Potential value of MR spectroscopic imaging for the radiosurgical management of patients with recurrent high-grade gliomas.

Previous studies have shown that metabolic information provided by 3D Magnetic Resonance Spectroscopy Imaging (MRSI) could affect the definition of target volumes for radiation treatments (RT). This study aimed to (i) investigate the effect of incorporating spectroscopic volumes as determined by MRSI on target volume definition, patient selection eligibility, and dose prescription for stereotactic radiosurgery treatment planning; (ii) correlate the spatial extent of pre-SRS spectroscopic abnormality and treatment volumes with areas of focal recurrence as defined by changes in contrast enhancement; and (iii) examine the metabolic changes following SRS to assess treatment response. Twenty-six patients treated with Gamma Knife radiosurgery for recurrent glioblastoma multiforme (GBM) were retrospectively evaluated. All patients underwent both MRI and MRSI studies prior to SRS. Follow-up MRI exams were available for all 26 patients, with MRI/MRSI available in only 15/26 patients. We observed that the initial CNI 2 contours extended beyond the pre-SRS CE in 25/26 patients ranging in volume from 0.8 cc to 18.8 cc (median 5.6 cc). The inclusion of the volume of CNI 2 extending beyond the CE would have increased the SRS target volume by 5-165% (median 23.4%). This would have necessitated decreasing the SRS prescription dose in 19/26 patients to avoid increased toxicity; the resultant treatment volume would have exceeded 20cc in five patients, thus possibly excluding those from RS treatment per our institutional practice. MRSI follow-up studies showed a decrease in Choline, stable Creatine, and increased NAA indicative of response to SRS in the majority of patients. When combined with patient survival data, metabolic information obtained during follow-up MRSI studies seemed to indicate the potential to help to distinguish necrosis from new/recurrent tumor; however, this should be further verified by biopsy studies.

A feasibility study of using conventional jaws to deliver IMRT plans in the treatment of prostate cancer.

The aim of this study is to investigate the feasibility of using conventional jaws to deliver inverse planned intensity-modulated radiotherapy (IMRT) plans for patients with prostate cancer. For ten patients, each had one three-dimensional conformal plan (3D plan) and seven inverse IMRT plans using direct aperture optimization. For IMRT plans using conventional jaws (JO plans), the number of apertures per beam angle was set from two to seven while three apertures per beam angle were set for the multi-leaf collimator (MLC) plans. To evaluate each planning method, we compared average dose volume histograms (DVH), the conformal index (COIN), total number of segments and total number of monitor units. Among the JO plans with the number of apertures per beam angle varying from two to seven, no difference was observed in the average DVHs, and the plan conformal index became saturated after four apertures per beam angle. Subsequently, JO plans with four apertures per beam angle (JO-4A) were compared with 3D and MLC plans. Based on the average DVHs, no difference was found among 3D, JO-4A and MLC plans with regard to the planning target volume and rectum, but the DVHs for the bladder and penile bulb were significantly better with inverse IMRT plans than those with 3D plans. When compared with the plan conformity, the average COIN values for 3D, JO-4A and MLC plans were 0.61 +/- 0.07, 0.73 +/- 0.05 and 0.83 +/- 0.05, respectively. In conclusion, inverse IMRT plans using conventional jaws are clinically feasible, achieving better plan quality than 3D-CRT plans.

An algorithm for shifting MLC shapes to adjust for daily prostate movement during concurrent treatment with pelvic lymph nodes.

Concurrent treatment of the prostate and the pelvic lymph nodes encounters the problem of the prostate gland moving independently from the pelvic lymph nodes on a daily basis. The purpose of this study is to develop a leaf-tracking algorithm for adjustment of IMRT portals without requirement of online dose calculation to account for daily prostate position during concurrent treatment with pelvic lymph nodes. A leaf-shifting algorithm was developed and programmed to adjust the positions of selected MLC leaf pairs according to prostate movement in the plane perpendicular to each beam angle. IMRT plans from five patients with concurrent treatment of the prostate and pelvic lymph nodes were selected to test the feasibility of this algorithm by comparison with isocenter-shifted plans, using defined dose endpoints. When the prostate moved 0.5, 1.0, and 1.5 cm along the anterior/posterior direction, the average doses to 95% of the prostate (D95%) for the iso-shift plans were similar to the MLC-shift plans, (54.7, 54.4, and 54.1 Gy versus 54.5, 54.3, and 53.9 Gy, respectively). The corresponding D95% averages to the pelvic lymph nodes were reduced from the prescription dose of 45 Gy to 42.7, 38.3, and 34.0 Gy for iso-shift plans (p = 0.04 for each comparison), while the D95% averages for the MLC-shift plans did not significantly differ from the prescription dose, at 45.0, 44.8, and 44.5 Gy. Compensation for prostate movement along the superior/inferior direction was more complicated due to a limiting MLC leaf width of 1.0 cm. In order to concurrently treat the prostate and pelvic lymph nodes with the prostate moving independently, shifting selected MLC leaf pairs may be a more practical adaptive solution than shifting the patient.

