Analysis of treatment planning time and optimization parameters for inverse planning for intracavitary and interstitial brachytherapy in uterine cervical cancer

Abstract

Purpose: This study aimed to investigate the effect of inverse planning parameters on dose-volume indices in brachytherapy for uterine cervical cancer.

Methods: Fourteen consecutive patients with cervical cancer who received intracavitary and interstitial brachytherapy (IC/ISBT) were selected. Tandem, ovoid, and interstitial needles were used in all cases. The evaluation plans were recalculated from the first fraction of clinical brachytherapy plans. The correlation between the 11 dose optimization parameters of inverse planning and the 13 dose-volume indices was evaluated. The parameters were adjusted in five levels, and dose optimization was performed in hybrid inverse planning optimization (HIPO). Spearman's rank correlation and multiple regression analyses were conducted to assess the association between the parameters and the indices. The indices included clinical target volume (CTV) dose, organ-at-risk (OAR) dose, homogeneity, and conformity. Additionally, the correlation between optimization parameters and calculation time was investigated, along with a technique for efficiently generating treatment plans.

Results: "CTV Max Weight" and "OAR Max Weight" were the key parameters significantly affecting the indices. Increasing "CTV Max Weight" improved homogeneity but reduced the target coverage. The effect of "OAR Max Weight" on the dose reduction of CTV_HR D₉₀ (β = -0.59) was more significant than that on the dose reduction of OAR D_2cc (β = -0.21). In addition, adjusting "CTV Min Weight" and "CTV Volume" could reduce the hyper-dose sleeve without increasing the OAR dose. A large number of normal tissue sampling points could negatively affect the dose distributions and increase the calculation times.

Conclusion: "CTV Max Weight" and "OAR Max Weight" were the most influential parameters in HIPO, significantly affecting dose-volume indices in IC/ISBT for uterine cervical cancer. Additionally, parameters that regulate the hyper-dose sleeve and needle-delivered dose were identified. The quality of treatment planning can be maintained and planning time reduced by appropriately optimizing these parameters.

Keywords: HIPO; IC/ISBT; brachytherapy; cervical cancer; inverse planning; optimization.

FIGURE 1

Association between the optimization parameters and the dose‐volume indices based on the Spearman's rank correlation analysis. The colors represent the different values of the correlation coefficient (r): red indicates a positive correlation (0 < r < 1); blue, a negative correlation (−1 < r < 0); and white, no correlation (r = 0).

FIGURE 2

Effect of each optimization parameter on the dose‐volume indices based on the multiple regression analysis. The colors represent the different values of the standardized regression coefficient (β): red indicates a positive effect (0 < β < 1); blue, a negative effect (−1 < β < 0); and white, no effect (β = 0).

FIGURE 3

Example of the effect of (a) “CTV Max Weight” and (b) “OAR Max Weight” on the dose distributions. The green, red, and blue lines represent the isodose curves of 300, 600, and 1200 cGy, respectively. The region of light blue is CTV_HR. The brown, yellow, and red regions represent the rectum, bladder, and sigmoid colon, respectively. The arrows indicate where the 600 cGy isodose curve shrinks.

FIGURE 4

Effect of “CTV Min Weight” on the dose‐volume indices. Association between “CTV Min Weight” and (a) CTV_HR D₉₀; (b) CTV_HR V_CTV,100; (c) Dose nonuniformity index; (d) Homogeneity index.

FIGURE 5

Effect of “CTV Volume” on the dose‐volume indices. Association between “CTV Min Weight” and (a) CTV_HR D₉₀; (b) rectum D_2cc; (c) Dose nonuniformity index; (d) Homogeneity index.

FIGURE 6

Effect of the normal tissue sampling points on the dose‐volume indices. Association between “Normal tissue sampling points” and (a) CTV_HR D₉₀; (b) rectum D_2cc; (c) conformity index; (d) calculation time by HIPO. Circles represent the longest calculation time for the largest tumor volume (163.5 cc).