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Purpose: To evaluate the potential benefits of intensity modulated tangential beams in the irradiation of the intact breast. Methods and Materials: Three-dimensional treatment planning was performed on five left and five right breasts using standard wedged and intensity modulated (IM) tangential beams. Optimal beam parameters were chosen using beams-eye-view display. For the standard plans, the optimal wedge angles were chosen based on dose distributions in the central plane calculated without inhomogeneity corrections, according to our standard protocol. Intensity-modulated plans were generated using an inverse planning algorithm and a standard set of target and critical structure optimization criteria. Plans were compared using multiple dose distributions and dose volume histograms for the planning target volume (PTV), ipsilateral lung, coronary arteries, and contralateral breast. Results: Significant improvements in the doses to critical structures were achieved using intensity modulation. Compared with a standard-wedged plan prescribed to 46 Gy, the dose from the IM plan encompassing 20% of the coronary artery region decreased by 25% (from 36 to 27 Gy) for patients treated to the left breast; the mean dose to the contralateral breast decreased by 42% (from 1.2 to 0.7 Gy); the ipsilateral lung volume receiving more than 46 Gy decreased by 30% (from 10% to 7%); the volume of surrounding soft tissue receiving more than 46 Gy decreased by 31% (from 48% to 33%). Dose homogeneity within the target volume improved greatest in the superior and inferior regions of the breast (approximately 8%), although some decrease in the medial and lateral high-dose regions (approximately 4%) was also observed. Conclusion: Intensity modulation with a standard tangential beam arrangement significantly reduces the dose to the coronary arteries, ipsilateral lung, contralateral breast, and surrounding soft tissues. Improvements in dose homogeneity throughout the target volume can also be achieved, particularly in the superior and inferior regions of the breast. It remains to be seen whether the dosimetric improvements achievable with IMRT will lead to significant clinical outcome improvements.

To determine the long-term effects of 75.6- and 81.0-Gy doses of three-dimensional conformal radiation therapy in a dose-escalation study in patients with stage T2c-T3 prostatic cancer.Fifty patients received an initial 75.6-Gy dose, and the dose in 46 patients was subsequently escalated to 81.0-Gy. Median follow-up was 60 and 40 months, respectively.The rates of effects of acute toxicity during the course of treatment were similar for both dose levels. Among the 96 patients, the rate of grade 2 morbidities necessitating medication to relieve acute symptoms was 17% (16 patients) for rectal and 36% (35 patients) for urinary morbidities. All other patients had either no or grade 1 morbidities. Fourteen patients (15%) developed late grade 2 rectal morbidities. There were no differences in 5-year actuarial rates of late grade 2 rectal or urinary morbidities among patients who received 75.6 Gy versus those who received 81.0 Gy. One patient treated with 81.0 Gy developed a grade 3 urethral stricture, which was resolved with dilatation.Tumor dose escalation beyond conventional radiation doses for localized prostatic cancer is feasible when delivered with three-dimensional conformal radiation therapy, with no increase in morbidity in normal tissue.

