SU-GG-T-14: Nomogram Derivation for Cs 131 Prostate Seed Implant

Integral Radiation Dose to Normal Structures with Conformal External Beam Radiation

The size of sebaceous glands in relation to the size of hair follicles on the heads of some small mammals (Insectivore, Chiroptera, Rodentia)

Purpose: To investigate the technical and dosimetric properties of IMRT plans for prostate carcinoma using standard beam orientations with mixed beam energies. Method and Materials: A cohort of 10 patients previously treated with IMRT for prostate carcinoma was selected. For each patient, three additional IMRT plans were calculated using a standard coplanar 5‐beam arrangement and compared to the delivered treatment plan. All plans used 6 MV for the anterior oblique fields and the posterior field, while the two posterior oblique fields were either 10, 15, or 18 MV. An identical set of constraints was used for the PTV and organs at risk (rectum and bladder) and all plans were normalized to give 95% PTV coverage at 100% of the prescription dose. Competing plans were analyzed based on DVH, conformity index, total body integral dose (photon and neutron), and the total and compartmentalized number of MU. Results: Equivalent conformity of dose to the PTV was achieved in all plans. The photon integral dose and number of MU, however, were significantly higher for plans with only 6 MV beams, while the 6/18 MV combination resulted in the lowest value for each of these parameters. For the 6/15 and 6/18 MV plans, the number of MU to be delivered with the pair of higher energy beams was found to be comparable to the number of MU delivered in a standard 4‐ or 6‐field 3DCRT beam arrangement. Conclusion: While the neutron integral dose rapidly increases beyond 10 MV, its magnitude in a mixed beam energy plan has been shown to be comparable to the neutron dose encountered in 3DCRT for prostate carcinoma. The resulting decrease in photon integral dose achieved in IMRT plans with mixed beam energies may thus suggest the selection of higher energy for those beams requiring the greatest tissue penetration.

e16158 Background: Intensity Modulated Radiotherapy (IMRT) is used in carcinoma prostate to achieve better target coverage with less dose to critical organs thereby permitting dose escalation and eventually better therapeutic ratio. There is scarcity of literature evaluating beam number and energy in prostate cancer treated by IMRT. Aim of our study is to identify the optimal number of beams and energy in the boost treatment of prostate cancer by IMRT. Methods: Ten patients were included in this study. Initially a dose of 45 Gy in 25 fractions was delivered to the prostate, seminal vesicles and the nodes by 3DCRT. A boost dose of 27 Gy in 15 fractions was planned to the prostate and the seminal vesicles by sliding window IMRT. Four different sets of IMRT plans: 5, 7 and 9 field with 6 MV; 7 field with 15 MV were generated. The dose constraints to the critical structures and the target volume were based on standard guidelines. The mean dose, maximum dose, volume receiving the prescribed dose (V100), volume receiving > 107% and <95% of the prescribed dose were analyzed for CTV and planning target volume (PTV). The mean dose, volume receiving 100%, 50% and 30% of the prescribed dose were analyzed for the bladder and the rectum. Similarly, the mean dose and the maximum dose to the right and left femoral heads, monitor units (MU) and the integral dose were analyzed. SPSS V10.0 software was used for statistical analysis. Results: The seven beam plan provide better dose homogeneity to PTV and lesser doses to critical structures like bladder, rectum and femoral heads when 6 MV photon is used. 15 MV photons further improve the dose homogeneity and decrease the dose to critical structures. The Monitoring Units required to deliver the treatment and the Integral dose is significantly reduced by using 15 MV compared to 6 MV. Conclusions: The optimal IMRT plan for boost planning in carcinoma prostate is that uses 7 beams and higher energy. This yields lesser dose to surrounding critical structures with better target conformity. [Table: see text] No significant financial relationships to disclose.

OBJECTIVEStereotactic radiosurgery (SRS) is characterized by high levels of conformity and steep dose gradients from the periphery of the target to surrounding tissue. Clinical studies have backed up the importance of these factors through evidence of symptomatic complications. Available data suggest that there are threshold doses above which the risk of symptomatic radionecrosis increases with the volume irradiated. Therefore, radiosurgical treatment plans should be optimized by minimizing dose to the surrounding tissue while maximizing dose to the target volume. Several metrics have been proposed to quantify radiosurgical plan quality, but all present certain weaknesses. To overcome limitations of the currently used metrics, a novel metric is proposed, the efficiency index (η50%), which is based on the principle of calculating integral doses: η50% = integral doseTV/integral dosePIV50%.METHODSThe value of η50% can be easily calculated by dividing the integral dose (mean dose × volume) to the target volume (TV) by the integral dose to the volume of 50% of the prescription isodose (PIV50%). Alternatively, differential dose-volume histograms (DVHs) of the TV and PIV50% can be used. The resulting η50% value is effectively the proportion of energy within the PIV50% that falls into the target. This value has theoretical limits of 0 and 1, with 1 being perfect. The index combines conformity, gradient, and mean dose to the target into a single value. The value of η50% was retrospectively calculated for 100 clinical SRS plans.RESULTSThe value of η50% for the 100 clinical SRS plans ranged from 37.7% to 58.0% with a mean value of 49.0%. This study also showed that the same principles used for the calculation of η50% can be adapted to produce an index suitable for multiple-target plans (Gη12Gy). Furthermore, the authors present another adaptation of the index that may play a role in plan optimization by calculating and minimizing the proportion of energy delivered to surrounding organs at risk (OARη50%).CONCLUSIONSThe proposed efficiency index is a novel approach in quantifying plan quality by combining conformity, gradient, and mean dose into a single value. It quantifies the ratio of the dose "doing good" versus the dose "doing harm," and its adaptations can be used for multiple-target plan optimization and OAR sparing.

