Tumor dose specification of I-125 seed implants

A new calculation procedure for interstitial implants with I-125 seeds is presented. The total activity of I-125 seeds required to deliver a net minimum dose of 1 Gy is also given as a function of implant volume for various seed spacings. This will permit clinicians to prescribe a specific dose for patients as a primary or boost treatment. Elongation and shape corrections are studied. Discussions and comparisons with other dosimetric calculations such as "matched" peripheral dose, average minimum dose, and minimum peripheral dose are made. The results indicate that for a fixed total activity the net minimum dose is relatively insensitive to variations in seed spacing and individual seed activity.     

Dosimetry accuracy as a function of seed localization uncertainty in permanent prostate brachytherapy: increased seed number correlates with less variability in prostate dosimetry

The variation of permanent prostate brachytherapy dosimetry as a function of seed localization uncertainty was investigated for I-125 implants with seed activities commonly employed in contemporary practice. Post-implant imaging and radiation dosimetry data from nine patients who underwent permanent prostate brachytherapy served as the source of clinical data for this simulation study. Gaussian noise with standard deviations ranging from 0.5 to 10 mm was applied to the seed coordinates for each patient dataset and 1000 simulations were performed at each noise level. Dose parameters, including D90, were computed for each case and compared with the actual dosimetry data. A total of 81 000 complete sets of post-brachytherapy dose volume statistics were computed. The results demonstrated that less than 5% deviation of prostate D90 can be expected when the seed localization uncertainty is 2 mm, whereas a seed localization uncertainty of 10 mm yielded an average decrease in D90 of 33 Gy. The mean normalized decrement in the prostate V100 was 10% at 5 mm uncertainty. Implants with greater seed number and larger prostate volume correlated with less sensitivity of D90 and V100 to seed localization uncertainty. Estimated target volume dose parameters tended to decrease with increasing seed localization uncertainty. The bladder V100 varied more significantly both in mean and standard deviation as compared to the urethra V100. A larger number of implanted seeds also correlated to less sensitivity of the bladder V100 to seed localization uncertainty. In contrast, the deviation of urethra V100 did not correlate with the number of implanted seeds or prostate volume.     

Dose perturbation effects in prostate seed implant brachytherapy with I-125

EGSnrc Monte Carlo simulation was used to investigate dose perturbation effects in prostate seed implant brachytherapy using 125I radioactive seeds used in implant brachytherapy. Dose perturbation effects resulting from the seed mutual attenuation in a prostate seed implant consisting of 27 seeds were investigated. The results showed that for 125I seeds implanted into the prostate at 1.00 cm, 0.75 cm and 0.50 cm apart (uniform spacing), the dose perturbation effects are up to 10%. The volume of the target occupied by the 10% dose difference between the full Monte Carlo simulation and the single seed superposition model decreases with increasing seed spacing. Despite the differences between the Monte Carlo simulation and the simple superposition, there was no significant change in the dose volume histogram for 1 cm and 0.75 cm seed spacing. However, there was a significant change in the dose volume histogram when the seed spacing was 0.5 cm. An analysis of the external volume index (EI), coverage index (CI) and homogeneity index (HI) also showed that there is no difference in these indexes for the 1.00 cm and 0.75 cm seed spacing between the simple superposition model and the full Monte Carlo simulation. Compared to the full Monte Carlo simulations, the simple superposition model overestimated EI, CI and HI by 7%, 5% and 4% respectively for the 0.50 cm seed spacing.     

The influence of brachytherapy dose heterogeneity on estimates of alpha/beta for prostate cancer