Single-fraction stereotactic radiosurgery for intracranial targets.

Stereotactic radiosurgery (SRS) is a technique for treating intracranial lesions with a high dose of ionizing radiation, usually in a single session, using a stereotactic apparatus for accurate localization and patient immobilization. This article describes several modalities of SRS and some of its applications, particularly for intracranial lesions.

Method to account for dose fractionation in analysis of IMRT plans: modified equivalent uniform dose.

PURPOSE: To propose a modified equivalent uniform dose (mEUD) to account for dose fractionation using the biologically effective dose without losing the advantages of the generalized equivalent uniform dose (gEUD) and to report the calculated mEUD and gEUD in clinically used intensity-modulated radiotherapy (IMRT) plans. METHODS AND MATERIALS: The proposed mEUD replaces the dose to each voxel in the gEUD formulation by a biologically effective dose with a normalization factor. We propose to use the term mEUD(D(o))(/n(o)) that includes the total dose (D(o)) and number of fractions (n(o)) and to use the term mEUD(o) that includes the same total dose but a standard fraction size of 2 Gy. A total of 41 IMRT plans for patients with nasopharyngeal cancer treated at our institution between October 1997 and March 2002 were selected for the study. The gEUD and mEUD were calculated for the planning gross tumor volume (pGTV), planning clinical tumor volume (pCTV), parotid glands, and spinal cord. The prescription dose for these patients was 70 Gy to >95% of the pGTV and 59.4 Gy to >95% of the pCTV in 33 fractions. RESULTS: The calculated average gEUD was 72.2 +/- 2.4 Gy for the pGTV, 54.2 +/- 7.1 Gy for the pCTV, 26.7 +/- 4.2 Gy for the parotid glands, and 34.1 +/- 6.8 Gy for the spinal cord. The calculated average mEUD(D(o))(/n(o)) using 33 fractions was 71.7 +/- 3.5 Gy for mEUD(70/33) of the pGTV, 49.9 +/- 7.9 Gy for mEUD(59.5/33) of the pCTV, 27.6 +/- 4.8 Gy for mEUD(26/33) of the parotid glands, and 32.7 +/- 7.8 Gy for mEUD(45/33) of the spinal cord. CONCLUSION: The proposed mEUD, combining the gEUD with the biologically effective dose, preserves all advantages of the gEUD while reflecting the fractionation effects and linear and quadratic survival characteristics.

Investigation of using a power function as a cost function in inverse planning optimization.

The purpose of this paper is to investigate the use of a power function as a cost function in inverse planning optimization. The cost function for each structure is implemented as an exponential power function of the deviation between the resultant dose and prescribed or constrained dose. The total cost function for all structures is a summation of the cost function of every structure. When the exponents of all terms in the cost function are set to 2, the cost function becomes a classical quadratic cost function. An independent optimization module was developed and interfaced with a research treatment planning system from the University of North Carolina for dose calculation and display of results. Three clinical cases were tested for this study with various exponents set for tumor targets and sensitive structures. Treatment plans with these exponent settings were compared, using dose volume histograms. The results of our study demonstrated that using an exponent higher than 2 in the cost function for the target achieved better dose homogeneity than using an exponent of 2. An exponent higher than 2 for serial sensitive structures can effectively reduce the maximum dose. Varying the exponent from 2 to 4 resulted in the most effective changes in dose volume histograms while the change from 4 to 8 is less drastic, indicating a situation of saturation. In conclusion, using a power function with exponent greater than 2 as a cost function can effectively achieve homogeneous dose inside the target and/or minimize maximum dose to the critical structures.

