Anniversary Paper: the role of medical physicists in developing stereotactic radiosurgery.

This article is a tribute to the pioneering medical physicists over the last 50 years who have participated in the research, development, and commercialization of stereotactic radiosurgery (SRS) and stereotactic radiotherapy utilizing a wide range of technology. The authors have described the evolution of SRS through the eyes of physicists from its beginnings with the Gamma Knife in 1951 to proton and charged particle therapy; modification of commercial linacs to accommodate high precision SRS setups; the multitude of accessories that have enabled fine tuning patients for relocalization, immobilization, and repositioning with submillimeter accuracy; and finally the emerging technology of SBRT. A major theme of the article is the expanding role of the medical physicist from that of advisor to the neurosurgeon to the current role as a primary driver of new technology that has already led to an adaptation of cranial SRS to other sites in the body, including, spine, liver, and lung. SRS continues to be at the forefront of the impetus to provide technological precision for radiation therapy and has demonstrated a host of downstream benefits in improving delivery strategies for conventional therapy as well. While this is not intended to be a comprehensive history, and the authors could not delineate every contribution by all of those working in the pursuit of SRS development, including physicians, engineers, radiobiologists, and the rest of the therapy and dosimetry staff in this important and dynamic radiation therapy modality, it is clear that physicists have had a substantial role in the development of SRS and theyincreasingly play a leading role in furthering SRS technology.

Dynamic collimator optimization compared with fixed collimator angle in arc-based stereotactic radiotherapy: a dosimetric analysis.

OBJECT: The purpose of this study was to determine the effect of static and dynamic collimator optimization when using a micromultileaf collimator (mMLC) in dynamic-arc stereotactic radiosurgery (SRS) by evaluating the dose to healthy peritumoral tissue. METHODS: Thirty patients previously treated for intracranial lesions with the BrainLAB mMLC underwent retrospective replanning. Three collimator optimization strategies were compared for a simulated SRS treatment plan, as follows: Strategy 1, static collimation fixed at 90 degrees throughout arcs; Strategy 2, static collimator settings optimized for each arc; and Strategy 3, dynamic collimator settings optimized every 10 degrees throughout treatment arcs. Dose-volume histograms for a 0.7-cm shell of healthy peritumoral tissue were quantitatively compared. Collimator optimization schemes (Strategies 2 and 3) significantly decreased the volume of peritumoral tissue that is irradiated when compared with static collimation at 90 degrees (Strategy 1). The volume was reduced by 40.6% for Strategy 2 (95% confidence interval [CI] +/- 11) and by 47.1% for Strategy 3 (95% CI +/- 8.1) at the 95% isodose; by 28.4% for Strategy 2 (95% CI +/- 4.9) and 39.1% for Strategy 3 (95% CI +/- 6) at the 90% isodose; and by 18.2% for Strategy 2 (95% CI +/- 8.1) and 25.4% for Strategy 3 (95% CI +/- 7.1) at the 80% isodose. Serial collimator optimization throughout the treatment arcs (Strategy 3) reduced the mean volume of peritumoral tissue irradiated when compared with static collimator optimization (Strategy 2), by 16.1% (95% CI +/- 1.5) at 95% isodose, by 11.7% (95% CI +/- 1) at 90% isodose, and by 8.2% (95% CI +/- 1.2) at 80% isodose regions. In specific cases, linear or polynomial functions were formulated to optimize collimator settings dynamically throughout treatment arcs. CONCLUSIONS: Dynamic collimator optimization during arc-based SRS decreases the volume of healthy peritumoral tissue treated with high doses of radiation and appears to be an effective method of improving target conformality. This study is the first step toward determination of a smoothing function algorithm to allow for true dynamic collimation during SRS.

IMRT fluence map editing to control hot and cold spots.

Manually editing intensity-modulated radiation therapy (IMRT) fluence maps effectively controls hot and cold spots that the IMRT optimization cannot control. Many times, re-optimizing does not reduce the hot spots or increase the cold spots. In fact, re-optimizing only places the hot and cold spots in different locations. Fluence-map editing provides manual control of dose delivery and provides the best treatment plan possible. Several IMRT treatments were planned using the Varian Eclipse planning system. We compare the effects on dose distributions between fluence-map editing and re-optimization, discuss techniques for fluence-map editing, and analyze differences between fluence editing on one beam vs. multiple beams. When editing a beam's fluence map, it is essential to choose a beam that least affects dose to the tumor and critical structures. Editing fluence maps gives an advantage in treatment planning and provides controlled delivery of IMRT dose.

