Matt Tobler

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.

Graduated block technique for the treatment of paranasal sinus tumors.

Cancers of the head and neck often present difficult dosimetric challenges; tumors of the paranasal sinuses, often advanced at diagnosis, pose several problems in treatment planning. Adequate coverage of involved areas often necessitates inclusion of the ipsilateral orbit due to direct extension of disease; sparing the uninvolved contralateral orbit may be difficult, especially if the superior nasal cavity and ethmiod sinus must be treated. We will report on a technique that allows delivery of a relatively homogeneous dose to a treatment volume that includes the paranasal sinus and ipsilateral orbit, with significant sparing of the anterior chamber of the contralateral eye. This technique uses a heavily weighted anterior field designed to deliver 100% to a plane posterior to the lens of each eye. From this plane posteriorly, lateral wedged fields are employed to increase the dose as the anterior contribution decreases. To achieve maximum homogeneity would require a wedge angle of greater than 60 degrees, the maximum wedge commonly available. To overcome this restraint, this technique uses multiple lateral wedged fields whose anterior field edges graduate in a posterior direction allowing for further compensation of the anterior fields fall-off. Film densitometry using a Rando phantom* is used to verify the technique.

Design and production of wax compensators for electron treatments of the chest wall

The primary aim of electron treatment planning for the post mastectomy chest wall is to encompass the volume between the skin surface and the lung-rib interface while limiting dose to the lung. Electron energies for treatment of the chest wall are chosen based on the thickness of tissue between these two areas. Surgical defects or surface irregularities often result in differing thicknesses of tissue across the treatment volume, and patient-specific compensation is necessary to achieve the desired dose distribution. This is true whether the treatment plan is designed using fixed or rotational electrons to treat the chest wall. These clinical requirements are often met using custom shaped wax of varying thickness which conforms to the chest surface. This paper will discuss the treatment planning process used to design these compensators, creation and use of an exact duplicate of the patients chest wall to aid in the production of these compensators, the production process itself, and verification of the completed compensator.

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 conformality. 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 the experience gained here, we provide recommended workflows for each ITPS to best exploit the fluence‐smoothing features of the system. PACS numbers: 87.56.bd, 87.56.N‐

Dosimetric evaluation of a specialized radiotherapy treatment technique for scleredema

Scleredema adultorum of Buschke is an unusual manifestation of diabetes mellitus that can result in painful indurations and thickening of the skin with associated limitation of motion, and has been previously reported to be responsive to radiotherapy. We report on the method employed to treat a severely affected patient by means of opposed photon fields and multiple electron fields encompassing his entire neck, arms, and thorax.

Dosimetry and treatment planning of complex electron arc therapy

Electron arc therapy has been primarily utilized for treatment of the post-mastectomy chest wall. Its clinical usefulness and benefits have been proven. Casting procedures and blocking techniques have been discussed. We have successfully applied electron arc therapy to the treatment of the nasal cavity and associated nodal regions. When compared to the chest wall, the smaller radii and pronounced irregularity of the surface anatomy of the facial region presents multiple new dosimetric and treatment planning challenges. We will discuss these challenges, along with those encountered in the casting procedures and reproduction of patient characteristics required for the execution of this treatment. Many aspects of dosimetry were utilized in unique ways to produce the treatment plans, cast and bolus needed. Due to the insertion of bolus into the nostrils, a unique hollow bite block was constructed to allow patient respiration during treatment. Film dosimetry was used to verify computer predictions and to compare this electron arc treatment technique with alternative fixed electron beam techniques. The potential benefits and difficulties of this technique will be discussed.

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.