Proton magnetic resonance spectroscopy imaging in the evaluation of patients undergoing gamma knife surgery for Grade IV glioma.

OBJECT: The purpose of this study was to assess the differences in spatial extent and metabolic activity in a comparison of a radiosurgical target defined by conventional strategies that utilize the enhancing lesion and a metabolic lesion defined by proton magnetic resonance spectroscopy (MRS) imaging. The authors evaluated whether these differences manifest themselves in the clinical outcome of patients and assessed the value of incorporating 1H-MRS imaging-derived spatial information into the treatment planning process for gamma knife surgery (GKS). METHODS: Twenty-six patients harboring Grade IV gliomas who had previously been treated with external-beam radiation therapy were evaluated by comparing the radiosurgically treated lesion volume with the volume of metabolically active tumor defined on 1H-MRS imaging. The cohort was evenly divided into two groups based on the percentage of overlap between the radiosurgical target and the metabolic lesion volumes. Patients with a percentage of overlap greater than 50% with respect to the metabolic lesion volume were classified as low risk and those with an overlap less than 50% were classified as high risk. Kaplan-Meier estimators were calculated using time to progression and survival as dependent variables. The metabolite levels within the metabolic lesion were significantly greater than those within the radiosurgical target (p < or = 0.001). The median survival was 15.7 months for patients in the low-risk group and 10.4 months for those in the high-risk group. This difference was statistically significant (p < 0.01). CONCLUSIONS: Analysis of the results of this study indicates that patients undergoing GKS may benefit from the inclusion of 1H-MRS imaging in the treatment planning process.

A study of planning dose constraints for treatment of nasopharyngeal carcinoma using a commercial inverse treatment planning system.

PURPOSE: The purpose of this study was to develop and test planning dose constraint templates for tumor and normal structures in the treatment of nasopharyngeal carcinoma (NPC) using a specific commercial inverse treatment planning system. METHODS AND MATERIALS: Planning dose constraint templates were developed based on the analyses of dose-volume histograms (DVHs) of tumor targets and adjacent sensitive structures by clinically approved treatment plans of 9 T1-2 and 16 T3-4 NPC patients treated with inverse planned intensity-modulated radiation therapy (IP-IMRT). DVHs of sensitive structures were analyzed by examining multiple defined endpoints, based on the characteristics of each sensitive structure. For each subgroup of patients with T1-2 and T3-4 NPC, the resulting mean values of these defined endpoint doses were considered as templates for planning dose constraints and subsequently applied to a second group of patients, 5 with T1-2 NPC and 5 with T3-4 NPC. The 10 regenerated plans (called new plans) were compared to the original clinical plans that were used to treat the second group of patients, based on plan conformity index and DVHs. RESULTS: The conformity indices of the new plans were comparable to the original plans with no statistical difference (p = 0.85). Among the serial sensitive structures evaluated, there was a significant decrease with the new plans in the dose to the spinal cord when analyzed by the maximum dose (p = 0.001), doses encompassing 1 cc of the spinal cord volume (p = 0.001) and 3 cc of the spinal cord volume (p = 0.001). There was no significant difference in the mean maximum dose to the brainstem between the new plans and the original plans (p = 0.36). However, a significant difference in the mean maximum dose to the brainstem was seen among the different T-stages (p = 0.04). A decrease with the new plan to the brainstem in the doses encompassing 5% and 10% of the volume was of borderline statistical significance (p = 0.08 and p = 0.06, respectively). There were no statistical differences between the new plans and the original plans in the mean doses to the chiasm, optic nerve, or eye for each of the endpoints considered. For parallel sensitive structures in the new plans, there was a significant increase in the average mean dose to the parotid glands (p = 0.01), a decrease that was of borderline significance in the average mean dose to the temporomandibular joint (p = 0.07), but no difference in the average mean dose to the ear. CONCLUSIONS: The statistical analysis showed that new plans are comparable to the original plans for most of the sensitive structures except for a trade-off between a dose reduction to the spinal cord in the new plans and an increase in the mean dose to the parotid glands. These tested planning dose constraint templates can serve as good "starting points" for an inverse plan of NPC using a specific commercial inverse treatment planning system.

A forward-planned treatment technique using multisegments in the treatment of head-and-neck cancer.