The sensitivity of estimates of alpha/beta for prostate tumours to dose heterogeneity in 125I brachytherapy implants, as well as to variation in selected radiobiological parameters, is analysed. The tumour control probabilities of brachytherapy and external beam radiotherapy are equated for ranges of alpha, Tpot, RBE and external beam dose. For each combination of parameters, the equality is used to derive the value of alpha/beta. Different clinical (non-uniform) brachytherapy dose distributions, and three uniform brachytherapy dose distributions (120, 144 and 160 Gy) are used. For 'nominal' input parameter values of Tpot = 45 days, alpha = 0.2 Gy(-1), RBE = 1.4, and an external beam dose of 70 Gy, the values obtained for alpha/beta ranged between 2.1 and 12.3 Gy for all of the clinical DVHs, between 2.1 and 3.8 Gy for the better quality clinical implants and between 1.0 and 1.8 Gy for the uniform brachytherapy doses. When only 2% of the volume receiving the lowest dose is omitted from the clinical DVHs, the estimated alpha/beta values ranged between 1.4 and 2.1 Gy. When ranges of input parameters were also considered, the overall range of alpha/beta values for the clinical brachytherapy dose distributions lay between 1.1 and 12.3 Gy for the three best clinical implants, and between 0.7 and 6.3 Gy for uniform doses. We conclude that estimation of alpha/beta without taking into account dose heterogeneity and inter-patient variation may underestimate the actual value alpha/beta.     

The role of high dose rate (HDR) brachytherapy in advanced non-small cell lung cancer

The aim of this study was to determine the influence of brachytherapy on the prognosis in advanced NSCLC, elaboration of clinical criteria useful in patients qualification to brachytherapy and the radiation method optimization. Between January 1994 and June 1998, 325 patients with symptomatic inoperable endobronchial obstructing lung cancer received brachytherapy alone or combined with external beam irradiation with palliative or radical intent. Patients were given 1 to 4 temporary Ir-192 endobronchial implants at the site of obstruction. Implant doses ranged from 6 to 12 Gy specified at a radius of 1 cm from the centre of the source. Total implant doses ranged from 6 to 24 Gy. Depending on radical or palliative intents external beam irradiation doses ranged from 20 to 60 Gy. The results were compared with results achieved in control group (N = 191) treated exclusively with external beam irradiation. Patients who received combined treatment revealed higher frequency of release or disappearance of hemoptysis, dysponea and atelectasis. Also the duration of clinical remission was significantly longer with combined treatment. In the group treated with radical intent higher percentage of patients with total tumor regression in both endoscopic and radiologic view was observed after tele--and brachytherapy compared with teleradiotherapy alone (respectively 29.8% and 19.4%). The group treated with palliative intent brachytherapy alone provided response rates comparable to those achieved with external beam irradiation alone or tele and brachytherapy. The relative risk of fatal pulmonary haemorrhage (FPH) and radiation induced bronchitis (RIB) were higher when NTD > 70 Gy, brachytherapy and laser therapy were administered and in patients with lobar bronchus infiltration. Knowledge of risk doses of FPA and RIB allow to optimise brachytherapy in patients with advanced NSCLC.     

Monte Carlo-aided dosimetry of the Source Tech Medical Model STM1251 I-125 interstitial brachytherapy source

We have used Monte Carlo photon transport simulations to calculate the dosimetric parameters of a new 125I seed, the Source Tech Medical Model STM125I source for interstitial brachytherapy. We followed the recommendations of the AAPM Task Group 43 and determined the following parameters: dose-rate constant, radial dose function, anisotropy function, anisotropy factor, and anisotropy constant. The recently (January 1999) revised National Institute of Standards and Technology I-125 standard for air-kerma strength calibration was taken into account as well as updated interaction cross-section data. The calculated dose-rate constant, when normalized to the simulated wide-angle, free-air chamber measurement of air-kerma strength, is 0.980 cGy h(-1) U(-1). The calculated radial dose function for the Model STM 1251 source is more penetrating than that of the model 6711 seed (by 18% at 5 cm distance), but agrees closely (within statistical errors) with that of the model 6702 seed up to distances of 10 cm. The STM125I source anisotropy functions indicate that its dose distribution is somewhat more anisotropic than that of the model 6702 and 6711 seeds at 1 cm distance but is comparable at larger distances. The Model STM125I anisotropy constant is very similar to that of the model 6711, 6702, and MED363I A/M seeds.     

