Abstract

Ka Fai Cheng¹, Oliver Tsz Hung Lam¹

¹Hong Kong Sanatorium and Hospital, Radiotherapy, Hong Kong, China

Compare and evaluate the dosimetric characteristics for cranium Stereotactic Radiosurgery (SRS) among Volumetric Modulated Arc Therapy (VMAT), Tomotherapy (Tomo) and Cyberknife (CK) techniques.

In a human phantom image, four spherical targets with the same location but different diameter of 1cm (0.52cc), 1.5cm (1.76cc), 3cm (14.14cc) and 5cm (65.6cc) were created. For each target size, with prescription of 20Gy in 1 fraction, five plans of different techniques, VMAT, Tomo, CK Fixed cone (CK Fixed), CK Iris Variable Aperture Collimator (CK Iris) and CK Multi-leaf Collimator (CK MLC) were created. All the plans were controlled to have at least 98% target volume coverage, Dmax < 25Gy and new conformity index (nCI) < 1.1, excepts 1cm target using VMAT, Tomo and CK-MLC which nCI ranged from 1.5 to 2. Gradient Index (GI**) and the distances of 50% and 70% isodose line (IL) from target edge*** were calculated. Theoretical beam-on time was compared.

*nCI = Target Volume x Volume of prescribed isodose / (Target Volume within prescribed isodose)²

**GI = Volume of 50% prescribed isodose / Volume of prescribed isodose

*** The distance of %IL from target edge = ³√{%IL volume/(4π/3)} – Target diameter/2

In general, CK plans had lower GI than VMAT and Tomo plans and this GI advantage decreased with increasing target size. Among the CK techniques, CK Fixed had the lowest GI for 1cm target. For 1.5cm and 3cm targets, CK Fixed and CK Iris had comparable GI and which were lower than CK MLC. The GI advantage of CK Fixed and CK Iris over CK MLC decreased with increasing target size. For 5 cm target, CK MLC had both advantages of lower GI and shorter beam-on time.

The distance of 70% IL/50% IL from the target provided a reference for expecting the maximum dose of a nearby organ. For example, for a 3cm spherical target using CK Fixed technique, maximum dose of an organ 0.3cm away from the target will be 70% of prescribed dose and maximum dose of an organ 0.55cm away from target will be 50% of prescribed dose.

CK had superior dose falloff compared to VMAT and Tomo. This advantage is most notable in the 1cm and 1.5cm target and least in the 5cm target. This suggested the dosimetric advantage of CK over VMAT and Tomo reduced as target size increases.Target size should be one of the considerations to selecting which CK techniques. CK Fixed seemed suitable for target size with diameter ≤ 1.5cm. CK Iris was suitable for target sizes between 1.5cm and 3cm. For target diameter ≥ 5cm, CK MLC might be the best choice.

In selecting treatment techniques, more factors other than dosimetry should be also considered, like treatment time, setup accuracy, imaging techniques, etc. For example, for a 3cm sphere target and patient in pain, CK MLC could be a better choice than CK Iris which sacrificed dosimetry slightly to reduce 24 mins beam-on time.