alone D2cc BladderrIC-IS D2cc bladderiIC-IS D2cc RectumrIC alone D2cc RectumrIC-IS D2cc rectum1 (3)7.67.67.94.64.74.82.42.62.72 (4)7.77.98.34.54.94.72.82.72.83 (3)6.867.15.76.16.14.13.944 (4)8.7895.75.95.82.83.13.15 (3)75.46.96.25.55.83.73.93.96 (3)7.27.17.35.75.75.73.33.23.3Average (CI)7.6 (6.3–9.4)7.1 (4.9–9.3)7.8 (6.3–9.8)5.3 (3.7–6.4)5.5 (4.1–6.2)5.5 (3.9–6.2)3.1 (2.2–4.2)3.2 (2.1–4.1)3.3 (2.2-4.1)p - value0.0000080.780.14 Open table in a new tab