Conventional (2D) Versus Conformal (3D) Techniques in Radiotherapy for Malignant Pediatric Tumors: Dosimetric Perspectives.

OBJECTIVES In pediatric radiotherapy, the enhanced radiosensitivity of the developing tissues combined with the high overall survival, raise the possibility of late complications. The present study aims at comparing two dimensional (2D) and three dimensional (3D) planning regarding dose homogeneity within target volume and dose to organs at risk (OARs) to demonstrate the efficacy of 3D in decreasing dose to normal tissue. MATERIAL AND METHODS Thirty pediatric patients (18 years or less) with different pediatric tumors were planned using 2D and 3D plans. All were CT scanned after proper positioning and immobilization. Structures were contoured; including the planning target volume (PTV) and organs at risk (OARs). Conformal beams were designed and dose distribution analysis was edited to provide the best dose coverage to the PTV while sparing OARs using dose volume histograms (DVHs) of outlined structures. For the same PTVs conventional plans were created using the conventional simulator data (2-4 coplanar fields). Conventional and 3D plans coverage and distribution were compared using the term of V95% (volume of PTV receiving 95% of the prescribed dose), V107% (volume of PTV receiving 107% of the prescribed dose), and conformity index (CI) (volume receiving 90% of the prescribed dose/PTV). Doses received by OARs were compared in terms of mean dose. In children treated for brain lesions, OAR volume received 90% of the dose (V90%) and OAR score were calculated. RESULTS The PTV coverage showed no statistical difference between 2D and 3D radiotherapy in terms of V95% or V107%. However, there was more conformity in 3D planning with CI 1.43 rather than conventional planning with CI 1.86 (p-value <0.001). Regarding OARs, 3D planning shows large gain in healthy tissue sparing. There was no statistical difference in mean dose received by each OAR. However, for brain cases, brain stem mean dose and brain V90% showed better sparing in 3D planning (brain stem mean dose was 61% in 2D and 51% in 3D (pvalue 0.0001) and Brain V90% was 17.6 in 2D and 8.6 in 3D (p-value 0.001). Similarly, there was overall significant decrease in doses receive by healthy tissues in term of OARs score which was 0.24 in 3D and 0.40 in 2D planning (p-value 0.0001). CONCLUSION This study confirms that 3D conformal radiotherapy is more effective than 2D conventional radiotherapy in decreasing dose to normal tissue without compromising dose distribution, homogeneity and dose coverage to PTV. KEY WORDS Conformal radiation therapy - Pediatric tumors - Dosimetry.