Purpose : Local failure is a major obstacle to the cure of locally advanced non small-cell lung cancer. Three-dimensional conformal radiation therapy (3-DCRT) selects optimal treatment parameters to increase dose to tumor and reduce normal tissue dose, potentially representing an enhancement of the therapeutic ratio of radiation therapy for lung cancer. We performed this analysis of 45 non-small cell lung cancer patients treated with 3-DCRT alone, to evaluate the ability of computer derived lung dose volume histograms to predict serious pulmonary toxicity, to assess the feasibility of this approach, and to examine the resulting survival. Methods : There were 28 males (62%) and 17 females (38%). The median age was 65 (range: 38–82). Tumor stage was Stage I/II in 13%, IIIa in 42%, and IIIb in 44%. The histology was squamous in 44%, adenocarcinoma in 36%, and other non-small cell histologies in the others. Only 47% of patients, had combined favorable prognostic factors (i.e. KPS ≤ 80, and ≤5% wt. loss). The median dose of radiation to gross disease was 70.2 Gy (range: 52.2–72 Gy) delivered in fractions of 1.8 Gy, 5 days per week. Results : Seven patients did not complete 3-DCRT due to disease progression outside the port. Follow-up data are mature: the median follow up of the 6 survivors is 43.5 months (35–59). Thoracic progression occurred in 46%. Median survival (all 45 patients.) is 15.7 months and survival is 32% at 2 years and 12% at 59 months. Pulmonary toxicity ≥grade 3 occurred in 9% of patients. Dose volume histograms were available in 31 patients and showed a correlation between risk of pulmonary toxicity and indices of dose to lung parenchyma. Grade 3 or higher pulmonary toxicity occurred in 38% (38) of patients with > 30% of lung volume receiving ≥25 Gy, versus 4% (123) of patients with ≤30% lung receiving ≥25 Gy ( P = 0.04). Grade 3 or higher pulmonary toxicity occurred in 29% (414) of patients with a predicted pulmonary normal tissue complication probability of 12% or higher versus 0% (0/17) in patients with a predicted probability of less than 12% ( P = 0.03). Conclusions : Despite adverse prognostic criteria median survival is encouraging and may be higher than some combined modality approaches. Dose volume histogram parameters may be useful to determine the maximum dose for individual patients and thereby permit avoidance of toxicity.

Purpose : We have described previously a model for delivering computer-controlled radiation treatments. We report here on the implementation and first year's clinical experience with such treatments using a 50 MeV medical microtron. Methods and Materials : The microtron is equipped with a multileaf collimator and is capable of setting up and treating a sequence of fixed fields called segments, under computer control. An external computer derives machine parameters for the segments from a three-dimensional treatment planning system, transfers them to the microtron control computer, checks the machine settings before allowing dose delivery to begin, and records the treatment. We describe the patient treatment methodology, portal film acquisition, electronic portal imaging, and quality assurance. Results : Patient treatments began in July 1992, comprising six-segment conformal treatments of the prostate. Using the recorded treatment data, the system performance has been examined and compared to other treatment machines. The average treatment time is 10 min, of which 4 min is for computer-controlled setup and irradiation; the remaining time is for patient positioning and checking of clearances. Long-term reproducibility of computer-controlled setup of the gantry and multileaf position is better than 0.5° and 1 mm, respectively. Termination due to a machine fault has occurred in 5.5% of treatments, improving to 2.5% in recent months. Conclusion : Our initial experience indicates that computer-controlled segmental therapy can be performed reliably on a routine basis. Treatment times with the microtron are significantly shorter than with conventional linacs, and setup accuracy is consistent with that needed for conformal therapy. We believe that treatment times can be further improved through software upgrades and integration of electronic portal imaging.

Purpose : A 50 MeV racetrack microtron has been installed and tested at Memorial Sloan-Kettering Cancer Center. It is designed to execute multi-segment conformal therapy automatically under computer control using scanned X ray and electron beams from 10 to 50 MeV. Prior to acceptance of the machine from the manufacturer, formal reliability testing was carried out. Only in this way could confidence be gained in its usefulness for routine 3D computer-controlled conformal therapy. Materials and Methods : To assess reliability, a set of 25 multi-segment test cases, each consisting of 10 to 17 fixed segments, was developed. The field arrangements and modalities for some of the test cases were identical to 3D conformal treatments that were being delivered with multiple static fields on conventional linear accelerators at our institution. Other cases were designed to explore reliability under more complex sets of conditions. These cases were “treated” repeatedly during a total period of 45 hours, over 5 days. During the treatments, ion chambers attached to the head of the machine provided dosimetric data for each field. Data from sensors connected to every set-up parameter (for example, couch positions, gantry angle, collimator leaf positions, etc.) were recorded and verified by an external computer. Results : While preliminary tests indicated an interlock rate of 5%, final reliability test results demonstrated an interlock fault rate of approximately 0.5%. The reproducibility of dosimetric data and geometric setup parameters was within specifications. As an example, leaf position reproducibility in the patient plane was within 0.5 mm for 97% of the setups. The times required to carry out treatments were recorded and compared with the times to carry out identical treatments on a conventional linear accelerator with cerrobend blocks. Areas where additional time savings can be achieved were identified. Conclusion : As an integral part of acceptance testing, the Scanditronix MM50 was rigorously tested for reliability. The machine successfully passed these tests, providing increased confidence in its usefulness for routine 3D conformal therapy.