PURPOSE: To describe in detail a forward-planned multisegment technique (FPMS) as an alternative treatment method for patients who are not suitable for inverse-planned intensity-modulated radiation therapy (IP-IMRT), or for situations where IP-IMRT is not available in a medical clinic. METHODS AND MATERIALS: Between April 1995 and February 2002, 38 primary head-and-neck patients were treated using the FPMS technique, which has evolved over the past 7 years at our medical center. In the most recent version of the FPMS technique, which includes 5 patients examined in this analysis, the primary tumor and the upper neck nodes were treated with 7 gantry angles, including an anterior, 2 lateral, 2 anterior oblique, and 2 posterior oblique beams with a total of 13 beam shapes formed by multileaf collimators (MLC), called MLC segments. The shape of each MLC segment was carefully designed, and the associated weights were optimized through manual iterations. The lower neck nodes and the supraclavicular nodes were treated with a split-beam anterior field, matched to the inferior border of the FPMS plan at the isocenter. With an autosequencing delivery system, all fields, including dynamic wedges, can be automatically treated. The dosimetric accuracy of this technique was verified with a phantom plan and measured with an ionization chamber, as well as film dosimetry. A sample FPMS plan is described in detail, and the average results for the 5 patients treated with FPMS are retrospectively compared to results for similar patients treated with IP-IMRT. RESULTS: The gross tumor volume was prescribed to 70 Gy (2.12 Gy/fraction) at the 88% isodose line, whereas the clinical target volume received a dose of 59.4 Gy (1.8 Gy/fraction) at the 75% isodose line. The maximum dose to the brainstem and spinal cord was below 54 and 45 Gy, respectively, comparable to IP-IMRT. The mean dose to the parotid glands was 32 Gy with FPMS vs. 26 Gy with IP-IMRT. Average delivery time was shorter for FPMS (15 min) than IP-IMRT (30 min), whereas the planning time depended on the expertise of the planner. Dosimetric accuracy for FPMS and IP-IMRT plans using phantom measurements was similar, within 1% of the phantom plan. With a median follow-up of 31 months, there was no local-regional recurrence, and the incidence of xerostomia is reduced compared to conventional techniques. CONCLUSION: FPMS achieved plans comparable to those for IP-IMRT and is an ideal alternative treatment technique for a center without the capabilities of IP-IMRT or for a patient who is not a suitable candidate, because of prolonged treatment time. The treatment outcomes from our clinical experience indicate that FPMS can achieve excellent local freedom from progression rates without causing excessive toxicity. Lastly, IP-IMRT plans should be comparable to, if not better than, FPMS plans in the treatment of head-and-neck cancer.

3D MRSI for resected high-grade gliomas before RT: tumor extent according to metabolic activity in relation to MRI.

PURPOSE: To evaluate the presence of residual disease after surgery but before radiotherapy (RT) in patients with high-grade glioma by MRI and magnetic resonance spectroscopy imaging (MRSI) and to estimate the impact of MRSI on the definition of postoperative target volumes for RT treatment planning. METHODS AND MATERIALS: Thirty patients (27 glioblastoma multiforme, 3 Grade III astrocytoma) underwent MRI and MRSI within 4 weeks after surgery but before the initiation of RT. The MRI data were manually contoured; the regions of interest included T(2)-weighted hyperintensity (T(2)), T(1)-weighted contrast enhancement (T(1)), and the resection cavity (RC). Levels of choline and N-acetyl-aspartate (NAA) in the three-dimensional MRSI data were analyzed on the basis of a choline-to-N-acetyl-aspartate index (CNI). The CNI and other metabolic indexes were superimposed on the MRI data as three-dimensional contours. Composite, conjoint, and disjoint volumes were defined for T(1) and T(2), with/without RC, and within the CNI contour, corresponding to a value of 2. In addition, follow-up MRI studies were examined for new onset contrast enhancement and compared with the initial spectroscopic findings obtained before RT. RESULTS: Substantial variation was found in the spatial relationship between the MRI and MRSI volumes. Ten patients had no contrast enhancement after surgery, and MRSI revealed abnormal metabolic activity in 8 of 10, averaging 20 cm(3) and extending 11-36 mm beyond the RC. In 20 patients with contrast-enhancing lesions, substantial variation was found between T(1) and CNI2; metabolic activity fell outside the contrast enhancement in 19 patients, averaging 21 cm(3) and extending 8-33 mm beyond the contrast enhancement. For all patients, the T(2) encompassed most of the metabolic volume. However, the CNI2 extended beyond the T(2) in 6 of 10 patients without contrast enhancement (mean, 8 cm(3); maximum, 15-23 mm) and in 13 of 20 patients with contrast enhancement (mean, 7 cm(3); maximum, 8-22 mm), representing an increase in the T(2) volume by as much as 180% (median 13%) and 86% (median 14%) for non-contrast-enhancing and contrast-enhancing patients, respectively. Preliminary evaluation of the MRI follow-up examinations revealed correspondence of areas of new contrast enhancement with initial MRSI abnormalities in 8 of 10 non-contrast-enhancing patients. In addition, CNI volumes correlated inversely with the time to onset of new contrast enhancement. CONCLUSION: MRSI is a valuable diagnostic tool for the assessment of residual disease after surgical resection in high-grade glioma. The incorporation of areas of metabolic abnormality into treatment planning for postoperative patients would produce different sizes and shapes of target volumes for both primary and boost volumes. It also may encourage the use of nonuniform margins to define the extent of tumor cell infiltration, rather than the current use of uniform margins.

