Verification of proton range, position, and intensity in IMPT with a 3D liquid scintillator detector system.

PURPOSE: Intensity-modulated proton therapy (IMPT) using spot scanned proton beams relies on the delivery of a large number of beamlets to shape the dose distribution in a highly conformal manner. The authors have developed a 3D system based on liquid scintillator to measure the spatial location, intensity, and depth of penetration (energy) of the proton beamlets in near real-time. METHODS: The detector system consists of a 20 × 20 × 20 cc liquid scintillator (LS) material in a light tight enclosure connected to a CCD camera. This camera has a field of view of 25.7 by 19.3 cm and a pixel size of 0.4 mm. While the LS is irradiated, the camera continuously acquires images of the light distribution produced inside the LS. Irradiations were made with proton pencil beams produced with a spot-scanning nozzle. Pencil beams with nominal ranges in water between 9.5 and 17.6 cm were scanned to irradiate an area of 10 × 10 cm square on the surface of the LS phantom. Image frames were acquired at 50 ms per frame. RESULTS: The signal to noise ratio of a typical Bragg peak was about 170. Proton range measured from the light distribution produced in the LS was accurate to within 0.3 mm on average. The largest deviation seen between the nominal and measured range was 0.6 mm. Lateral position of the measured pencil beam was accurate to within 0.4 mm on average. The largest deviation seen between the nominal and measured lateral position was 0.8 mm; however, the accuracy of this measurement could be improved by correcting light scattering artifacts. Intensity of single proton spots were measured with precision ranging from 3 % for the smallest spot intensity (0.005 MU) to 0.5 % for the largest spot (0.04 MU). CONCLUSIONS: Our LS detector system has been shown to be capable of fast, submillimeter spatial localization of proton spots delivered in a 3D volume. This system could be used for beam range, intensity and position verification in IMPT.

Intensity modulated proton therapy treatment planning using single-field optimization: the impact of monitor unit constraints on plan quality.

PURPOSE: To investigate the effect of monitor unit (MU) constraints on the dose distribution created by intensity modulated proton therapy (IMPT) treatment planning using single-field optimization (SFO). METHODS: Ninety-four energies between 72.5 and 221.8 MeV are available for scanning beam IMPT delivery at our institution. The minimum and maximum MUs for delivering each pencil beam (spot) are 0.005 and 0.04, respectively. These MU constraints are not considered during optimization by the treatment planning system; spots are converted to deliverable MUs during postprocessing. Treatment plans for delivering uniform doses to rectangular volumes with and without MU constraints were generated for different target doses, spot spacings, spread-out Bragg peak (SOBP) widths, and ranges in a homogeneous phantom. Four prostate cancer patients were planned with and without MU constraints using different spot spacings. Rounding errors were analyzed using an in-house software tool. RESULTS: From the phantom study, the authors have found that both the number of spots that have rounding errors and the magnitude of the distortion of the dose distribution from the ideally optimized distribution increases as the field dose, spot spacing, and range decrease and as the SOBP width increases. From our study of patient plans, it is clear that as the spot spacing decreases the rounding error increases, and the dose coverage of the target volume becomes unacceptable for very small spot spacings. CONCLUSIONS: Constraints on deliverable MU for each spot could create a significant distortion from the ideally optimized dose distributions for IMPT fields using SFO. To eliminate this problem, the treatment planning system should incorporate the MU constraints in the optimization process and the delivery system should reliably delivery smaller minimum MUs.

Prompt gamma-ray emission from biological tissues during proton irradiation: a preliminary study.