Purpose: To achieve a clinically accepted optimum treatment plan in a reasonable time, IMRT parameters; energy, gantry angle, number of segments and intensity levels and collimator angle are iteratively attempted. Each parameter may provide complex and unstable solution. The selection of beam angle may provide better conformality that is studied with availability of beam angle optimizer (BAO) compared to the fixed beam angle. Method and Materials: Dosmietric comparison of fixed beam geometry (FBG) and BAO for IMRT plans is investigated for 17 patients. IMRTdosimetry plans (prostate, pancreas, brain and head and neck) with same target volumes were studied for both techniques. The head and neck and prostate PTV volumes ranged between 95.96–319.9 cm3 and 153.6–321.3 cm3 where as OAR were 8.3–47.8 cm3 and 68.3–469.2 cm3. In FBG, a standard coplanar 7–9 fields equally spaced gantry angles were used. In BAO the selection of gantry angle was chosen by the algorithm for the same number of beams. The desired dose volume constraints were kept the same for both techniques. Treatment planning was performed on Eclipse treatment planning system using beam data generated for Varian IX linear accelerator with 0.5 cm multileaf collimator.Results: The DVH for PTV are similar for both techniques but BAO provided superior sparing of the OARs in comparison with FBG. However, when the MUs were analyzed, FBG provided an average 16.9 % higher MU compared to the BAO. Conclusions: The comparison of the fixed beam and optimized beam geometry plans reveal identical DVHs of PTVs and much better sparing to OARs. However the differences in monitor units are significantly lower (7–36%) in BAO with reduction in treatment time. Such reduction in MU translates into the reduction in total body and integral dose. It is concluded therefore that BAO feature should be clinically used against FBG.

Stereotactic radiosurgery (SRS) delivered using a step-and-shoot approach, using either isocentric or non-isocentric radiation beams, typically results in distinct treatment delivery patterns across different SRS systems, reflected in treatment time. In this study, we performed a comparative analysis of dosimetric and radiobiological properties of the CyberKnifeTM(CK) and the Gamma KnifeTM(GK) systems in the management of vestibular schwannomas (VS). 
Approach: The treatment plan and delivery data of 21 patients underwent GK SRS for VS tumors were retrospectively evaluated and compared to corresponding treatment plan calculations performed for the CK. A a marginal dose of 12Gy was prescribed in each case. Corresponding Biological Effective Dose (BED) distributions were calculated and compared accounting for sublethal DNA damage repair during treatment delivery.
Main Results: Statistical analyses showed comparable plan quality indices, except for coverage, gradient index and R50, where GK plans exhibited higher coverage and lower gradient and R50 values. CK plans had higher minimum target doses, while D98 values were higher for GK. GK plans exhibited higher mean and maximum target doses due to lower prescription isodose levels. Integral doses (ID) were similar between systems. Treatment times were comparable, although Beam-On-Times were shorter for CK. CK plans presented higher minimum but lower maximum target BED values. Increased BED values were observed in target regions within the auditory canal and near the cochlea for GK plans. No significant differences were found in BED98, mean BED, and integral BED. The marginal BED decreased linearly with treatment time, following: BED(Gy2.47)= -0.29*T(min)+67.47 for both systems. A novel finding comprised the mutual dependency shown for target integral BED (IBED) increasing linearly with ID according to: IBED(mJ2.47)=5.63*ID(mJ). 
Significance: Based on the same VS patient cohort, our results suggest that the same dose prescription results in comparable time-resolved BED distributions across the two SRS systems.&#xD.

