P Higgins

Single dose versus fractionated stereotactic radiotherapy for recurrent high-grade gliomas

PURPOSE To evaluate the efficacy of stereotactic radiotherapy (SRT) in patients with recurrent high-grade gliomas by comparing two different treatment regimens, single dose or fractionated radiotherapy. METHODS AND MATERIALS Between April 1991 and January 1998, 71 patients with recurrent high-grade gliomas were treated with SRT. Forty-six patients (65%) were treated with single dose radiosurgery (SRS) and 25 patients (35 %) with fractionated stereotactic radiotherapy (FSRT). For the SRS group, the median radiosurgical dose of 17 Gy was delivered to the median of 50% isodose surface (IDS) encompassing the target. For the FSRT group, the median dose of 37.5 Gy in 15 fractions was delivered to the median of 85% IDS. RESULTS Actuarial median survival time was 11 months for the SRS group and 12 months for the FSRT group (p = 0.3, log-rank test). Variables predicting longer survival were younger age (p = 0.006), lower grade (p = 0.0006), higher Karnofsky Performance Scale (KPS) (p = 0.0005), and smaller tumor volume (p = 0.02). Patients in the SRS group had more favorable prognostic factors, with median age of 48 years, KPS of 70, and tumor volume of 10 ml versus median age of 53 years, KPS of 60, and tumor volume of 25 ml in the FSRT group. Late complications developed in 14 patients in the SRS group and 2 patients in the FSRT group (p<0.05). CONCLUSION Given that FSRT patients had comparable survival to SRS patients, despite having poorer pretreatment prognostic factors and a lower risk of late complications, FSRT may be a better option for patients with larger tumors or tumors in eloquent structures. Since this is a nonrandomized study, further investigation is needed to confirm this and to determine an optimal dose/fractionation scheme.

Recommendations for clinical electron beam dosimetry: Supplement to the recommendations of Task Group 25

The goal of Task Group 25 (TG-25) of the Radiation Therapy Committee of the American Association of.Physicists in Medicine (AAPM) was to provide a methodology and set of procedures for a medical physicist performing clinical electron beam dosimetry in the nominal energy range of 5-25 MeV. Specifically, the task group recommended procedures for acquiring basic information required for acceptance testing and treatment planning of new accelerators with therapeutic electron beams. Since the publication of the TG-25 report, significant advances have taken place in the field of electron beam dosimetry, the most significant being that primary standards laboratories around the world have shifted from calibration standards based on exposure or air kerma to standards based on absorbed dose to water. The AAPM has published a new calibration protocol, TG-51, for the calibration of high-energy photon and electron beams. The formalism and dosimetry procedures recommended in this protocol are based on the absorbed dose to water calibration coefficient of an ionization chamber at 60Co energy, N60Co(D,w), together with the theoretical beam quality conversion coefficient k(Q) for the determination of absorbed dose to water in high-energy photon and electron beams. Task Group 70 was charged to reassess and update the recommendations in TG-25 to bring them into alignment with report TG-51 and to recommend new methodologies and procedures that would allow the practicing medical physicist to initiate and continue a high quality program in clinical electron beam dosimetry. This TG-70 report is a supplement to the TG-25 report and enhances the TG-25 report by including new topics and topics that were not covered in depth in the TG-25 report. These topics include procedures for obtaining data to commission a treatment planning computer, determining dose in irregularly shaped electron fields, and commissioning of sophisticated special procedures using high-energy electron beams. The use of radiochromic film for electrons is addressed, and radiographic film that is no longer available has been replaced by film that is available. Realistic stopping-power data are incorporated when appropriate along with enhanced tables of electron fluence data. A larger list of clinical applications of electron beams is included in the full TG-70 report available at http://www.aapm.org/pubs/reports. Descriptions of the techniques in the clinical sections are not exhaustive but do describe key elements of the procedures and how to initiate these programs in the clinic. There have been no major changes since the TG-25 report relating to flatness and symmetry, surface dose, use of thermoluminescent dosimeters or diodes, virtual source position designation, air gap corrections, oblique incidence, or corrections for inhomogeneities. Thus these topics are not addressed in the TG-70 report.

