The Impact of Prostate and Seminal Vesicle Motion during Prostate Cancer Radiotherapy on Planning Margins

Abstract

The seminal vesicles (SVs) form part of the target in the radiotherapy (RT) treatment of prostate cancer, either because of their significant risk of involvement in locally advanced disease or because of overt clinical invasion. While prostate motion and appropriate margins have been documented, there is little information on SV motion. Movement of the SVs compared to the prostate, if uncorrected, can lead to geometrical target miss. The aim of this prospective study was to assess and compare SV motion with respect to prostate motion to ascertain the appropriate margins for each target. The study population consisted of 10 patients with locally advanced prostate cancer and considered to be at risk of SV invasion or with clinically T3b disease. The average age was 71 years, with average PSA and median Gleason scores of 10 μg/L and 8, respectively, and T stages from T2a to T3b. Following consent, 5 gold seeds were inserted trans-rectally into each patient. Of the 5 seeds, 3 were inserted into the prostate (right base, left mid gland, right apex), and 1 into each of the left and right SVs. All patients were treated to a dose of 78 Gy in 39 fractions with IMRT. Daily orthogonal 2D Electronic Portal Images (EPIs) were taken of each patient in treatment position prior to RT. Elekta I-View GT template matching software was used to verify EPIs with the reference image (CT-derived Digitally Reconstructed Radiograph). Online verification was performed for each fraction using the 3 prostate seeds. Matches of the left and right SV seeds were performed offline for each of the 39 fractions of all 10 patients. The total standard deviation (SD) of systematic errors (∑tot) and SD of random errors (σtot) were calculated, and the CTV-PTV margins for both the prostate and SVs were generated using a previously published formula (2.5∑tot + 0.7σtot). The average correctional shifts (with ranges) for the prostate and SVs in mm were: Prostate: Right-Left (RL) 0.9 (−4.6 to 6.5), Superior-Inferior (SI) −0.3 (−9.8 to 6.3), Anterior-Posterior (AP) −0.3 (−8.6 to 9.1) Left SV: RL 0.9 (−10.5 to 10.4), SI -1.4 (−10.8 to 8.8), AP -2.2 (−16.6 to 8.3) Right SV: RL 0.1 (−9.0 to 5.3), SI -1.1 (−9.4 to 6.3), AP -2.1 (−13.2 to 10.9). Required CTV-PTV margins in mm in the RL, SI and AP planes were: Prostate: 4.2, 6.1, 3.3 Left SV: 9.1, 5.4, 6.9 Right SV: 5.5, 5.4, 6.7. Positional variation of the prostate and SVs was observed during RT. We have found that SV motion is greater in the RL and AP directions than prostate motion. Consequently, the calculated PTV margins for the SVs were greater than the prostate margins, indicating that differential margins may be warranted.