In this paper, we present the results of a preliminary study of secondary 'prompt' gamma-ray emission produced by proton-nuclear interactions within tissue during proton radiotherapy. Monte Carlo simulations were performed for mono-energetic proton beams, ranging from 2.5 MeV to 250 MeV, irradiating elemental and tissue targets. Calculations of the emission spectra from different biological tissues and their elemental components were made. Also, prompt gamma rays emitted during delivery of a clinical proton spread-out Bragg peak (SOBP) in a homogeneous water phantom and a water phantom containing heterogeneous tissue inserts were calculated to study the correlation between prompt gamma-ray production and proton dose delivery. The results show that the prompt gamma-ray spectra differ significantly for each type of tissue studied. The relative intensity of the characteristic gamma rays emitted from a given tissue was shown to be proportional to the concentration of each element in that tissue. A strong correlation was found between the delivered SOBP dose distribution and the characteristic prompt gamma-ray production. Based on these results, we discuss the potential use of prompt gamma-ray emission as a method to verify the accuracy and efficacy of doses delivered with proton radiotherapy.

Assessment of the accuracy of an MCNPX-based Monte Carlo simulation model for predicting three-dimensional absorbed dose distributions.

In recent years, the Monte Carlo method has been used in a large number of research studies in radiation therapy. For applications such as treatment planning, it is essential to validate the dosimetric accuracy of the Monte Carlo simulations in heterogeneous media. The AAPM Report no 105 addresses issues concerning clinical implementation of Monte Carlo based treatment planning for photon and electron beams, however for proton-therapy planning, such guidance is not yet available. Here we present the results of our validation of the Monte Carlo model of the double scattering system used at our Proton Therapy Center in Houston. In this study, we compared Monte Carlo simulated depth doses and lateral profiles to measured data for a magnitude of beam parameters. We varied simulated proton energies and widths of the spread-out Bragg peaks, and compared them to measurements obtained during the commissioning phase of the Proton Therapy Center in Houston. Of 191 simulated data sets, 189 agreed with measured data sets to within 3% of the maximum dose difference and within 3 mm of the maximum range or penumbra size difference. The two simulated data sets that did not agree with the measured data sets were in the distal falloff of the measured dose distribution, where large dose gradients potentially produce large differences on the basis of minute changes in the beam steering. Hence, the Monte Carlo models of medium- and large-size double scattering proton-therapy nozzles were valid for proton beams in the 100 MeV-250 MeV interval.

Radiation safety survey on a flattening filter-free medical accelerator.

Dose rates at several locations outside a treatment room were measured for 6 and 18 MV photon beams from a Varian Clinac 21EX accelerator operated with and without a flattening filter. Also, dose rates in the treatment room due to activation were measured at 18 MV. An analysis of the measured data is presented. The results suggest that substantial reduction in doses outside the treatment room and lower activation can be achieved with a flattening-filter free accelerator.

Measurements of in-air output ratios for a linear accelerator with and without the flattening filter.

The in-air output ratio (Sc) for photon beams from linear accelerators describes the change of in-air output as a function of the collimator settings. The physical origin of the Sc is mainly due to the change in scattered radiation that can reach the point of measurement as the geometry of the head changes. The flattening filter (FF) and primary collimator are the major sources of scattered radiation. The change in amount of backscattered radiation from the collimator into the beam-monitoring chamber also contributes to the variation of output. In this work, we measured the Sc and backscatter factors (Sb) into the beam-monitoring chamber for a linear accelerator with and without the FF. We measured the Sc with a Farmer-type chamber in a miniphantom at the depth of 10 g/cm2 for 6- and 18-MV x-ray beams from a Varian Clinac 2100EX linear accelerator. The Sb were measured with a universal pulse counter and a diode array with build-in counting hardware and software. The head scatter component (Sh) was then derived from the relationship Sc= Sh x Sb, where Sb was the linear fit of measured results. Significant differences were observed for Sc with and without the FF. Within the range of experimental uncertainty, the Sb was similar with and without the FF. The variations in Sh differed significantly over the range of field sizes of 3 X 3 to 40 X 40 cm2 with and without the FF; for the 6-MV beam, it was 8% vs 3%, and for the 18-MV beam, 7% vs 1%. By analyzing the contributions of backscatter factor and total in-air output ratios with and without the FF, we directly gained insight into the contributions of different components to the total variations in Sc of a linear accelerator. Sc, Sb, and Sh are basic and useful dosimetric quantities for delivery of intensity-modulated radiation therapy using a linear accelerator operating in a mode without the FF.

