Evaluation of fluence-smoothing feature for three IMRT planning systems.

Commercially available intensity-modulated radiation therapy (IMRT) inverse treatment planning systems (ITPS) typically include a smoothing function which allows the user to vary the complexity of delivered beam fluence patterns. This study evaluated the behavior of three ITPSs when varying smoothing parameters. We evaluated four cases treated with IMRT in our clinic: sinonasal carcinoma (SNC), glioblastoma multiforme (GBM), base of tongue carcinoma (BOT), and prostate carcinoma (PST). Varian Eclipse v6.5, BrainLAB BrainScan v5.31, and Nomos Corvus v6.2 ITPSs were studied for the SNC, GBM, and PST sites. Only Eclipse and Corvus were studied for BOT due to field size constraints of the BrainLAB MM3 collimator. For each ITPS, plans were first optimized using vendor- recommended default "smoothing" values. Treatment plans were then reoptimized, exploring various smoothing values. Key metrics recorded included a delivery complexity (DC) metric and the Ian Paddick Conformality Index (IPCI). Results varied widely by vendor with regard to the impact of smoothing on complexity and conformality. Plans run on the Corvus ITPS showed the logically anticipated increase in DC as smoothing was decreased, along with associated improved organ-at-risk (OAR) sparing. Both Eclipse and BrainScan experienced an expected trend for increased DC as smoothing was decreased. However, this increase did not typically result in appreciably improved OAR sparing. For Eclipse and Corvus, and to a much lesser extent BrainScan, increases in smoothing decreased DC but eventually caused unacceptable losses in plan quality. Depending on the ITPS, potential benefits from optimizing fluence smoothing levels can be significant, allowing for increases in either efficiency or conformality. Because of variability in smoothing function behavior by ITPS, it is important that users familiarize themselves with the effects of varying smoothing parameters for their respective ITPS. Based on experience gained here, we provide recommended workflows for each ITPS to best exploit the fluence-smoothing features of the system.

Optimization of collimator parameters to reduce rectal dose in intensity-modulated prostate treatment planning.

The inability to avoid rectal wall irradiation has been a limiting factor in prostate cancer treatment planning. Treatment planners must not only consider the maximum dose that the rectum receives throughout a course of treatment, but also the dose that any volume of the rectum receives. As treatment planning techniques have evolved and prescription doses have escalated, limitations of rectal dose have remained an area of focus. External pelvic immobilization devices have been incorporated to aid in daily reproducibility and lessen concern for daily patient motion. Internal immobilization devices (such as the intrarectal balloon) and visualization techniques (including daily ultrasound or placement of fiducial markers) have been utilized to reduce the uncertainty of intrafractional prostate positional variation, thus allowing for minimization of treatment volumes. Despite these efforts, prostate volumes continue to abut portions of the rectum, and the necessary volume expansions continue to include portions of the anterior rectal wall within high-dose regions. The addition of collimator parameter optimization (both collimator angle and primary jaw settings) to intensity-modulated radiotherapy (IMRT) allows greater rectal sparing compared to the use of IMRT alone. We use multiple patient examples to illustrate the positive effects seen when utilizing collimator parameter optimization in conjunction with IMRT to further reduce rectal doses.

The application of electron beam delivery using dose rate variation and dynamic couch motion in conformal treatment of the cranial-spinal axis.

Radiation therapy to the cranial-spinal axis is typically targeted to the spinal cord and to the cerebrospinal fluid (CSF) in the subarachnoid space adjacent to the spinal cord and brain. Standard techniques employed in the treatment of the whole central nervous system do little to compensate for the varying depths of spinal cord along the length of the spinal field. Lateral simulation films, sagittal magnetic resonance imaging (MRI), or computerized tomography (CT) are used to estimate an average prescription depth for treatment along the spine field. However, due to the varying depth of the target along the spinal axis, even with the use of physical compensators, there can be considerable dose inhomogeneity along the spine field. With the advent of treatment machines that have full dynamic capabilities, a technique has been devised that will allow for more conformal dose distribution along the full length of the spinal field. This project simulates this technique utilizing computer-controlled couch motion to deliver multiple small electron beams of differing energies and intensities. CT planning determines target depth along the entire spine volume. The ability to conform dose along the complete length of the treatment field is investigated through the application of superpositioning of the fields as energies and intensities change. The positioning of each beam is registered with the treatment couch dynamic motion. This allows for I setup in the treatment room rather than multiple setups for each treatment position, which would have been previously required. Dose-volume histograms are utilized to evaluate the dose delivered to structures in the beam exit region. This technique will allow for precise localization and delivery of a homogeneous dose to the entire CSF space.