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Administration of yttrium-90 microspheres via the hepatic artery is an attractive approach to selectively delivertherapeuticdoses of radiation to livermalignancies. This procedure allows deliveringradiation absorbed doses in excess of 100 Gy to the tumors without significant liver toxicity. The microsphere therapy involves different specialties including medicaloncology,radiationoncology, nuclear medicine, interventional radiology, medical physics, and radiation safety. We have treated 80 patients with nonresectable hepatic tumors with yttrium-90 microspheres during the past two years on an institutional study protocol. The nominal radiation absorbed dose to the tumor in this study was 150 Gy. Required activity was calculated based on the nominal radiation absorbed dose and patient’s liver volume obtained from the CT scan, assuming a uniform distribution of the microspheres within the liver. Microspheres were administered via a catheter placed into the hepatic artery. The actual radiation absorbed doses to tumors and normal liver tissue were calculated retrospectively based on the patient’s 99 m Tc-MAA study and CT scans. As expected, the activity uptake within the liver was found to be highly nonuniform and multifold tumor to nontumor uptake was observed. A partition model was used to calculate the radiation absorbed dose within each region. For a typical patient the calculated radiation absorbed doses to the tumor and liver were 402 and 118 Gy, respectively. The radiation safety procedure involves confinement of the source and proper disposal of the contaminated materials. The average exposure rates at 1 m from the patients and on contact just anterior to the liver were 6 and 135 uSv/h, respectively. The special physics and dosimetry protocol developed for this procedure is presented.

Purpose: Intensity-modulated arc therapy (IMAT) is a method for delivering intensity-modulated radiation therapy (IMRT) using rotational beams. During delivery, the field shape, formed by a multileaf collimator (MLC), changes constantly. The objectives of this study were to ( 1 ) clinically implement the IMAT technique, and ( 2 ) evaluate the dosimetry in comparison with conventional three-dimensional (3D) conformal techniques. Methods and Materials: Forward planning with a commercial system (RenderPlan 3D, Precision Therapy International, Inc., Norcross, GA) was used for IMAT planning. Arcs were approximated as multiple shaped fields spaced every 5–10° around the patient. The number and ranges of the arcs were chosen manually. Multiple coplanar, superimposing arcs or noncoplanar arcs with or without a wedge were allowed. For comparison, conventional 3D conformal treatment plans were generated with the same commercial forward planning system as for IMAT. Intensity-modulated treatment plans were also created with a commercial inverse planning system (CORVUS, Nomos Corporation). A leaf-sequencing program was developed to generate the dynamic MLC prescriptions. IMAT treatment delivery was accomplished by programming the linear accelerator (linac) to deliver an arc and the MLC to step through a sequence of fields. Both gantry rotation and leaf motion were enslaved to the delivered MUs. Dosimetric accuracy of the entire process was verified with phantoms before IMAT was used clinically. For each IMAT treatment, a dry run was performed to assess the geometric and dosimetric accuracy. Both the central axis dose and dose distributions were measured and compared with predictions by the planning system. Results: By the end of May 2001, 50 patients had completed their treatments with the IMAT technique. Two to five arcs were needed to achieve highly conformal dose distributions. The IMAT plans provided better dose uniformity in the target and lower doses to normal structures than 3D conformal plans. The results varied when the comparison was made with fixed gantry IMRT. In general, IMAT plans provided more uniform dose distributions in the target, whereas the inverse-planned fixed gantry treatments had greater flexibility in controlling dose to the critical structures. Because the field sizes and shapes used in the IMAT were similar to those used in conventional treatments, the dosimetric uncertainty was very small. Of the first 32 patients treated, the average difference between the measured and predicted doses was −0.54 ± 1.72% at isocenter. The 80%–95% isodose contours measured with film dosimetry matched those predicted by the planning system to within 2 mm. The planning time for IMAT was slightly longer than for generating conventional 3D conformal plans. However, because of the need to create phantom plans for the dry run, the overall planning time was doubled. The average time a patient spent on the table for IMAT treatment was similar to conventional treatments. Conclusion: Initial results demonstrated the feasibility and accuracy of IMAT for achieving highly conformal dose distributions for different sites. If treatment plans can be optimized for IMAT cone beam delivery, we expect IMAT to achieve dose distributions that rival both slice-based and fixed-field IMRT techniques. The efficient delivery with existing linac and MLC makes IMAT a practical choice.