Deconvolution of detector size effect for small field measurement

Parametrization of the small fields employed in stereotactic applications is a painstaking process involving extensive film dosimetry to achieve acceptable beam edge definition. Use of cylindrical or spherical detectors for profile measurements would simplify data acquisition but add a volume averaging artifact to beam edge definition. We demonstrate a simple approach to unfolding the chamber size artifact from measured small beam profiles using typical cylindrical chambers. In comparison with film measurements we have found good agreement when the detector response function is deconvoluted from the measured profiles, although the amount of correction needed is fairly minimal for the detectors studied.

Patient selection criteria for the treatment of brain metastases with stereotactic radiosurgery

In this study we evaluate prognostic factors that predict local-regional control and survival following stereotactic radiosurgery (SRS) in patients with brain metastasis and establish guidelines for patient selection. Our evaluation is based on 73 patients with brain metastasis treated with SRS at the University of Minnesota between March 1991 and November 1995. The ability of stereotactic radiosurgery to improve local control in patients with brain metastases is confirmed in our study in which only 6 of 62 patients failed locally after SRS, with an actuarial local progression-free survival of 80% at 2 years. Variables that predicted worse prognosis were larger tumor size (p=0.05) for local progression-free survival and multiplicity of metastasis (p=0.03) and infratentiorial location of metastases (p=0.006) for regional progression-free survival. Absence of extracranial disease, KPS ≥ 70, and single intracranial metastasis were significant predictors of longer survival. Patients who fulfill all three criteria will survive longer after SRS (MS=17.7 months) and will most likely benefit from the increase local control in the brain achieved by SRS. Survival in patients who do not meet any of these criteria is very poor (MS=1.5 months), and these patients are less likely to benefit from this treatment. Careful selection of patients for SRS is warranted.

In vivo diode dosimetry for routine quality assurance in IMRT.

Due to the complexity of IMRT dosimetry, dose delivery evaluation is generally done using a treatment plan in which the optimized fluence distribution has been transferred to a test phantom for accessibility and simplicity of measurement. The actual patient doses may be reconstructed in vivo through the use of electronic portal imaging devices or films, but the assessment of absolute dose from these measurements is time-consuming and complicated. In our clinic we have instituted the use of routine diode dosimetry for IMRT patients following the same procedure used for standard radiation therapy patients in which each new treatment field is checked at the start of treatment. For standard cases the dose at dmax is calculated as part of the monitor unit calculation. For the IMRT cases, the dose contribution to the dmax depth for each field is taken from the treatment plan. We found that about 90% of the diode measurements agreed to within +/- 10% of the planned doses (45/51 fields) and 63% (32/51 fields) achieved +/- 5% agreement. By using this direct in vivo method to verify the clinical doses delivered, we have been able to make a uniform startup procedure for all patients while simplifying our IMRT QA process.

Evaluation of surface and superficial dose for head and neck treatments using conventional or intensity-modulated techniques

With increased use of intensity-modulated radiation therapy (IMRT) for head and neck treatment questions have arisen as to selection of an optimum treatment approach when either superficial sparing or treatment is desired. Other work has pointed out the increased superficial dose resulting from obliquity effects when multiple tangential beams are applied to head and neck treatment, as is the general case in IMRT planning. Helical tomotherapy might be expected to result in even further enhanced superficial dose compared with conventional bilateral field treatment. We have designed a typical right oropharynx target volume in an anthropomorphic head and neck phantom. Three different treatment techniques have been used to optimally treat this target, including bilateral static fields, eight-field IMRT and helical tomotherapy. The phantom was immobilized in a standard treatment position and treated on a Varian 2300cd linear accelerator and on a Hi-Art Helical Tomotherapy unit. 1 mm3 lithium-fluoride thermoluminescent dosimeters (TLDs) were placed on the surface of the phantom at a number of axial test positions. Film strips (Kodak EDR2) were either wrapped around the surface or sandwiched within the phantom. Measured doses at the surface and as a function of depth are compared with the planning system predictions for each treatment technique. The maximum surface doses on the proximal treatment side, averaged from TLDs and films, were measured to be 69-82% of the target dose with the bilateral fields yielding the lowest surface doses (69%), tomotherapy about 2% more than that (71%) and IMRT 13% more (82%). Anterior to the target volume, doses are always low for bilateral treatment. In this case the minimum anterior surface dose (chin area) was 6% of the prescription dose from that technique as compared with 26% and 35% from the IMRT and tomotherapy methods, respectively. The Eclipse and Tomotherapy planning systems both modelled deep and superficial doses well. Surface doses were better modelled by Eclipse at the test points, while the tomotherapy plans consistently overestimated the measured doses by 10% or more. Depth dose measurements, extracted from embedded films, indicated the depth of dose build-up to >99% to be the shallowest for IMRT (2-5 mm) followed by tomotherapy (5-8 mm) and bilateral fields (10-15 mm). The amount of surface dose is clearly technique dependent and should be taken into account in the planning stage.