Comparison of treatment plans using intensity-modulated radiotherapy and three-dimensional conformal radiotherapy for paranasal sinus carcinoma.

PURPOSE: To compare intensity-modulated radiotherapy (IMRT) treatment planning with three-dimensional conformal radiotherapy (3D-CRT) planning for paranasal sinus carcinoma. MATERIALS AND METHODS: Treatment plans using traditional 3-field technique, 3D-CRT planning, and inverse planning IMRT were developed for a case of paranasal sinus cancer requiring adjuvant radiotherapy. Plans were compared with respect to dose conformality, dose-volume histograms, doses to critical normal tissues, and ease of treatment delivery. RESULTS: The inverse-planned IMRT technique was more conformal around the tumor target volume than conventional techniques. The dose-volume histograms demonstrated significantly better critical normal-tissue sparing with the IMRT plans, while able to deliver a minimum dose of 60 Gy to the clinical tumor volume and 70 Gy to the gross tumor volume. Acute toxicities in our analysis were minimal. CONCLUSIONS: IMRT planning provided improved tumor target coverage when compared to 3D-CRT treatment planning. There was significant sparing of optic structures and other normal tissues, including the brainstem. Inverse planning IMRT provided the best treatment for all paranasal sinus carcinomas, but required stringent immobilization criteria. Further studies are needed to establish the true clinical advantage of this modality.

Comparison of intensity-modulated radiosurgery with gamma knife radiosurgery for challenging skull base lesions.

PURPOSE: To quantitatively compare intensity-modulated radiosurgery (IMRS) using 3-mm mini-multileaf collimation with gamma knife radiosurgery (GKRS) plans for irregularly shaped skull base lesions in direct proximity to organs at risk (OAR). METHODS AND MATERIALS: Ten challenging skull base lesions originally treated with GKRS were selected for comparison with IMRS using inverse treatment planning and 3-mm mini-multileaf collimation operating in step-and-shoot delivery mode. The lesions ranged in volume from 1.6 to 32.2 cm(3) and were treated with 9-20 GK isocenters (mean 13.2). The IMRS plans were designed with the intent to, at minimum, match the GKRS plans with regard to OAR sparing and target coverage. For each case, IMRS plans were generated using 9 coplanar, 11 equally spaced noncoplanar, and 11 OAR-avoidant noncoplanar beams; the best of these approaches with respect to target conformality, sparing of OAR, and maintaining coverage was selected for comparison with the original GKRS plan. RESULTS: Assuming no patient motion or setup error, IMRS provided comparable target coverage and sparing of OAR and an improved conformity index at the prescription isodose contour but sometimes less conformity at lower isodose contours compared with the actual GKRS plan. All IMRS plans produced less target dose heterogeneity and shorter estimated treatment times compared with the GKRS plans. CONCLUSION: Compared with GKRS for complex skull base lesions, IMRS plans using a 3-mm mini-multileaf collimator achieved comparable or sometimes improved target coverage, conformity, and critical structure sparing with shorter estimated treatment times.

A self consistent normalized calibration protocol for three dimensional magnetic resonance gel dosimetry.