Decreased prevalence of immediate hypersensitivity (atopy) in a cancer population.

It has been suggested that the atopic population has decreased risk of cancer. This investigation examined the cumulative prevalence of atopy in a population with neoplastic disease and compared this with the prevalence of atopy in an age-matched control group and with published estimates of atopy in the general peopulation. Seventy-four patients with neoplastic disease and 86 patients without cancer were evaluated. The subjects were given a standard allergic questionnaire which evaluated them with regard to a history of allergic symptoms, hives, eczema, frequent colds, frequent unexplained rashes, hay fever, and asthma. All were skin tested with a representative group of regionally significant allergens. There was a 15-fold decrease in prevalence of atopy in the cancer population, compared with the control group and compared with published estimates of atopy in the general population.

Derivation of the distribution of extrafocal radiation for head scatter factor calculation.

Head scatter factors for high energy photon beams from linear accelerators can be modeled using a two-source model consisting of focal and extrafocal radiation. The focal radiation can be approximated as a point source, and the distribution of the extrafocal radiation is a two-dimensional (2D) radial symmetric function. Various methods, including analytical, Monte Carlo, and empirical trial functions, have been used to determine the radial symmetric function of extrafocal radiation distribution. This article describes a method for directly determining the extrafocal radiation distribution without assuming any empirical trial function. The extrafocal radiation distribution is determined with measured head scatter factors for rectangular fields defined by the lower jaw (X) fixed at 40 cm and the upper jaw (Y) varying from 3 to 40 cm. The derivatives of the measured head scatter factors, with respect to the Y jaw position projected in the plane of extrafocal radiation, are proportional to the one-dimensional (1D) projection (also called the line spread function) of the extrafocal radiation distribution. Two methods are used to solve the radial function of extrafocal radiation from the 1D projection. The first method uses a 2D filtered backprojection algorithm, originally developed for parallel beam computed tomography reconstruction, to directly derive the radial dependence of the extrafocal radiation distribution. The method has been applied to 6 and 18 MV photon beams from a Siemens linear accelerator and has been tested by comparing measured and calculated head scatter factors for square and rectangular fields. The second method uses a Fourier transform followed by a Fourier-Bessel transform to solve the problem. The distributions of extrafocal radiation derived from these two methods are virtually identical.

Preliminary evaluation of implantable MOSFET radiation dosimeters.

In this paper, we report on measurements performed on a new prototype implantable radiation detector that uses metal-oxide semiconductor field effect transistors (MOSFETs) designed for in vivo dosimetry. The dosimeters, which are encapsulated in hermetically sealed glass cylinders, are used in an unbiased mode during irradiation, unlike other MOSFET detectors previously used in radiotherapy applications. They are powered by radio frequency telemetry for dose measurements, obviating the need for a power supply within each capsule. We have studied the dosimetric characteristics of these MOSFET detectors in vitro under irradiation from a 60Co source. The detectors show a dose reproducibility generally within 5% or better, with the main sources of error being temperature fluctuations occurring between the pre- and post-irradiation measurements as well as detector orientation. A better temperature-controlled environment leads to a reproducibility within 2%. Our preliminary in vitro results show clearly that true non-invasive in vivo dosimetry measurements are feasible and can be performed remotely using telemetric technology.

CT-guided interstitial implantation of gynecologic malignancies.

