Seminal vesicle interfraction displacement and margins in image guided radiotherapy for prostate cancer

Abstract

Background: To analyze interfraction motion of seminal vesicles (SV), and its motion relative to rectal and bladder filling.

Methods and materials: SV and prostate were contoured on 771 daily computed tomography "on rails" scans from 24 prostate cancer patients undergoing radiotherapy. Random and systematic errors for SV centroid displacement were measured relative to the prostate centroid. Margins required for complete geometric coverage of SV were determined using isotropic expansion of reference contours. SV motion relative to rectum and bladder was determined.

Results: Systematic error for the SV was 1.9 mm left-right (LR), 2.9 mm anterior-posterior (AP) and 3.6 mm superior-inferior (SI). Random error was 1.4 mm (LR), 2.7 mm (AP) and 2.1 mm (SI). 10 mm margins covered the entire left SV and right SV on at least 90% of fractions in 50% and 33% of patients and 15 mm margins covered 88% and 79% respectively. SV AP movement correlated with movement of the most posterior point of the bladder (mean R2 = 0.46, SD = 0.24) and rectal area (mean R2 = 0.38, SD = 0.21).

Conclusions: Considerable interfraction displacement of SV was observed in this cohort of patients. Bladder and rectal parameters correlated with SV movement.

Figure 1

a) Margins required to cover the entire left seminal vesicle. Results for all 24 patients were plotted individually. b) Margins required to cover the inferior 2.5 cm portion of left seminal vesicle, c) entire right seminal vesicle and d) inferior 2.5 cm portion of right seminal vesicle. Same line colour has been used for each individual patients in Figures 1a – d.

Figure 2

Percentage of patients where the entire or the inferior 2.5 cm of SV is within the PTV on at least 90% of the fractions. Data from all patients including those with SVs ≤ 2.5 cm in length.

Figure 3

Relationship between the geometrical margin to cover SV and systematic error (average movement of the seminal vesicle centroid from its position in the reference scan, expressed as the length of the 3D vector). Three fractions were excluded as outliers which give margins that fit approximately 90% of all fractions.

Figure 4

Correlation of SV AP movement with movement of the most posterior point of the bladder.

Figure 5

Correlation of SV AP movement with change in rectal area using Method 1 of rectal area evaluation. Data for both LSV and RSV for all patients were plotted individually. A negative value on the x axis represents anterior movement of SV as rectal area increases.