In a clinical setting, mixed and inconsistent results have been reported using Magnetic Resonance Relaxation imaging of irradiated aqueous polymeric gels as a three-dimensional dosimeter, for dose verification of conformal radiation therapy. The problems are attributed to the difficulty of identifying an accurate dose calibration protocol for each delivered gel at the radiation site in a clinical setting. While careful calibration is done at the gel manufacturing site in a controlled laboratory setting, there is no guarantee that the dose sensitivity of the gels remains invariant upon delivery, irradiation, magnetic resonance imaging and storage at the clinical site. In this study, we have compared three different dose calibration protocols on aqueous polymeric gels for a variety of irradiation scenarios done in a clinical setting. After acquiring the three-dimensional proton relaxation maps of the irradiated gels, the dose distributions were generated using the off-site manufacturer provided calibration curve (Cal-1), the on-site external tube gel calibration (Cal-2) and the new on-site internal normalized gel calibration (Cal-3) protocols. These experimental dose distributions were compared with the theoretical dose distributions generated by treatment-planning systems. We observed that the experimental dose distributions generated from the Cal-1 and Cal-2 protocols were off by 10% to 40% and up to 200% above the predicted maximum dose, respectively. On the other hand, the experimental dose distributions generated from the Cal-3 protocol matched reasonably well with the theoretical dose distributions to within 10% difference. Our result suggests that an independent on-site normalized internal calibration must be performed for each batch of gel dosimeters at the time of MR relaxation imaging in order to account for the variations in dose sensitivity caused by various uncontrollable conditions in a clinical setting such as oxygen contamination, temperature changes and shelf life of the delivered gel between manufacturing and MR acquisitions.

Issues in optimization for planning of intensity-modulated radiation therapy.

The clinical use of intensity-modulated radiation therapy (IMRT) is expanding rapidly in academic and, more recently, in community-based radiotherapy centers due to a high level of clinician interest, improving reimbursement patterns, and the availability of the tools required to plan and deliver IMRT plans. These tools include inverse planning optimization algorithms and linear accelerator control systems with automated, multifield delivery capabilities. The hazards of this new technology are due primarily to the nonintuitive nature of the inverse planning process and the highly complex methods of delivery required for IMRT dose delivery. Important efforts are being made to define the required quality assurance for these computer-optimized IMRT plans and to find ways to reduce their complexity without reducing the quality of the resulting plans. By minimizing the complexity of these dose plans, one also minimizes the treatment time and the probability of dose delivery errors. Methods of optimization and evaluation of dose plans and practical considerations in inverse planning are discussed. In addition, this article points out the potential hazards of inverse-planned IMRT and discusses methods by which the complexity of these plans might be reduced.

Investigation of the use of MOSFET for clinical IMRT dosimetric verification.

(Received 22 October 2001; accepted for publication 26 March 2002; published 22 May 2002) With advanced conformal radiotherapy using intensity modulated beams, it is important to have radiation dose verification measurements prior to treatment. Metal oxide semiconductor field effect transistors (MOSFET) have the advantage of a faster and simpler reading procedure compared to thermoluminescent dosimeters (TLD), and with the commercial MOSFET system, multiple detectors can be used simultaneously. In addition, the small size of the detector could be advantageous, especially for point dose measurements in small homogeneous dose regions. To evaluate the feasibility of MOSFET for routine IMRT dosimetry, a comprehensive set of experiments has been conducted, to investigate the stability, linearity, energy, and angular dependence. For a period of two weeks, under a standard measurement setup, the measured dose standard deviation using the MOSFETs was +/- 0.015 Gy with the mean dose being 1.00 Gy. For a measured dose range of 0.3 Gy to 4.2 Gy, the MOSFETs present a linear response, with a linearity coefficient of 0.998. Under a 10 x 10 cm2 square field, the dose variations measured by the MOSFETs for every 10 degrees from 0 to 180 degrees is +/- 2.5%. The percent depth dose (PDD) measurements were used to verify the energy dependence. The measured PDD using the MOSFETs from 0.5 cm to 34 cm depth agreed to within +/- 3% when compared to that of the ionization chamber. For IMRT dose verification, two special phantoms were designed. One is a solid water slab with 81 possible MOSFET placement holes, and another is a cylindrical phantom with 48 placement holes. For each IMRT phantom verification, an ionization chamber and 3 to 5 MOSFETs were used to measure multiple point doses at different locations. Preliminary results show that the agreement between dose measured by MOSFET and that calculated by Corvus is within 5% error, while the agreement between ionization chamber measurement and the calculation is within 3% error. In conclusion, MOSFET detectors are suitable for routine IMRT dose verification.