PURPOSE: To establish the efficacy of computed tomography (CT)-based planning and analysis of transperineal implants. METHODS AND MATERIALS: For patients with bulky disease or geometrically unfavorable anatomy, transperineal interstitial implantation of gynecologic tumors offers an alternative to standard intracavitary techniques. Control of dose rate and total dose distributions to produce a homogenous, low dose rate implant presents a challenge to the radiation oncologist in these complex implants, as does the relationship of these distributions to the patients's anatomy. We have used CT imaging following needle implantation, prior to source loading, in 25 patients (28 implants), as an aid in both the planning of the implant and the analysis of the dosimetry. RESULTS: The spatial relationship between the needles and the normal anatomy can be clearly defined, despite the presence of some artifacts. Tumor volume is less clearly visualized but the adequacy of needle placement can be assessed and adjusted if necessary. Modifications of the planned source placement, based upon the location of specific needles and critical structures, can be made prior to loading the patient. Dose rate and total dose distributions are displayed with the appropriate anatomy on axial images and on reconstructed sagittal and coronal planes. Multiple points of dose specification for the rectum and the bladder are easily defined. Dose rate adjustment can be made by selectively changing the activity associated with a particular needle or needles. Multiple implants as well as external beam irradiation can also be integrated. CONCLUSIONS: CT-based dosimetry has permitted intelligent planning decisions to be made prior to and during these implants. It has further allowed more accurate anatomically based dosimetric analysis, with visualization and control of dose rate and total dose distributions displayed together with the patient's anatomy. This more elaborate analysis should ultimately lead to a better understanding of the reasons for local control and complications and their relationships to dose rate, total dose, and volume.

Comparision of four different dose specification methods for high-dose-rate intracavitary radiation for treatment of cervical cancer.

PURPOSE: To compare the dose delivered to target tissues and dose-limiting structures as defined by specific dose points with high-dose-rate intracavitary brachytherapy using tandem and ring or tandem and ovoids applicators, and to provide a reasonable approach to dose optimization. METHODS AND MATERIALS: Dosimetry was obtained using four different dose specifications: (1) 100% of the dose prescribed in a tapered fashion along the tandem and 140% at the ovoid/ring surface, (2) 100% of the dose prescribed along the tandem and 100% at the ovoid/ring surface, (3) 100% of the dose prescribed to point A without any additional applicator specification points, and (4) nonoptimized plan using relative dwell weighting to simulate classic Fletcher low-dose-rate (LDR) loading with the dose specified at point A. Point doses were recorded at A, B, and T (cervical tumor point), ICRU rectum, and ovoid/ring surface. RESULTS: For the tandem and ovoids applicators, significant differences were found among the four different dose specification methods for point T and vaginal mucosal doses. When the dose was optimized to point A alone, the ovoid dwell weights were reduced, resulting in higher point T doses and underdosing of the vaginal mucosa. Fixed weighting based on Fletcher LDR loading specifications resulted in higher vaginal mucosa doses. For the tandem and ring applicators, significant differences were observed for vaginal mucosal doses and the ICRU rectal dose. Optimization to point A alone resulted in widely varying dosimetric distributions and vaginal mucosa doses up to 632% of the prescription dose. With nonoptimized fixed weighting, the vaginal wall dose and ICRU rectal dose were increased. CONCLUSION: Prescribing to dose optimization points in a tapered fashion along the tandem and at the ovoid/ring surface results in a pear-shaped dose distribution resembling classic LDR systems. The other dose specification methods may result in underdosing of important target tissues or overdosing of adjacent dose-limiting structures.

Comparison of high dose-rate and low dose-rate dose distributions for vaginal cylinders.

