Biomedical subjects

Ping Xia

Publications and source records attributed to Ping Xia.

31 records · Page 2Linked to original sources

PubMed2003

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.

Aged↗

PubMed2003

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.

Brain Neoplasms↗

PubMed2003

Guidance document on delivery, treatment planning, and clinical implementation of IMRT: report of the IMRT Subcommittee of the AAPM Radiation Therapy Committee.

Intensity-modulated radiation therapy (IMRT) represents one of the most significant technical advances in radiation therapy since the advent of the medical linear accelerator. It allows the clinical implementation of highly conformal nonconvex dose distributions. This complex but promising treatment modality is rapidly proliferating in both academic and community practice settings. However, these advances do not come without a risk. IMRT is not just an add-on to the current radiation therapy process; it represents a new paradigm that requires the knowledge of multimodality imaging, setup uncertainties and internal organ motion, tumor control probabilities, normal tissue complication probabilities, three-dimensional (3-D) dose calculation and optimization, and dynamic beam delivery of nonuniform beam intensities. Therefore, the purpose of this report is to guide and assist the clinical medical physicist in developing and implementing a viable and safe IMRT program. The scope of the IMRT program is quite broad, encompassing multileaf-collimator-based IMRT delivery systems, goal-based inverse treatment planning, and clinical implementation of IMRT with patient-specific quality assurance. This report, while not prescribing specific procedures, provides the framework and guidance to allow clinical radiation oncology physicists to make judicious decisions in implementing a safe and efficient IMRT program in their clinics.

Algorithms↗

PubMed2002

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.

Female↗

PubMed2002

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.

Humans↗

PubMed2002

Intensity-modulated radiotherapy in the treatment of nasopharyngeal carcinoma: an update of the UCSF experience.

PURPOSE: To update our experience with intensity-modulated radiotherapy (IMRT) in the treatment of nasopharyngeal carcinoma (NPC). METHODS AND MATERIALS: Between April 1995 and October 2000, 67 patients underwent IMRT for NPC at the University of California-San Francisco (UCSF). There were 20 females and 47 males, with a mean age of 49 (range 17-82). The disease was Stage I in 8 (12%), Stage II in 12 (18%), Stage III in 22 (33%), and Stage IV in 25 (37%). IMRT was delivered using three different techniques: 1) manually cut partial transmission blocks, 2) computer-controlled auto-sequencing segmental multileaf collimator (SMLC), and 3) sequential tomotherapy using a dynamic multivane intensity modulating collimator (MIMiC). Fifty patients received concomitant cisplatinum and adjuvant cisplatinum and 5-FU chemotherapy according to the Intergroup 0099 trial. Twenty-six patients had fractionated high-dose-rate intracavitary brachytherapy boost and 1 patient had gamma knife radiosurgery boost after external beam radiotherapy. The prescribed dose was 65-70 Gy to the gross tumor volume (GTV) and positive neck nodes, 60 Gy to the clinical target volume (CTV), 50-60 Gy to the clinically negative neck, and 5-7 Gy in 2 fractions for the intracavitary brachytherapy boost. Acute and late normal tissue effects were graded according to the Radiation Therapy Oncology Group (RTOG) radiation morbidity scoring criteria. The local progression-free, local-regional progression-free, distant metastasis-free rates, and the overall survival were calculated using the Kaplan-Meier method. RESULTS: With a median follow-up of 31 months (range 7 to 72 months), there has been one local recurrence at the primary site. One patient failed in the neck. Seventeen patients developed distant metastases; 5 of these patients have died. The 4-year estimates of local progression-free, local-regional progression-free, and distant metastases-free rates were 97%, 98%, and 66% respectively. The 4-year estimate of overall survival was 88%. The worst acute toxicity documented was as follows: Grade 1 or 2 in 51 patients, Grade 3 in 15 patients, and Grade 4 in 1 patient. The worst late toxicity was Grade 1 in 20 patients, Grade 2 in 15 patients, Grade 3 in 7 patients, and Grade 4 in 1 patient. At 3 months after IMRT, 64% of the patients had Grade 2, 28% had Grade 1, and 8% had Grade 0 xerostomia. Xerostomia decreased with time. At 24 months, only one of the 41 evaluable patients had Grade 2, 32% had Grade 1, and 66% had Grade 0 or no xerostomia. Analysis of the dose-volume histograms (DVHs) showed that the average maximum, mean, and minimum dose delivered were 79.3 Gy, 74.5 Gy, and 49.4 Gy to the GTV, and 78.9 Gy, 68.7 Gy, and 36.8 Gy to the CTV. An average of only 3% of the GTV and 3% of the CTV received less than 95% of the prescribed dose. CONCLUSION: Excellent local-regional control for NPC was achieved with IMRT. IMRT provided excellent tumor target coverage and allowed the delivery of a high dose to the target with significant sparing of the salivary glands and other nearby critical normal tissues.

Adolescent↗

PubMed2002

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.

Calibration↗

PubMed2002

Managing nasopharyngeal carcinoma with intracavitary brachytherapy: one institution's 45-year experience.

