Evaluation of pelvic lymph node coverage of conventional radiotherapy fields based on bony landmarks in high risk prostate cancer patients using virtual simulation
Keywords:
Shielding, nodal contouringAbstract
Virtual simulation refers to a method of delineating the tumour or treatment field. Field placement has resulted in more irregular shaped smaller fields as compared to conventional radiotherapy treatment fields for prostate cancer. The aim of this study was to evaluate pelvic lymph node coverage of conventional radiotherapy fields based on bony landmarks in high risk prostate cancer patients using virtual simula- tion based nodal mapping by using blood vessels as surrogate markers.Materials and methods Forty patients with high risk stage T3N1 prostate cancer underwent virtual simulation using a computed tomography (CT) scanner. Gross tumor volume (GTV), clinical target volume (CTV) and planning target volume (PTV) were outlined on the unenhanced CT images. Unenhanced images were used as part of the institutional protocol. All pelvic lymph nodes were contoured by using pelvic vessels as surrogate markers. The vessel contours were hidden by an option available in the planning software of the CT scanner. Thereafter conventional radiotherapy fields were drawn on digital reconstructed images (DRIs). The hidden vessel-contours were made visible again and distances were measured at different points of antero-posterior (AP) and lateral fields. Distances > 5 mm or more between the contoured nodes and the field borders were considered acceptable.
Results The antero-posterior (AP) fields showed inadequate coverage of the obturator lymph nodes at the level of the acetabulum (mean distance 2.0 mm p value 0.002). The lateral fields showed inadequate coverage of the sacral lymph nodes at the level of the second sacral vertebra (mean distance -0.47 mm p value 0.003) .
Conclusion The conventional pelvic fields for high risk prostate cancer do not give optimal nodal coverage. It is of utmost importance that the blocks to shield the rectum and femoral heads are fabricated with precision in order to achieve optimal nodal coverage.
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