PURPOSE: The identification of appropriate high dose-rate parameters required to produce a "uniform" dose distribution on the surface of a vaginal cylinder. The high dose-rate dose distribution is then compared to the traditional low dose-rate dose distributions obtained with Burnett cylinders. METHODS AND MATERIALS: Dose distributions were calculated for 2, 3, and 3.5 cm diameter Burnett cylinders with and without crossing sources. Three models for the high dose-rate cylinders were developed and compared. High dose-rate dose distributions were calculated for 2, 3, and 3.5 cm diameter cylinders with and without anisotropic corrections for various dose specification points. RESULTS: Low dose-rate distributions are not uniform over the surface of the applicator. The exact distribution depends upon cylinder diameter and upon the exact source loading. High dose rate dose distributions can be configured to provide for a "uniform" dose on the surface, if an apex dose specification point is used together with dose specification points on the surface of the applicator opposite each dwell position. CONCLUSIONS: The conversion of low dose rate techniques to high dose rate techniques for vaginal cylinders involves an appreciation of the details of dose distributions of both approaches. The comparison between traditional low dose-rate distributions and high dose-rate distributions shows that, unlike the low dose-rate distributions, a relatively uniform high dose-rate distribution can be obtained independent of cylinder diameter. The clinical significance of the differences in the low dose-rate and high dose-rate dose distributions remains to be determined by long-term follow up of patients treated with high dose-rate techniques.

Microdosimetric single event spectra of Ytterbium-169 compared with commonly used brachytherapy sources and teletherapy beams.

Ytterbium-169 has been developed as a possible replacement for Iridium-192 and Iodine-125. The Theory of Dual Radiation Action predicts that the initial slope of the cell survival curve and therefore the relative biological effect at low dose rate is proportional to dose average lineal energy, yd, which is the microscopic analog of the dose average linear energy transferred. The quality factor used in radiation protection has been shown to be a function of the frequency average lineal energy, yf. Single event microdosimetric spectra for 60Co, 137Cs, 192Ir, 125I and 169Yb were measured in air and at several depths in phantom with a Rossi proportional counter. These spectra show marked differences between sources. The microscopic analogs of the track average and dose average LET, (yd and yf, respectively) differ between isotopes by factors of two or even higher in comparison to megavoltage electron beams. These yd's and yf's for 169Yb are consistently higher when compared to 60Co or 137Cs but are approximately equal to those for 125I. Values of yf and yd for 192Ir are intermediate between 60Co and 169Yb. The Theory of Dual Radiation Action predicts a low dose rate RBE (assuming a 1 micron effective site diameter) compared to 60Co (in air) of: 1.00 for 137Cs, 1.29 for 192Ir, 1.60 for 169Yb and 1.77 for 125I.

Comparison of traditional low-dose-rate to optimized and nonoptimized high-dose-rate tandem and ovoid dosimetry.

PURPOSE: Few dose specification guidelines exist when attempting to perform high-dose-rate (HDR) dosimetry. The purpose of this study was to model low-dose-rate (LDR) dosimetry, using parameters common in HDR dosimetry, to achieve the "pear-shape" dose distribution achieved with LDR tandem and ovoid applications. METHODS AND MATERIALS: Radiographs of Fletcher-Suit LDR applicators and Nucletron "Fletcher-like" HDR applicators were taken with the applicators in an idealized geometry. Traditional Fletcher loadings of 3M Cs-137 sources and the Theratronics Planning System were used for LDR dosimetry. HDR dosimetry was performed using the Nucletron Microselectron HDR UPS V11.22 with an Ir-192 source. Dose optimization points were initially located along a line 2 cm lateral to the tandem, beginning at the tandem tip at 0.5-cm intervals, ending at the sail, and optimized to 100% of the point A dose. A single dose optimization point was also placed laterally from the center of each ovoid equal to the radius of the ovoid (ovoid surface dose). For purposes of comparison, dose was also calculated for points A and B, and a point located 1 cm superior to the tandem tip in the plane of the tandem, (point F). Four- and 6-cm tandem lengths and 2.0-, 2.5-, and 3.0-cm ovoid diameters were used for this study. Based on initial findings, dose optimization schemes were developed to best approximate LDR dosimetry. Finally, radiographs were obtained of HDR applications in two patients. These radiographs were used to compare the optimization schemes with "nonoptimized" treatment plans. RESULTS: Calculated doses for points A and B were similar for LDR, optimized HDR, and nonoptimized HDR. The optimization scheme that used tapered dose points at the tandem tip and optimized a single ovoid surface point on each ovoid to 170% of point A resulted in a good approximation of LDR dosimetry. Nonoptimized HDR resulted in higher doses at point F, the bladder, and at points lateral to the tandem tip than both the optimized plan or the LDR plan. CONCLUSION: Optimized HDR allows specification of dose to points of interest, can approximate LDR dosimetry, and appears superior to nonoptimized HDR treatment planning, at least at the tandem tip. An optimization scheme is presented that approximates LDR dosimetry.