Calculation of depth dose and dose per monitor unit for irregularly shaped electron fields

A new dosimetric quantity, the lateral build-up ratio (LBR), has been introduced to calculate depth dose distribution for any shaped field. Factors to account for change in incident fluence with collimation are applied separately. The LBR data for a small circular field are used to extract radial spread of the pencil beam, sigma(r), as a function of depth and energy. By using the relationship between LBR, sigma(r), energy and depth, a formalism is developed to calculate dose per monitor unit for any shaped field. Criteria for lateral scatter equilibrium are also developed which are useful in clinical dosimetry.

Helical tomotherapy targeting total bone marrow - first clinical experience at the University of Minnesota.

Total body irradiation (TBI) has been widely utilized as part of the conditioning regimen for hematopoietic cell transplantation [1]. However, with traditional TBI techniques the entire body is irr...

Single dose versus fractionated stereotactic radiotherapy for meningiomas

OBJECTIVE To evaluate the safety and efficacy of stereotactic radiosurgery (SRS) compared to fractionated stereotactic radiation therapy (FSRT) for meningiomas treated over a seven year period. METHODS AND MATERIALS Of the 53 patients (15 male and 38 female) with 63 meningiomas, 35 were treated with SRS and the 18 patients with tumors adjacent to critical structures or with large tumors were treated with FSRT. The median doses for the SRS and the FSRT groups were 1400 cGy (500-4500 cGy) and 5400 cGy (4000-6000 cGy) respectively. Median target volumes for SRS and FSRT were 6.8 ml and 8.8 ml respectively. The median follow-up for the SRS and FSRT groups were 38 months (4.1-97 months) and 30.5 months (6.0-63 months) respectively. RESULTS The five-year tumor control probability (TC) for benign versus atypical meningiomas were 92.7% vs. 31% (P = .006). The three-year TC were 92.7% vs. 93.3% for SRS vs. FSRT groups respectively (P = .62). For benign meningiomas, the three-year TC were 92.9% vs. 92.3% for the SRS group (29 patients) vs. FSRT group (14 patients) respectively (P = .77). Two patients in the SRS group and one in the FSRT group developed late complications. CONCLUSION Preliminary data suggest that SRS is a safe and effective treatment for patients with benign meningiomas. Fractionated stereotactic radiation therapy with conventional fractionation appeared to be an effective and safe treatment alternative for patients not appropriate for SRS. A longer follow-up is required to determine the long-term efficacy and the toxicity of these treatment modalities.

Patient specific 3D printed phantom for IMRT quality assurance

The purpose of this study was to test the feasibility of a patient specific phantom for patient specific dosimetric verification.Using the head and neck region of an anthropomorphic phantom as a substitute for an actual patient, a soft-tissue equivalent model was constructed with the use of a 3D printer. Calculated and measured dose in the anthropomorphic phantom and the 3D printed phantom was compared for a parallel-opposed head and neck field geometry to establish tissue equivalence. A nine-field IMRT plan was constructed and dose verification measurements were performed for the 3D printed phantom as well as traditional standard phantoms.The maximum difference in calculated dose was 1.8% for the parallel-opposed configuration. Passing rates of various dosimetric parameters were compared for the IMRT plan measurements; the 3D printed phantom results showed greater disagreement at superficial depths than other methods.A custom phantom was created using a 3D printer. It was determined that the use of patient specific phantoms to perform dosimetric verification and estimate the dose in the patient is feasible. In addition, end-to-end testing on a per-patient basis was possible with the 3D printed phantom. Further refinement of the phantom construction process is needed for routine use.