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.

Optimization of the primary collimator settings for fractionated IMRT stereotactic radiotherapy.

Advances in field-shaping techniques for stereotactic radiosurgery/radiotherapy have allowed dynamic adjustment of field shape with gantry rotation (dynamic conformal arc) in an effort to minimize dose to critical structures. Recent work evaluated the potential for increased sparing of dose to normal tissues when the primary collimator setting is optimized to only the size necessary to cover the largest shape of the dynamic micro multi leaf field. Intensity-modulated radiotherapy (IMRT) is now a treatment option for patients receiving stereotactic radiotherapy treatments. This multisegmentation of the dose delivered through multiple fixed treatment fields provides for delivery of uniform dose to the tumor volume while allowing sparing of critical structures, particularly for patients whose tumor volumes are less suited for rotational treatment. For these segmented fields, the total number of monitor units (MUs) delivered may be much greater than the number of MUs required if dose delivery occurred through an unmodulated treatment field. As a result, undesired dose delivered, as leakage through the leaves to tissues outside the area of interest, will be proportionally increased. This work will evaluate the role of optimization of the primary collimator setting for these IMRT treatment fields, and compare these results to treatment fields where the primary collimator settings have not been optimized.

The application of dynamic field shaping and dynamic dose rate control in conformal rotational treatment of the prostate.

The current philosophy of dose escalation in the treatment of prostate cancer has forced the treatment planner to re-evaluate his/her planning approach. Precise and accurate delivery of dose to the prostate while maintaining the required dose limits to the normal critical structures, such as the rectum, has become increasingly difficult in light of these escalated doses. Conformal treatment techniques allow the treatment planner to precisely shape each individual treatment field so that desired volume coverage and normal tissue sparing can be achieved. In addition to these beam-shaping advantages, adjustment of an individual beam's weighting also helps to create the desired distribution and tissue sparing. Rotational therapy "simulates" treatment with multiple beams and angles, similar to the thought process behind conformal treatment technique. With rotational therapy, however, the treatment planner's inability to provide adequate beam shaping and weighting adjustment has placed limits on its value as a viable planning option. The introduction of computer-controlled treatment machines, which allow dynamic adjustment of the field shape with the rotation of the beam, makes it possible to re-evaluate rotational therapy as a potential option. Similarly, the treatment planner's ability to change field weighting can be accomplished by the application of dynamic dose rate control, allowing a rotational beam to deliver a weighting similar to that possible with conformal fixed-field techniques. Dose-volume histogram data will be used to evaluate doses delivered to the prostate, rectum, and bladder using rotational therapy with dynamic field shape and dynamic dose rate control as a treatment planning option. The dose delivery and normal tissue-sparing potential of this technique compared to coplanar and noncoplanar conformal fixed-field techniques will also be presented.

Intensity-modulated photon arc therapy for treatment of pleural mesothelioma.

Radiotherapy plays a key role in the definitive or adjuvant management of patients with mesothelioma of the pleural surface. Many patients are referred for radiation with intact lung following biopsy or subtotal pleurectomy. Delivery of efficacious doses of radiation to the pleural lining while avoiding lung parenchyma toxicity has been a difficult technical challenge. Using opposed photon fields produce doses in lung that result in moderate-to-severe pulmonary toxicity in 100% of patients treated. Combined photon-electron beam treatment, at total doses of 4250 cGy to the pleural surface, results in two-thirds of the lung volume receiving over 2100 cGy. We have developed a technique using intensity-modulated photon arc therapy (IMRT) that significantly improves the dose distribution to the pleural surface with concomitant decrease in dose to lung parenchyma compared to traditional techniques. IMRT treatment of the pleural lining consists of segments of photon arcs that can be intensity modulated with varying beam weights and multileaf positions to produce a more uniform distribution to the pleural surface, while at the same time reducing the overall dose to the lung itself. Computed tomography (CT) simulation is critical for precise identification of target volumes as well as critical normal structures (lung and heart). Rotational arc trajectories and individual leaf positions and weightings are then defined for each CT plane within the patient. This paper will describe the proposed rotational IMRT technique and, using simulated isodose distributions, show the improved potential for sparing of dose to the critical structures of the lung, heart, and spinal cord.

