Anatomy-based volumetric modulated arc therapy for a prostate cancer patient with a hip prosthesis

High-atomic number materials such as those used in making artificial bones or prostheses, spinal cord fixation devices and dental fillings may be located in patients undergoing radiotherapy. These devices may have a major influence on absorbed dose distributions in the body, as they cause significant beam attenuation and scattering. A report of the American Association of Physicists in Medicine described the clinical dosimetry methodology used for a patient with a hip prosthesis and recommended beam arrangements without passing through the prosthesis [1]. Volumetric modulated arc therapy (VMAT) has been increasingly used for the treatment of patients with prostate cancer because it is associated with fewer monitor units and higher patient throughput than those in step-and-shoot intensity modulated radiotherapy [2]. However, to the authors’ best knowledge, no reports have been published regarding VMAT for a prostate cancer patient with a hip prosthesis. The purpose of this letter is to propose a simple anatomy-based VMAT strategy for a prostate cancer patient with a hip prosthesis, using ERGO ++ (Elekta, Stockholm Sweden) [3], a treatment planning system (TPS) based on hollow-out conformation radiotherapy [4]. ERGO++VMAT allows the delivery of a uniform dose to the prostate target, while efficiently protecting the rectum and other healthy organs. As a routine prostate VMAT planning procedure in ERGO+ +, either conformal or conformal avoidance fields were applied when the target was in front of the rectum, or when the rectum was in front of the target, respectively. Subsequently, VMAT optimization was performed by determining the best beam intensity variations for all the beam directions under the given dose constraints. Our findings were as follows: (i) a clinically acceptable plan was successfully established by considering a unilateral hip prosthesis and contralateral femur in addition to the rectum when conformal avoidance fields were defined, and (ii) a single-rotation VMAT required three separate subarcs, thereby avoiding beam angles that directly went through the prosthesis and femur. A VMAT plan was established using ERGO ++ version 1.7.4. For the patient with a prosthesis, Versys HA/TCP Fiber Metal Midcoat Stem, Multipolar Cup, and Nitride Head/Neck (Zimmer, USA) were used in place of the right femur, which had a stem thickness of 14 mm, a head size of 52 mm, and a neck size of 3.5 × 22 mm. The stem was made of a titanium alloy (90% titanium), and the head was made of a cobalt-chromium alloy. The clinical target volume (CTV) consisted of the entire prostate and partial seminal vesicles, as visualized on the CT axial images. A planning target volume (PTV) was obtained by adding a margin of 5 mm in the dorsal direction and a margin of 8 mm in the other directions to the CTV. A planning organ-at-risk volume (PRV) was obtained by adding an isotropic margin of 2 mm to the prosthesis for more complete beam shielding. A multileaf collimator (MLC) margin of 6 mm was further applied to compensate for the beam penumbra. Figure 1 shows the beam’s eye views for a singlerotation anatomy-based VMAT plan. We found that an arc of 360° needed to be divided into 3 separate subarcs at the beam angles, where the PRV or left femur was positioned right in front of the PTV. Conformal fields were applied when the PTV was in front of the PRV, rectum or left femur, whereas conformal avoidance fields were applied when the PRV, rectum or left femur was in front of the PTV. When the PRV was in front of the PTV (in subarcs 1 and 2), the PRV was fully shielded. Initially, the left femur was not shielded; therefore, two separate subarcs were used, resulting in significantly higher doses in the left femur and left-side pelvic bone. To reduce the doses in these regions, the left femur was shielded, thereby leading to a 3-subarc plan. For the best radiation shield, the Journal of Radiation Research, 2013, 54, 398–400 Letter to the Editor doi: 10.1093/jrr/rrs101 Advance Access Publication 29 November 2012