The authors have devised a computer controlled translational couch to administer total body irradiation accurately and safely. In this technique, patients comfortably rest on a couch in supine and prone positions and are transported slowly through a narrow beam with the gantry in upright position. Dose to the patient is determined by the couch velocity that is calculated based on physical parameters such as patient's dimensions, beam geometry and machine dose rate. In the authors' design, the couch velocity is continuously updated for possible machine dose rate fluctuations to eliminate dose non-uniformity within the patient. The translational couch technique provides better dose uniformity within the patient compared to fixed beam techniques, and allows a more precise shielding block placement for organs at risk. At the same time, it presents special challenges for dosimetry calculations. A dosimetry parameter is introduced that converts moving beam output to fixed beam output factor. Based on this factor, a simple dosimetry calculation method has been implemented that takes advantage of conventional dosimetry parameters, eliminating extensive dosimetry measurements. Comprehensive dose measurements within phantoms confirmed the validity of the calculations method. Further improvements in dose uniformity within the patient by modulating couch velocity with patient thickness and shielding the organs at risk are discussed.

Purpose The purpose of this work is to explore the possibility of using intensity-modulated radiation therapy (IMRT) to deliver the boost dose to the tumor bed simultaneously with the whole-breast IMRT to reduce the radiation treatment time by 1–2 weeks. Methods and materials The biologically effective dose (BED) for different treatments was calculated using the linear-quadratic (LQ) model with parameters previously derived for breast cancer from clinical data (α/β = 10Gy, α = 0.3Gy −1 ). A potential doubling time of 15 days (from in vitro measurements) for breast cancer and a generic α/β ratio of 3 Gy for normal tissues were used. A series of regimens that use IMRT as initial treatment and an IMRT simultaneous integrated boost (SIB) were derived using biologic equivalence to conventional schedules. Possible treatment plans with IMRT SIB to the tumor bed were generated for 2 selected breast patients, 1 with a shallow tumor and 1 with a deep-seated tumor. Plans with a simultaneous integrated electron boost were also generated for comparison. Dosimetric merits of these plans were evaluated based on dose volume histograms. Results A commonly used conventional treatment of 45 Gy (1.8 Gy × 25) to the whole breast and then a boost of 20 Gy (2 Gy × 10) is biologically equivalent to an alternative plan of 1.8 Gy × 25 to the whole breast with a 2.4 Gy × 25 SIB to the tumor bed. The new regime reduces treatment time from 7 to 5 weeks. For the patient with a deep-seated tumor, the IMRT plans reduce the volume of the breast that receives high doses (compared with the conventional photon boost plan) and provides good coverage of the target volumes. Conclusion It is biologically and dosimetrically feasible to reduce the overall treatment time for breast radiotherapy by using an IMRT simultaneous integrated boost. For selected patient groups, IMRT plans with a new regimen can be equal to or better than conventional plans.

Stereotactic Gamma Knife radiosurgery has been widely used for treating trigeminal neuralgia (TN). A single large fractional dose of 7000 to 9000 cGy is commonly prescribed as the maximum dose for these treatments. For this reason, if a small percentage of the prescribed dose such as 2–3 % scattered to the eye, it could reach or even exceed the tolerance dose of the lens. For several TN cases, we found that the Leksell Gamma Plan system calculates the lens dose about 0.5–2 % of the maximum dose independent of the use of eye shielding. These dose values are significantly high and it motivated us to investigate the lens dose for the TN patients treated with stereotactic Gamma Knife radiosurgery. Phantom studies and in vivo dosimetry measurements were carried out for six patients treated at our institution. The average dose to the lens ipsilateral to the treated nerve was measured to be 7.7±0.6 cGy. Based on the biological model of Lyman and Emami [Int. J. Radiat. Oncol. Biol. Phys. 21, 109–122 (1991)], the probability of the lens complication (cataract) was determined to be 0.1%. Our findings suggest that few TN patients would develop cataracts after receiving Gamma Knife radiosurgery. PACS number(s): 87.53.–j, 87.66.–a

Intensity-modulated arc therapy, or MAT, is one of the methods for delivering intensity-modulated radiation treatments to achieve high dose conformity. Instead of delivering intensity-modulated beams with fixed gantry angles [1], IMAT delivers optimized dose distributions by rotating the radiation beam around the patient. During delivery, the field shape, which is formed by a multileaf collimator (MLC), changes constantly as determined by the required intensity distributions at different beam angles. Intensity distributions at all angles around the patient are achieved with multiple overlapping arcs of different field apertures and gantry speed. Therefore, IMAT is also different from tomotherapy[2,3], which use intensity modulated fan beams rotating around the patient to delivery the treatment slice-by-slice. As with tomotherapy, IMAT combines intensity modulation and rotational delivery. Detailed description of the technique was reported by Yu in a previous article [4].