Clinical Outcomes of Upfront Stereotactic Radiosurgery Alone for Patients With Greater Than 4 Brain Metastases

In this series, eighteen patients with Graves' disease were treated with 8000 rads (80 Gy) of radioiodine (131I), using the new high resolutional ultrasonic scanner for the determination of the accurate weight of the thyroid gland. The mean dose of radioiodine administered orally was 4.6 +/- 3.0 mCi (170.2 +/- 110.0 MBq) and 133.7 +/- 44.6 microCi/g (4.95 +/- 1.65 MBq). At one year after treatment, twelve of eighteen patients (66.7%) became euthyroid, five (27.8%) remained hyperthyroid and one (5.6%) became hypothyroid. Analysis of various factors which may be related to the effect of radioiodine therapy revealed that the weight of the thyroid gland in the hyperthyroid and euthyroid groups was significantly different (61.7 +/- 33.5 g vs. 25.1 +/- 9.1 g, p less than 0.05). Furthermore, all patients with larger glands (more than sixty grams) remained hyperthyroid, while the incidence of euthyroidism was as high as 80% in patients with smaller glands (less than forty grams). Although the number of patients studied was small, these results indicate that a larger thyroid gland requires a larger radioiodine dose per gram of tissue than a smaller gland, suggesting that the therapeutic radiation dose should be graded according to the gland size even when the gland size is accurately estimated by ultrasound. Further study is required to determine the appropriate radiation dose graded according to the gland size.

In this study, dose rate distribution around a spherical 137Cs pellet source, from a low‐dose‐rate (LDR) Selectron remote afterloading system used in gynecological brachytherapy, has been determined using experimental and Monte Carlo simulation techniques. Monte Carlo simulations were performed using MCNP4C code, for a single pellet source in water medium and Plexiglas, and measurements were performed in Plexiglas phantom material using LiF TLD chips. Absolute dose rate distribution and the dosimetric parameters, such as dose rate constant, radial dose functions, and anisotropy functions, were obtained for a single pellet source. In order to investigate the effect of the applicator and surrounding pellets on dosimetric parameters of the source, the simulations were repeated for six different arrangements with a single active source and five non‐active pellets inside central metallic tubing of a vaginal cylindrical applicator. In commercial treatment planning systems (TPS), the attenuation effects of the applicator and inactive spacers on total dose are neglected. The results indicate that this effect could lead to overestimation of the calculated F(r,θ), by up to 7% along the longitudinal axis of the applicator, especially beyond the applicator tip. According to the results obtained in this study, in a real situation in treatment of patients using cylindrical vaginal applicator and using several active pellets, there will be a large discrepancy between the result of superposition and Monte Carlo simulations.PACS number: 87.53.Jw

Dosimetry of the breast for determining carcinogenic risk in mantle irradiation

In Silico Dosimetric Comparison of Intensity-Modulated Proton Radiotherapy (IMPT) vs. Intensity-Modulated Photon Radiotherapy (IMRT) in the Preoperative Treatment of Retroperitoneal Sarcoma

To evaluate the outcomes based on gland size between robotically assisted radical prostatectomy (RALP) and open RP (RRP), as larger prostates might increase the difficulty of RP. We reviewed 660 patients who had RALP and 340 who had RRP from May 2003 to August 2006; the patients were divided into two groups, with a prostate of >75 and <or=75 g. The clinical characteristics, surgical approach, perioperative and postoperative outcomes were evaluated. Patients with large prostates were significantly older (P < 0.001), but had a lower pathological stage (RALP, P = 0.046, and RRP, P = 0.008) than patients with small glands, regardless of technique. There was no difference in length of stay or transfusion rates between the groups. A large prostate increased the operative duration of RALP (P < 0.001) but not of RRP. For both RALP and RRP, positive margin rates were lower with larger glands (RALP, P = 0.014; RRP, P = 0.033). Overall, the positive margin rates were lower with RALP (9.9% and 19.0%) than RRP (18.5% and 35.5%) among patients with larger or smaller (P < 0.001) glands, respectively. Prostates of >or=75 g had fewer positive margins than smaller glands, regardless of surgical technique. There was also a significant decrease in positive margin rate in among prostates of >75 g in favour of RALP. Thus, RALP appears to be comparable with RRP for patients with large glands, and might reduce the positive margin rate.

Feather mites (Arachnida: Acari: Astigmata) feed mainly on secretions of the uropygial gland of birds. Here, we use analyses corrected for phylogeny and body size to show that there is a positive correlation between the size of this gland and mite abundance in passerine birds at an interspecific level during the breeding season, suggesting that the gland mediates interactions between mites and birds. As predicted on the basis of hypothesized waterproofing and antibiotic functions of uropygial gland secretions, riparian/marsh bird species had larger glands and higher mite loads than birds living in less mesic terrestrial environments. An unexpected pattern was a steeper relationship between mite load and gland size in migratory birds than in residents. If moderate mite loads are beneficial to a host but high loads detrimental, this could create complex selection regimes in which gland size influences mite load and vice versa. Mites may exert selective pressures on gland size of their hosts that has resulted in smaller glands among migratory bird species, suggesting that smaller glands may have evolved in these birds to attenuate a possible detrimental effect of feather mites when present in large numbers.

Radiation Induced Lymphopenia is Associated with the Effective Dose to the Circulating Immune Cells (EDIC) for Breast Cancer