A dosimetric comparison of different conventional 2D & 3D treatment plans of palliative spinal bone irradiation

Abstract

Background: External beam radiotherapy is a common and effective modality in treating patients with spinal metastases. Simple field arrangements would be used in conventional 2D radiotherapy including single posterior and anterior-posterior posterior-anterior (AP-PA) opposing fields. Different prescription method, field size and beam weighting was used commonly, and were investigated in this study. 3D conformal radiation therapy (3DCRT) will also be investigated. Dosimetric comparison on dose homogeneity in target volume and the organ at risk (OARs) dose was performed using three-dimensional dosimetric data.

Methods: Computed tomography simulation scans of 15 patients with thoracic spinal bone metastases who received conventional 2D radiotherapy were analyzed retrospectively. Five different treatment plans were created for each patient using different radiotherapy technique: (1) single posterior field with oncologists’ prescribed field size; (2) single posterior field with ICRU prescription point; (3) single posterior field with local prescription method (85% isodose level prescribed to Dmax); (4) AP-PA opposing fields with AP-PA ratio = 1:2; (5) AP-PA opposing fields with equal weighting. Conformity, homogeneity, minimum, maximum and mean dose of planning target volume (PTV) were compared. (6) 3DCRT with posterior and posterior oblique wedged fields. OARs dose including spinal cord, esophagus, heart, lung and liver dose was analyzed.

Results: In OAR sparing, all OARs were safe and well tolerated in all plans. In PTV dosimetric results, all single posterior and AP-PA opposing technique did not demonstrate satisfactory PTV coverage and homogeneity. While 3DCRT achieved the best PTV homogeneity (homogeneity index 0.11 compared with 0.15-0.35 in other plans) and PTV coverage (PTV min dose 17.1 ± 2.8 and PTV max dose 22.4 ± 0.4 Gy).

Conclusion: The 3DCRT achieved the best coverage and homogeneity of PTV. Moreover, it can well spare the mid-line structures including the heart and esophagus with the doses to other OARs were below the tolerance limit. Therefore, the optimal and recommended treatment technique to thoracic spinal irradiation would be the 3DCRT treatment technique if it is flexible to do so.