Purpose : The specific aim of 3-dimensional conformal radiation therapy is to improve the target dose distribution while concomitantly reducing normal tissue dose. Such an approach should permit dose escalation until the limits of acceptable normal tissue toxicity are reached. To evaluate the feasibility of tumor dose escalation for nine patients with lung cancer, we determined the dose distribution to the target and normal tissues with 3-dimensional conformal radiation therapy and conventional planning. Methods and Materials : Plans were compared to assess adequacy of dose delivery to target volumes, dose-volume histograms for normal tissue, and normal tissue complication probabilities (NTCP) for nine patients with lung tumors. Results : The mean percentage of gross disease which received ≤ 70.2 Gy with 3-dimensional conformal radiation therapy (31)CRT) was 40% of the mean percentage of gross disease which received ≤ 70.2 Gy with conventional treatment planning (CTP). The mean NTCP for lung parenchyma with 3DCRT was 36% of the mean NTCP with CTP. The mean esophageal NTCP with 3DCRT was 88% of the mean NTCP with CTP. Conclusion : This preliminary analysis suggests that three dimensional conformal radiation therapy may provide superior delivery of high dose radiation with reduced risk to normal tissue, suggesting that this approach may have the potential to improve the therapeutic ratio of high dose radiation therapy for lung cancer.

A biologically based model is developed to predict radiation-induced normal tissue complication probability (NTCP) in inhomogeneously irradiated organs such as the lung or the kidney. The organ is assumed to be composed of independent functional subunits (FSUs) organized with a parallel architecture and it is assumed that the complication is produced only if a sufficiently large number of FSUs (the "functional reserve") are destroyed. A general expression for NTCP is derived, as well as simple expressions for the mean and standard deviation of the radiation damage to the FSUs. It is demonstrated that these results for inhomogeneous irradiation reproduce those of the serial and tumor control models when the functional reserve consists of one or all of the FSUs, respectively. When the number of FSUs is large, the dose response for organs with identical characteristics is very steep. Since clinical dose-response curves may arise from populations with varying functional reserves and radiosensitivities, we derive expressions for the NTCP for inhomogeneously irradiated organs that incorporate such variations.

A biologically based model is developed for normal tissue complication probability as a function of dose and irradiated volume fraction for organs such as the kidney and the lung. The organ is assumed to be composed of functional subunits (FSUs) which are arranged in a parallel architecture. The complication is produced only if a sufficiently large fraction of the FSUs are inactivated by radiation and an FSU is inactivated only when all the clonogenic cells within it are killed. The linear-quadratic model is used for the dose-response of individual cells within an FSU. The predictions of this model are compared with those of an empirical power law function for uniform whole and partial organ irradiation.

The role of 3-D treatment planning for carcinoma of the nasopharynx was assessed in a four institution study. Two patients were worked up and had an extensive number of CT scans on which target volumes and normal tissues were defined. Treatment planning was then performed using state of the art dose planning systems for these patients to assess the value of the new technology. In general, it was demonstrated that multi-field conformal plans could achieve good tumor dose coverage, while at the same time reducing normal tissue doses, compared to standard treatment planning techniques. The role of inhomogeneity corrections, beam energy, and the use of CT vs. simulation films for defining target volumes were also discussed. In addition, techniques to evaluate 3-D plans for the nasopharynx were considered, and some analysis of this problem is presented in this paper.

During external beam radiotherapy, normal tissues are irradiated along with the tumor. Radiation therapists try to minimize the dose of normal tissues while delivering a high dose to the target volume. Often this is difficult and complications arise due to irradiation of normal tissues. These complications depend not only on the dose but also on volume of the organ irradiated. Lyman has suggested a four-parameter empirical model which can be used to represent normal tissue response under conditions of uniform irradiation to whole and partial volumes as a function of the dose and volume irradiated. In this paper, Lyman's model has been applied to a compilation of clinical tolerance data developed by Emami et al. The four parameters to characterize the tissue response have been determined and graphical representations of the derived probability distributions are presented. The model may, therefore, be used to interpolate clinical data to provide estimated normal tissue complication probabilities for any combination of dose and irradiated volume for the normal tissues and end points considered.