A theoretical derivation of the nomograms for permanent prostate brachytherapy

This study calculates the required minimum radioactivity to deliver a prescribed dose of radiation to a target using radioisotopes in permanent prostate brachytherapy. Assuming the radioactivity to be in a continuous form, an integral equation--Fredholm equation of the first kind, can be formulated with the radioactivity density used as the variable. The density distribution to produce a uniform volume dose rate is determined using a quadrature method and the radial profile behaves smoothly from the zero radius, and peaks sharply approaching the volume boundary. The density for Pd-103 is about 1.5 times that of I-125 due to its higher spatial attenuation. A nomogram is the relationship between the total activity per unit dose (A) and the dimension of the volume (d). Expressing the nomogram as A=c X dn U/Gy, then (c,n)= [(0.0098, 2.09) I-125] and [(0.031, 2.25) Pd-103]. Compared with the Memorial nomogram, (c,n)=[(0.011,2.2) I-125] and [(0.036,2.56) Pd-103], or that quoted by AAPM TG64, (c,n)=[(0.014,2.05) I-125] and [(0.056,2.22) Pd-103], our calculation determined an average 33% and 35% decrease for I-125, and 89% and 77% decrease for Pd-103, respectively. Two reasons for the extra total activity found in the Memorial and AAPM nomograms are: (a) An imperfect clinical situation limited by the restraints of implant techniques (e.g., use of templates) associated with the presence of adjacent normal organs, and (b) source discretization into seeds. When radioactivity is clumped as discrete seeds, higher activity is needed because of "wastage" in two aspects: (a) Dose cold-spots at intersource spaces, (b) hot-spots around the sources. Thus in theory, use of lower activity seeds will require less total activity to deliver a prescribed dose. Based on our study, Pd-103 delivers a higher therapeutic ratio and a lower integral dose to the patient compared to I-125.     

Planning evaluation of radiotherapy for complex lung cancer cases using helical tomotherapy

Lung cancer treatment is one of the most challenging fields in radiotherapy. The aim of the present study was to investigate what role helical tomotherapy (HT), a novel approach to the delivery of highly conformal dose distributions using intensity-modulated radiation fan beams, can play in difficult cases with large target volumes typical for many of these patients. Tomotherapy plans were developed for 15 patients with stage III inoperable non-small-cell lung cancer. While not necessarily clinically indicated, elective nodal irradiation was included for all cases to create the most challenging scenarios with large target volumes. A 2 cm margin was used around the gross tumour volume (GTV) to generate primary planning target volume (PTV2) and 1 cm margin around elective nodes for secondary planning target volume (PTV1) resulting in PTV1 volumes larger than 1000 cm3 in 13 of the 15 patients. Tomotherapy plans were created using an inverse treatment planning system (TomoTherapy Inc.) based on superposition/convolution dose calculation for a fan beam thickness of 25 mm and a pitch factor between 0.3 and 0.8. For comparison, plans were created using an intensity-modulated radiation therapy (IMRT) approach planned on a commercial treatment planning system (TheraplanPlus, Nucletron). Tomotherapy delivery times for the large target volumes were estimated to be between 4 and 19 min. Using a prescribed dose of 60 Gy to PTV2 and 46 Gy to PTV1, the mean lung dose was 23.8+/-4.6 Gy. A 'dose quality factor' was introduced to correlate the plan outcome with patient specific parameters. A good correlation was found between the quality of the HT plans and the IMRT plans with HT being slightly better in most cases. The overlap between lung and PTV was found to be a good indicator of plan quality for HT. The mean lung dose was found to increase by approximately 0.9 Gy per percent overlap volume. Helical tomotherapy planning resulted in highly conformal dose distributions. It allowed easy achievement of two different dose levels in the target simultaneously. As the overlap between PTV and lung volume is a major predictor of mean lung dose, future work will be directed to control of margins. Work is underway to investigate the possibility of breath-hold techniques for tomotherapy delivery to facilitate this aim.     