Forward planning using multileaf collimation as a replacement for patient tissue compensation.

In treatment planning, a dosimetrist may encounter a technique that would best be treated by including some type of compensation to correct for tissue or depth variations throughout the field, allowing for a more homogeneous dose distribution. Recent innovations, such as intensity-modulated radiotherapy (IMRT), have been introduced in an effort to address these issues. In many institutions, however, the treatment planning capabilities available may not accommodate consideration of such new technologies. The treatment planner is therefore left to determine how to incorporate these concepts with the current technologies available. While compensation may be an option, this may not always be possible due to the position of the beam or to actual mechanical restraints. Some institutions may also lack the ability and equipment to consider compensation at all. The answer is forward planning IMRT. This concept combines current forward planning techniques with multiple asymmetrically blocked treatment fields, varying the intensity of the beam from a given orientation to produce the desired treatment plan.

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.

Dose calculation errors due to inaccurate representation of heterogeneity correction obtained from computerized tomography.

Computerized tomography (CT) is used routinely in evaluating radiation therapy isodose plans. With the introduction of 3D algorithms such as the voxel raytrace, which determines inhomogeneity corrections from actual CT Hounsfield numbers, caution must be used when evaluating isodose calculations. Artifacts from contrast media and dental work, radiopaque markers placed by the treatment planner, and changing bowel and rectal air patterns all have the potential to introduce error into the calculation due to inaccurate assessment of high or low density. Radiopaque makers such as x-spot BB's or solder wire are placed externally on the patient. Barium contrast media introduced at the time of simulation may be necessary to visualize specific anatomical structures on the CT images. While these localization and visualization tools may be necessary, it is important to understand the effects they may introduce in the planning process. Other problems encountered are patient specific and out of the control of the treatment planner. These include high- and low-density streaking caused by dental work, which produce computational errors due to overestimation, and small bowel and rectal air, the patterns of which change on a daily basis and may result in underestimation of structure density. It is important for each treatment planner to have an understanding of how this potentially tainted CT information may be applied in dose calculations and the possible effects they may have. At our institution, the voxel raytrace calculation is automatically forced any time couch angle is introduced. Errors in the calculation from the above mentioned situations may be introduced if a heterogeneity correction is applied. Examples of potential calculation errors and the magnitude of each will be discussed. The methods used to minimize these errors and the possible solutions will also be evaluated.

Electron arc irradiation of the postmastectomy chest wall in locally recurrent and metastatic breast cancer.

The purpose of this study was to evaluate local-regional control and overall survival in women with locally recurrent and metastatic breast cancer (MBC) treated with postmastectomy electron arc therapy. Postmastectomy electron arc irradiation was used to treat 39 women with isolated local-regional recurrence of breast cancer following mastectomy, and 14 patients with MBC who had, or who were at high risk of, local-regional recurrence. After computed tomography treatment planning, patients were treated with electron arc radiotherapy to a median dose of 59.3 Gy. The median follow-up for alive patients was 45.4 months. For patients with local-regional recurrence, the 5-year local-regional control and overall survival rates were 74% and 43%, respectively. The 2-year overall survival was greater for those patients with a disease-free interval greater than 24 months when compared to patients with a disease-free interval less than 24 months (83% vs. 60%, respectively); however, the median survival was not significantly different (57.6 and 58.6 months, respectively). Patients with a solitary nodule at recurrence had an improved 5-year overall survival of 58% compared with 40% for patients with multiple lesions. For patients with metastatic disease, the 5-year local-regional control and overall survival rates were 76% and 31%, respectively. Local-regional control can be achieved in the majority of patients with local-regionally recurrent breast cancer (74%) or MBC (76%) who had, or who were, at high risk of local-regional recurrence treated with postmastectomy electron arc irradiation.