Correlation of treatment volume with milligram-hours for intracavitary applications for carcinoma of the cervix.

Following the recommendations of the European Curietherapy Group, the three-dimensional dose distribution corresponding to various milligram-hour volumes has been analyzed according to its length, width, and height dimensions. Thus, it is possible to state the dimensions of a number of isodose surfaces for a dose prescription given in milligram-hours. Problems associated with the exact placement of the three-dimensional dose distribution in relation to the patient's anatomy are discussed.

Improved dose localization with dual energy photon irradiation in treatment of lateralized intracranial malignancies.

Dual energy photon irradiation (6 MV and 20 MV) was compared to conventional treatment planning with 6 MV photons in a lateralized intracranial malignancy. Dose volume analysis was performed of both the tumor plus a 2 cm margin (target volume, TV) and normal tissues (NT). Parallel opposed treatment using weightings of 1:1, 1.5:1, and 2:1 were compared for 6 MV photons alone or in combination with 20 MV photons. Uniform treatment of the TV was accomplished within the 60 Gy isodose. Significant differences were observed, however, in NT volumes receiving greater than or equal to 60 Gy and 45-59 Gy. Dual photon energy reduced treatment of NT volumes to greater than or equal to 60 Gy by 13% (177 cm3 vs 204 cm3 in 2:1 weighting) to 70% (147 cm3 vs 498 cm3 in 1:1 weighting) for comparable plans. Dose optimization was also performed for both 6 MV alone or in combination with 20 MV photons. Usual approaches to achieve dose lateralization with conventional isocentric techniques were applied including parallel opposed 6 MV photons ipsilaterally weighted 3.4:1 (POP), and a 110 degrees arc rotational field used to limit treatment to the eye (ARC). Dual energy photon optimized plans included a three beam parallel opposed plan (TOP) and a mixed photon ipsilateral (IPSI) approach. The technique using parallel opposed 20 MV photons and ipsilateral 6 MV photons (TOP) used beam weightings of 1.1 (contralateral 20 MVX): 1.6 (ipsilateral 6 MVX): 1 (ipsilateral 20 MVX) to achieve dose optimization. The ipsilateral approach with 6 MVX and 20 MVX (IPSI) used beam weightings of 1:1.4, respectively. All optimized plans demonstrated a 41% (120 cm3; POP) to 53% (95 cm3; TOP) improvement over parallel opposed 6 MV photons weighted 2:1 (204 cm3) in NT volume receiving greater than or equal to 60 Gy. Comparison of optimized treatment showed the IPSI plan to be superior, treating 12% of NT volume to greater than or equal to 60 Gy and 38% to 45-59 Gy; the 6 MV POP plan resulted in NT volumes of 15% and 51%, respectively, for those dose levels. Dual photon energy irradiation of lateralized intracranial malignancies allows reduction of dose to normal tissue volumes while achieving excellent coverage of the target volume. Treatment planning should be performed in all lateralized intracranial lesions to achieve dose optimization exploiting depth dose characteristics.

Radiation Therapy Oncology Group: radiosurgery quality assurance guidelines.