A leaf sequencing algorithm to enlarge treatment field length in IMRT.

With MLC-based IMRT, the maximum usable field size is often smaller than the maximum field size for conventional treatments. This is due to the constraints of the overtravel distances of MLC leaves and/or jaws. Using a new leaf sequencing algorithm, the usable IMRT field length (perpendicular to the MLC motion) can be mostly made equal to the full length of the MLC field without violating the upper jaw overtravel limit. For any given intensity pattern, a criterion was proposed to assess whether an intensity pattern can be delivered without violation of the jaw position constraints. If the criterion is met, the new algorithm will consider the jaw position constraints during the segmentation for the step and shoot delivery method. The strategy employed by the algorithm is to connect the intensity elements outside the jaw overtravel limits with those inside the jaw overtravel limits. Several methods were used to establish these connections during segmentation by modifying a previously published algorithm (areal algorithm), including changing the intensity level, alternating the leaf-sequencing direction, or limiting the segment field size. The algorithm was tested with 1000 random intensity patterns with dimensions of 21 x 27 cm2, 800 intensity patterns with higher intensity outside the jaw overtravel limit, and three different types of clinical treatment plans that were undeliverable using a segmentation method from a commercial treatment planning system. The new algorithm achieved a success rate of 100% with these test patterns. For the 1,000 random patterns, the new algorithm yields a similar average number of segments of 36.9 +/- 2.9 in comparison to 36.6 +/- 1.3 when using the areal algorithm. For the 800 patterns with higher intensities outside the jaw overtravel limits, the new algorithm results in an increase of 25% in the average number of segments compared to the areal algorithm. However, the areal algorithm fails to create deliverable segments for 90% of these patterns. Using a single isocenter, the new algorithm provides a solution to extend the usable IMRT field length from 21 to 27 cm for IMRT on a commercial linear accelerator using the step and shoot delivery method.

Metabolic imaging of low-grade gliomas with three-dimensional magnetic resonance spectroscopy.

PURPOSE: The role of radiotherapy (RT) seems established for patients with low-grade gliomas with poor prognostic factors. Three-dimensional (3D) magnetic resonance spectroscopy imaging (MRSI) has been reported to be of value in defining the extent of glioma infiltration. We performed a study examining the impact MRSI would have on the routine addition of 2-3-cm margins around MRI T2-weighted hyperintensity to generate the treatment planning clinical target volume (CTV) for low-grade gliomas. METHODS AND MATERIALS: Twenty patients with supratentorial gliomas WHO Grade II (7 astrocytomas, 6 oligoastrocytomas, 7 oligodendrogliomas) underwent MRI and MRSI before surgery. The MRI was contoured manually; the regions of interest included T2 hyperintensity and, if present, regions of contrast enhancement on T1-weighted images. The 3D-MRSI peak parameters for choline and N-acetyl-aspartate, acquired voxel-by-voxel, were categorized using a choline/N-acetyl-aspartate index (CNI), a tool for quantitative assessment of tissue metabolite levels, with CNI 2 being the lowest value corresponding to tumor. CNI data were aligned to MRI and displayed as 3D contours. The relationship between the anatomic and metabolic information on tumor extent was assessed by comparing the CNI contours and other MRSI-derived metabolites to the MRI T2 volume. RESULTS: The limitations in the size of the region "excited" meant that MRSI could be used to evaluate only a median 68% of the T2 volume (range 38-100%), leaving the volume T2c. The CNI 2 volume (median 29 cm(3), range 10-73) was contained totally within the T2c in 55% of patients. In the remaining patients, the volume of CNI 2 extending beyond the T2c was quite small (median 2.3 cm(3), range 1.4-5.2), but was not distributed uniformly about the T2c, extending up to 22 mm beyond it. Two patients demonstrated small regions of contrast enhancement corresponding to the regions of highest CNI. Other metabolites, such as creatine and lactate, seem useful for determining less and more radioresistant areas, respectively. CONCLUSION: Metabolically active tumor, as detected by MRSI, is restricted mainly to the T2 hyperintensity in low-grade gliomas, but can extend outside it in a limited and nonuniform fashion up to 2 cm. Therefore, a CTV including T2 and areas of CNI extension beyond the T2 hyperintensity would result in a reduction in the size and a change in the shape of the standard clinical target volumes generated by adding uniform margins of 2-3 cm to the T2 hyperintensity.

