Development of a planning class solution for VMAT treatment of anal cancer

Abstract

Anal cancer target volumes are large and complex when organs at risk (OAR) are to be spared. Volumetric modulated arc therapy (VMAT) is a state of the art technique that should produce a significant improvement in OAR shielding compared to 3D conformal radiotherapy (3D-CRT). The purpose of this study was to develop a clinically acceptable VMAT planning class solution for anal cancer patients which is preferable to the present 3DCRT in terms of dose conformity and organ at risk sparing. A VMAT planning class solution was developed using a suitable treatment planning system (TPS). The study was based on retrospective CT datasets of 6 anal cancer patients who had been already treated with 3D-CRT; five patients were staged T1-T2/N0 and one patient T3-T4/N1. Three VMAT plans were generated for each patient using the class solution; a double arc plan with a flattening-filter beam (FF), a double arc plan with flattening-filter free (FFF) beam and a single arc plan (SA). The three plans generated for each patient were compared in terms of the dose conformity, homogeneity, target coverage, organs at risk sparing, monitor units per fraction (MU), treatment time (TTT) and number of segments (NS). The three investigated VMAT treatment techniques produced similar target conformity but offered significant improvement in OAR sparing compared to 3DCRT. The double arc VMAT plans achieved similar target coverage as the SA plan but were statistically significantly better in terms of organs at risk sparing, particularly in sparing the small bowel, femoral heads, bladder and healthy tissue. Mean and Standard Error of Mean TTT, MUs and NS for the FF beam were 233±23s/808±92/136±6; that of the FFF beam 179±25 s/960±156/151±5 and that of the SA plan 193±7 s/1096±72/148±7. The VMAT clinical plans offer a significant improvement over 3D-CRT plans in terms of dose conformity and organs at risk sparing. This should produce less radiation-related toxicity. Double arc VMAT is the recommended treatment modality. The use of a FFF beam reduces treatment time significantly and produces similar dosimetric results as a FF beam.