A multidisciplinary Radiation Therapy Oncology Group (RTOG) task force has developed quality assurance guidelines for radiosurgery. The purpose of the guidelines are fourfold: (1) To ensure that participating institutions have the proper equipment and appropriate technique(s) to administer radiosurgery; (2) to outline a standard data set for each treated patient to assess protocol compliance; (3) to define minor and major deviations in protocol treatment; and (4) to set forth clinical data necessary to determine treatment efficacy, including failure patterns, and treatment toxicity. These guidelines are being implemented into active and developing radiosurgery protocols.

Single and double plane implants: a comparison of the Manchester System with the Paris System.

A comparison between the Manchester System and the Paris System of interstitial dosimetry has been made in the case of single and double plane implants. The rules of both systems are reviewed. A brief description of the Paris System is presented in an appendix. Dose distributions for two different examples are presented in two orthogonal planes. The Paris System uses considerably fewer sources than the Manchester System. It results in a larger volume of high dose than the Manchester System. The use of Iridium-192 sources strength and source length can be adjusted represents a significant advantage. The Paris System attempts to adapt the implant configuration to the clinical situation as the target thickness is used to define the source separation and the target length is used to define the source length. The differences in the dose definition are discussed.

Dose distribution in total skin electron beam irradiation using the six-field technique.

Total skin low energy electron beam irradiation is used to treat superficially widespread skin lesions such as cutaneous T-cell lymphoma. Total skin irradiation involves delivering an adequate dose at a depth of 0.25 to 1.0 cm, while sparing underlying tissue. The dose distributions obtained when using a modified Stanford six-field technique depend upon the beam energy, the beam angle, the diameter and shape of the body part, and other variables. The dose distribution uniformity of six pairs of angulated electron beams has been studied as a function of beam energy, the gantry angle, +/- theta, above and below the horizontal and the diameter of a cylindrical polystyrene phantom. Depth doses and dose uniformity for single and multiple fields have been measured as a function of beam energy, phantom diameter and position.

Technical modifications in hyperfractionated total body irradiation for T-lymphocyte deplete bone marrow transplant.

The Medical College of Wisconsin implemented a major bone marrow transplant (BMT) program in July 1985. The type of transplants to be focused on were allogeneic T-lymphocyte deplete. Total body irradiation (TBI) was initially patterned after the Memorial method. Patients received total body irradiation in a sitting position at a dose rate of 20-25 cGy/minute with 50% attenuation lung blocks used both anterior/posterior and posterior/anterior. Electron boosting was utilized for the ribs beneath the lung blocks. Occasionally, lower extremity boosting was required because of the sitting position. A dose of 14 Gy was chosen since T-lymphocyte deplete bone marrow transplant data suggest the need for higher total doses to consistently obtain engraftment. This dose was given in 3 equal daily fractions over 3 days following conditioning chemotherapy. Six of 11 patients treated in this manner developed lethal pulmonary events. In response to the pulmonary toxicity, partial lung shielding was increased to 60% attenuation. In the next 107 patients receiving this program of total body irradiation there was a reduced incidence of fatal pulmonary events (10 cases of fatal idiopathic interstitial pneumonitis and 12 cases of fatal pulmonary infections) after a median follow-up of 9 months. This was an obvious improvement over the initial group. A significant level of hepato-renal toxicity was also observed with 14 Gy total body irradiation when no liver or kidney blocking was used. Of the first 20 patients treated, three cases of fatal veno-occlusive disease resulted. Subsequently, a 10% attenuation right sided liver block was added. Five of 98 patients treated with this block have developed fatal hepatic dysfunction, (median follow-up of 7.2 months). This incidence is not statistically different from the initial group but favors the use of the liver block. Some renal toxicity was also detected with the earlier regimen, especially in pediatric patients. Partial kidney blocking has been implemented to minimize this toxicity. Our current dose rate has been reduced to 8 cGy/minute in a further attempt to reduce organ toxicity. To date, this selective blocking has not adversely affected the excellent rate (96%) of first time engraftments.