Organ sparing by conformal avoidance intensity modulated radiation therapy for anal cancer: Dosimetric evaluation of coverage of pelvis and inguinal/femoral nodes

Abstract

4257 Background: To describe a novel and straightforward conformal avoidance IMRT technique for coverage of pelvis and inguinal/femoral nodes and to compare the dosimetry of the new method with that of other methods of radiation treatment. Methods: Data of 2 patients with anal cancer were used as example cases to illustrate details and advantages of conformal avoidance IMRT technique. Conventional photons with enface electrons design was created first, thereby providing “outermost boundaries” defined as planning target volume (PTV) for subsequent conformal avoidance IMRT design. Organs at risk (OARs) including femoral head and neck and external genitalia were contoured as conformal avoidance structures. A step-and-shoot inverse IMRT planning was generated. For dosimetric comparison, technique by modified segmental boost was also generated. These treatment techniques were evaluated by dose volume histogram (DVH) of PTV and OARs. Dose profiles at 4 different depths from each treatment planning were generated. Results: DVH of PTV shows that coverage of the PTV is comparable among 3 treatment techniques. Volume of PTV receiving more than 90% prescription dose are in the range of 94 to 98% for the 3 treatment techniques and all have only 0 to 2% of PTV receiving more than 110% prescription dose. DVH of OARs confirms that both femoral head and neck and external genitalia could be spared well by conformal avoidance IMRT. Although greater inhomogeneity of dose distribution within the PTV is noted by conformal avoidance IMRT as shown by dose profiles at 4 depths, the maximum doses at different depths are less than 115% which are comparable to the planning by modified segmental boost technique. Planning by photons and enface electrons technique however shows a greater dose variation up to 134% of the prescription dose at 1.5 cm depth along photon-electron match-line. Conclusions: To cover pelvis and inguinal/femoral nodes conformal avoidance IMRT is technically simple to simulate, plan and execute. Dosimetric study demonstrated that it achieves comparable PTV coverage compared with other approaches while at the same time significantly spares the surrounding OARs. No significant financial relationships to disclose.