More accurate fitting of 125I and 103Pd radial dose functions

In this study an improved functional form for fitting the radial dose functions, g(r), of 125I and 103Pd brachytherapy seeds is presented. The new function is capable of accurately fitting radial dose functions over ranges as large as 0.05 cm < or = r < or = 10 cm for 125I seeds and 0.10 cm < or = r < or = 10 cm for 103Pd seeds. The average discrepancies between fit and calculated data are less than 0.5% over the full range of fit and maximum discrepancies are 2% or less. The fitting function is also capable of accounting for the sharp increase in g(r) (upturn) seen for some sources for r < 0.1 cm. This upturn has previously been attributed to the breakdown of the approximation of the sources as a line, however, in this study we demonstrate that another contributing factor is the 4.5 keV characteristic x-rays emitted from the Ti seed casing. Radial dose functions are calculated for 18 125I seeds and 9 103pd seeds using the EGSnrc Monte Carlo user-code BrachyDose. Fitting coefficients of the new function are tabulated for all 27 seeds. Extrapolation characteristics of the function are also investigated. The new functional form is an improvement over currently used fitting functions with its main strength being the ability to accurately fit the rapidly varying radial dose function at small distances. The new function is an excellent candidate for fitting the radial dose function of all 103Pd and 125I brachytherapy seeds and will increase the accuracy of dose distributions calculated around brachytherapy seeds using the TG-43 protocol over a wider range of data. More accurate values of g(r) for r < 0.5 cm may be particularly important in the treatment of ocular melanoma.     

乳腺癌保乳术后三种瘤床同期加量放疗技术的剂量学比较

目的：比较乳腺癌保乳术后放疗中不同的瘤床同期加量（SimuRaneous Integrated Boost,SIB）技术剂量学特点,探讨其对靶区和正常组织受照剂量的影响。方法：选择12例接受保乳术的乳腺癌患者．为每例患者设计三种瘤床同期加量的放疗计划：适形野中野x射线瘤床加量计划（FIFx）、适形野中野电子线瘤床加量计划（FIFE）以及适形野加调强野的混合调强（HybridIMRT）计划,靶区PTV1、PTV2的处方剂量分别为50Gy和60Gy。比较各计划中靶区剂量特点和患侧肺、对侧乳腺及心脏受照剂量以及机器跳数的差异。结果：当瘤床深度大于4．5cm时,FIFE计划中PTV：的最小剂量显著降低,剂量均匀性明显变差;HybridIMRT的靶区适形指数明显高于其他两种计划（P〈O．05）,患侧肺及心脏受到的高剂量照射体积明显减少（P〈0．05）,低剂量受照体积在各计划中并无统计学差异;HybridIMRT的机器跳数较FIF_X和FIF_E分别减少43．8％（P〈0．05）和21．8％（P〈O．05）。结论：在乳腺癌保乳术后放疗中,当瘤床深度大于4．5cm时采用电子线进行加量容易造成瘤床深部的剂量不足。采用混合调强的瘤床同期加量技术能提高靶区剂量适形度,并能在不增加患侧肺和心脏低剂量受照体积的条件下显著降低其高剂量受照体积。     

The effect of seed anisotrophy on brachytherapy dose distributions using 125I and 103Pd

We have evaluated the effect of the anisotropy of individual seeds on dose distributions for permanent prostate implants using 125I and 103Pd. The dose distributions were calculated for various implants using both the line source and point source calculational formalisms, for two different models of 125I and 103Pd seeds. The dose distributions were compared using cumulative dose volume histograms (DVH) and cumulative difference dose volume histograms (deltaDVH) for the prostate target volume and for the rectum surface. The DVHs could not distinguish between the dose distributions from isotropic and non-isotropic seeds. However, the deltaDVHs were useful in determining the fraction of the target volume for which the difference between the dose distribution for line sources and for point sources exceeded a threshold value. The dose distributions were calculated (1) for all the seeds oriented co-linearly, along either the x-, y-, or z-axis, and (2) for the seeds at randomized orientations, more closely resembling the clinical situation. For all cases, there was a significant difference in the effect of seed anisotropy from the different seed types. For the geometrically simpler test cases with a small number of seeds, the effect of anisotropy on the dose distribution was too large to ignore for any of the seed types investigated. For the idealized pre-plan case, the effect was much smaller. For clinical prostate implants, the calculations done with seeds oriented co-linearly along the z-axis (needle implant axis) were a reasonable approximation for those from simulations of seeds with randomized orientations. Again, the effect of anisotropy varied drastically between different seed models, and also between different clinical cases. However, the effect of anisotropy must be considered in the context of all the other uncertainties in clinical brachytherapy treatments.     

Monte Carlo simulation of the dose distribution around 125I seeds

The Monte Carlo method was used to investigate dose distributions around the 3M Company model 6701 and model 6702 125I brachytherapy seeds. The transverse axis dose distributions of the two seed models were found to be nearly identical, but the longitudinal axis dose distributions differed significantly. Seed design influences upon dose distributions also were investigated.