Investigation of in vivo source tracking error thresholds for interstitial and intra-cavitary high-dose-rate cervical brachytherapy

Abstract

Purpose: The purpose of this study was to determine a comprehensive in vivo source tracking error thresholds in high-dose-rate (HDR) brachytherapy for cervical cancer. Achieving this enables the definition of an action level for imminent in vivo source tracking technologies and treatment monitoring devices, preventing clinically relevant changes to the applied dose.

Material and methods: Retrospective HDR interstitial (n = 10) and intra-cavitary (n = 20) cervical brachytherapy patients were randomly selected to determine the feasibility of implementing in vivo source tracking error thresholds. A script was developed to displace all dwell positions in each treatment plan, along all major axes from their original position. Dose-volume histogram (DVH) indices were calculated without re-optimization of modified plans to determine the appropriate in vivo source tracking error thresholds in each direction.

Results: In vivo source tracking error thresholds were directionally dependent; the smallest were found to be 2 mm in the anterior and posterior directions for both interstitial and intra-cavitary treatments. High-risk clinical treatment volume (HR-CTV) coverage was significantly impacted by displacements of 4 to 5 mm along each axis. Critically, there was a large variation in DVH metrics with displacement due to change in dwell weightings and patient anatomy.

Conclusions: Determining the dosimetric impact of dwell position displacement provides a clinical benchmark for the development of pre-treatment verification devices and an action level for real-time treatment monitoring. It was established that an in vivo source tracking error threshold needs to be patient-specific. In vivo source tracking error thresholds should be determined for each patient, and can be conducted with extension of the method established in this work.

Keywords: brachytherapy; dwell position error; gynecological; high-dose-rate; in vivo; source tracking.

Fig. 1

A) Interstitial treatment with needles and three applicator channels shown in magenta; B) Intra-cavitary treatment with three applicator channels. For interstitial (A) and intra-cavitary (B) treatment plans, the 100% isodose volume is shown in blue. Contours from patient dataset are displayed with HR-CTV volume in red, bladder in yellow, and rectum in brown

Fig. 2

Rotation of an intra-cavitary treatment plan about the tandem

Fig. 3

Dose given to bladder (A), rectum (B), and HR-CTV (C) by un-altered treatment plans. Box plots represent the distribution of doses provided by the un-altered treatment plans

Fig. 4

Relative change in bladder D_2cc, with displacement in the anterior-posterior (A), right-left (B), and superior-inferior (C) directions for intra-cavitary cases. Red dashed line denotes the threshold, a 10% relative increase in dose

Fig. 5

Relative change in rectum D_2cc, with displacement in the anterior-posterior (A), right-left (B), and superiorinferior (C) directions for intra-cavitary cases. Red dashed line denotes the threshold, a 10% relative increase in dose

Fig. 6

Relative change in HR-CTV D₉₀, with displacement in the anterior-posterior (A), right-left (B), and superior-inferior (C) directions for intra-cavitary cases. Red dashed line denotes the threshold, a 10% relative decrease in dose

Fig. 7

Change in dose due to applicator rotation. Red dashed line denotes the threshold, a 10% relative change in dose. A) Bladder, B) rectum, and C) HR-CTV

Fig. 8

Relative change in bladder D_2cc, with displacement in the anterior-posterior (A), right-left (B), and superior-inferior (C) directions for interstitial cases. Red dashed line denotes the threshold, a 10% relative increase in dose

Fig. 9

Relative change in rectum D_2cc, with displacement in the anterior-posterior (A), right-left (B), and superior-inferior (C) directions for interstitial cases. Red dashed line denotes the threshold, a 10% relative increase in dose

Fig. 10

Relative change in HR-CTV D₉₀, with displacement in the anterior-posterior (A), right-left (B), and superior-inferior (C) directions for interstitial cases. Red dashed line denotes the threshold, a 10% relative decrease in dose