PURPOSE: At our institution, we have been using intracavitary brachytherapy as a boost in selected cases of both primary and recurrent nasopharyngeal carcinoma. The local control, distant metastasis-free rate, overall survival, and morbidity are presented. METHODS AND MATERIALS: Between January 1, 1955, and August 2000, 576 patients with a diagnosis of nasopharyngeal carcinoma were seen at the department of radiation oncology, University of California-San Francisco, and 55 patients received intracavitary brachytherapy as one part of their treatment. All patients were treated with megavoltage external beam radiation, including 43 patients treated for initial disease and 12 for recurrence. Brachytherapy was routinely used for early cases of T1 and T2 lesions and selected cases of more advanced lesions, as well as recurrent lesions. The median age was 48 years (range 22-85 years); there were 17 women and 38 men, and 39 patients were ethnic Chinese, 13 were white, and 3 were other races. Stage at treatment (primary and recurrent) was I (n=13), II (n=18), III (n=19), and IV (n=5); 18 patients had concurrent chemotherapy. The brachytherapy applicators used were Rotterdam (n=24), balloon (n=16), ovoid (n=14), and ribbon (n=1). The dose rate was high (n=24), low (n=29), or pulsed (n=2). External beam doses ranged from 54 to 72 Gy for primary disease and 30 to 42 Gy for recurrent disease. Brachytherapy doses ranged from 5 to 7 Gy for high dose rate and 10 to 54 Gy for low dose rate. RESULTS: With a median follow-up of 36 months in those who were treated for primary carcinoma, the 5-year estimate of local control was 89%, the distant metastasis-free rate was 75%, and the overall survival estimate was 86%. Recurrent patients had a median follow-up of 50 months; the 5-year estimate of local control was 64%, the distant metastasis-free rate was 100%, and the overall survival estimate was 91%. Patients with Stage I or II disease had a longer overall survival compared with those with Stage III or IV (p=0.05). There was a significant difference in the rate of distant metastases due to nodal status (N0 vs. N1-N3, p=0.02) or to overall stage (I/II vs. III/IV, p=0.005). CONCLUSIONS: Intracavitary boost brachytherapy was found to be effective and well tolerated in selected cases of both primary and recurrent nasopharyngeal carcinoma.

Adult↗

PubMed2002

Determination and synchronisation of G1-phase of the cell cycle in 2- and 4-cell mouse embryos.

Incorporation of [3H]thymidine at different concentrations into mouse embryos at early developmental stages was determined by autoradiography. Methods to synchronise the G1-phase of mouse 2- and 4-cell embryos were also investigated. The results showed that the ability of embryos to incorporate [3H]thymidine increased with development. Embryos at the 4-cell stage were not labelled when the concentration of [3H]thymidine was lower than 5 microCi/ml, whereas the nuclei of embryos at morula and blastocyst stages began to show silver grains at a concentration of 0.1 microCi/ml of [3H]thymidine. After 2- and 4-cell mouse embryos were synchronised at the onset of G1-phase by treatment with low temperature or nocodazole, and DNA synthesis was detected by autoradiography, the duration of G1-phase was estimated. The result showed that 43% of the 2-cell embryos had a G1-phase of < or = 1 h, 22% had a G1-phase of < or = 2 h, 22% had a G1-phase of < or = 3 h and 13% had a G1-phase of < or = 4 h. The G1-phase in 85% of the 4-cell embryos was < or = 3 h, that in 8% of embryos was < or = 4 h and that in 7% of embryos was < or = 5 h. The toxicity of nocodazole on mouse embryo development was assessed based on both blastocyst formation and the number of blastomeres, and the results indicated that the effect of nocodazole on embryo development and cell cycle block was dose-dependent. The minimum concentration of nocodazole for metaphase block of mouse late 2-cell embryos was 0.05 microM, and the appropriate concentrations which did not impair development were 0.05-0.5 microM.

Animals↗

PubMed2002

Communication and sampling rate limitations in IMRT delivery with a dynamic multileaf collimator system.

The delivery of an intensity modulated radiation field with a dynamic multileaf collimator (MLC) requires precise correlation between MLC positions and cumulative monitor units (MUs). The purpose of this study is to investigate the precision of this correlation as a function of delivered MUs and dose rate. A semi-Gaussian shaped intensity profile and a simple geometric intensity pattern consisting of four square segments were designed to deliver a total of 1, 4, 16, 64, and 100 MUs at three different dose rates of 100, 400, and 600 MU/min. The semi-Gaussian intensity pattern was delivered using both sliding window and step and shoot techniques. The dose profiles of this intensity pattern were measured with films. The four square intensity pattern was delivered using step and shoot and conventional delivery techniques for comparison. Because of geometrical symmetry, the dose to each segment in this intensity pattern is expected to be the same when the same MU is assigned to each segment. An ionization chamber was used to measure the dose in the center of each of the four square segments. For the semi-Gaussian shaped profile, significant artifacts were observed when the profile was delivered with small MUs and/or at a high dose rate. For the four square intensity pattern, the dose measured in each segment presented a large variation when delivered with small MUs and a high dose rate. The variation increases as the MU/segment decreases and as the dose rate increases. These MU and dose rate dependencies were not observed when the intensity pattern was delivered using a conventional delivery technique. The observed distortion of the semi-Gaussian profile and dose variations among the segments of the four square intensity pattern are explained by considering the sampling rate and the communication time lag between the control systems. Finally, clinical significance is discussed.

Algorithms↗

PubMed2002

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.

Algorithms↗

PubMed2002

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.

Algorithms↗

PubMed2002

[Changes of soil physical, chemical and ecological factors under mechanized cultivation].

Three-years agricultural mechanization extension project in Huang-Huai-Hai regions showed that the application of comprehensive agricultural technologies which included the return of straw and stalk to field by mechanization, deep application of fertilizer, deep plough and soil no-tillage with mulch, had an obvious biological effects. In comparing with traditional cultivation, the comprehensive mechanized cultivation could decrease soil bulk density by 0.08 g.cm-3, increase soil organic mater by 12%, improve moisture utilization by 10.1-13.6%, and increase the grain yields of wheat and corn by 1218 kg.hm-2.

Agriculture↗