Skin toxicity due to intensity-modulated radiotherapy for head-and-neck carcinoma.

PURPOSE: To investigate the cause of acute skin toxicity observed in the treatment of head-and-neck cancer with extended-field intensity-modulated radiotherapy (EF-IMRT). METHODS AND MATERIALS: EF-IMRT was used to treat head-and-neck cancer, with the gross target volume receiving 70 Gy and the clinical target volume 60 Gy. A thermoplastic mask covering the head, neck, and shoulder was used for immobilization. Dosimetric studies were conducted to investigate the possible causes of the skin reactions, such as the bolus effect of the mask, the use of multiple tangential beams with IMRT plans, and the way in which the physicians contoured the lymph nodes. The dose-volume histograms of conventional opposed-lateral fields were compared with that of the multiple tangential EF-IMRT fields. IMRT plans with neck nodes contoured up to and including the skin surface were compared with plans that contoured the neck nodes 5 mm away from the skin surface. In addition, IMRT plans defining the skin as a sensitive structure were compared with plans that did not define the skin as a sensitive structure. All plans were created using an anthropomorphic Rando phantom, and the skin doses were measured with and without the mask. In each measurement, 6 thermoluminescent dosimeters (TLDs) were placed at the lateral and medial surfaces of the neck. RESULTS: For all four plans, the measured skin doses with the mask were consistently higher than those without the mask. The average dose increase was about 18% owing to the bolus effect of the mask. Multiple tangential fields used in IMRT plans contributed to an increase in skin dose by about 19% and 27%, with and without the mask, respectively. If the skin of the neck was contoured as a sensitive structure for dose optimization, the volume of skin that received >45 Gy was further reduced by about 20%. Five patients immobilized with head and shoulder masks were treated with EF-IMRT plans with the neck nodes carefully delineated away from the skin surface. The neck skin was identified as a sensitive structure for dose optimization. Grade 1 toxicity was observed in 3 patients, Grade 2 in 1 patient, and Grade 3 in 1 patient toward the end of treatment. CONCLUSION: Multiple factors contributed to the observed acute skin reaction for head-and-neck cancer patients treated with EF-IMRT. By taking into consideration the skin as a sensitive structure during inverse planning, it was possible to reduce the skin dose to a tolerable level without compromising tumor target coverage.

The effect of beam energy and number of fields on photon-based IMRT for deep-seated targets.

PURPOSE: To examine the influence of energy and number of beams on nontarget dose when using intensity-modulated radiation therapy (IMRT) to treat deep-seated targets. METHODS AND MATERIALS: Ten patients with prostate cancer (36-226 cc) treated locally to 75.6 Gy were studied. IMRT plans were created for 6-, 10-, and 18-MV photons using 4, 6, 9, and 11 coplanar nonopposed fields. Plans, normalized to cover 95% of the target volume, were analyzed using: (a) conformity index (CI) at 105%, 100%, 95%, 90%, 80%, 70%, 50% of prescribed dose; (b) prescription isodose line (PI); (c) minimum dose to target (Tar(min)); (d) maximum dose to tissue (Tis(max)); (e) dose to rectum/bladder/penis bulb; (f) integral nontarget dose (ID). Because CI evaluates dose independent of location, tissue also was divided into "near region" (NR: 1-cm-thick shell surrounding target) and "far region" (FR: tissue minus NR) volumes that were evaluated at the same levels as CI. RESULTS: The target and sensitive structure metrics were the same for all plans. However, although there was little difference in NR volume exposed to dose, regardless of energy or number of fields, there was a significant increase in FR volume exposed to dose, at all levels, for low energy/few field plans compared to high energy/many fields (e.g., > 50 cc >or= 65 Gy). This effect disappeared with >or= 9 fields regardless of energy. CONCLUSION: With IMRT, the use of 6 MV photons with less than 9 fields may result in an increase in dose in regions distant from the target volume (e.g., near the skin surface), even though the CI and sensitive structure metrics may indicate good conformance of high dose to the target volume itself. The clinical significance of this increased dose distant from the target, in terms of complications, remains to be determined.