Purpose : Local failure is a major obstacle to the cure of locally advanced non-small-cell lung cancer. 3-Dimensional conformal radiation therapy (3-DCRT) selects optimal treatment parameters to increase dose to tumor and reduce normal tissue dose, potentially permitting dose escalation. There are several ongoing trials of dose escalation using 3-dimensional conformal radiation therapy for non-small-cell lung cancer. We performed this analysis to determine if data derived from dose volume histograms could be used as the basis for designing the method of dose escalation in these trials. Methods and materials : Between 1990 and 1993, 31 patients were treated with 3-DCRT and had complete normal tissue dose volume histograms created as part of the planning process. The stage distribution was stage I/II 13%, stage IIIa in 45%, and stage IIIb in 42%. The median radiation dose to gross disease was 70.2 Gy (52.2-72 Gy). Elective mediastinal irradiation (50.4 Gy) was administered to 52% (16/31) of patients. Results : The major toxicity encountered in this experience was pulmonary. Dose-volume-histogram data were used to analyze the predictors of toxicity and showed a correlation between risk of pulmonary toxicity and indices of dose to lung parenchyma. Grade 3 or higher pulmonary toxicity occurred in 38% (3/8) of pts with > 30% of lung volume receiving ≥ 25 Gy, versus 4% (1/23) of pts. with ≤ 30% lung receiving ≥ 25 Gy (p=0.04). Grade 3 or higher pulmonary toxicity occurred in 29% (4/14) of patients with a predicted pulmonary normal tissue complication probability of 12% or higher versus 0% (0/17) in patients with a predicted probability of less than 12% (p=0.03). The single fatality occurred in a patient with a calculated pneumonitis probability of 85% and a high percent (49%) lung volume receiving > = 25 Gy. Conclusion : This preliminary experience demonstrates a correlation between lung dose-volume-histogram data and the risk of severe pulmonary toxicity. This provides an opportunity to modify the method of radiation dose escalation. Dose-volume-histogram data can allow escalation according to the risk to the lung parenchyma (which is the major organ of concern) rather than escalation according to tumor dose levels. Because of the major inter-patient variability of intrathoracic tumor bulk and anatomic distribution, this strategy is intuitively appropriate. This approach may facilitate completion of dose escalation studies and identification of maximum tolerable pulmonary dose levels.

The first of a new generation of microtron accelerators has been installed and tested. It is currently in use for multisegment conformal radiotherapy at our institution. The unit produces x rays and electrons from 10 to 50 MeV in 5 MeV increments. It incorporates a 64 leaf, doubly focused multileaf collimator (MLC), which can be used to shape x-ray and electron beams. Both x-ray and electron beams are produced by magnetically scanning the electron beams from the accelerator. The new generation unit incorporates a purging magnet to sweep away any primary or secondary electrons that pass through the target(s). In this paper, the beam characteristics of the accelerator that were studied during acceptance testing are described. Representative examples of depth doses, beam profiles, output factors, and elementary beam distributions are presented and discussed, in comparison with the earlier generation of microtron accelerators and with other radiotherapy machines.

The characteristics of photon beams from the Scanditronix MM50 radiation therapy machine that are necessary for treatment planning are described. The MM50 uses a scanning beam instead of a conventional flattening filter to achieve flat dose distributions. At each beam energy, a scan pattern is chosen, depending on the field size; the small scan pattern (S) is used for field sizes up to 10 x 10 cm, the medium scan pattern (M) is used for field sizes up to 20 x 20 cm, and the large scan pattern (L) is used for the larger field sizes. The dose distributions of the beams associated with the 10 MV S, M, and L scan patterns, the 25 MV S, M, and L patterns, and the 50 MV S and M patterns are described. The data reported includes central axis data, beam profiles, and output factors. In addition to the measured data, our dose calculation model requires a pencil beam kernel for each beam. The kernel is constructed using the average photon energy spectrum, which is generated using a Monte Carlo simulation of the MM50. The simulation, based on EGS4, is also used to generate the radial variation of fluence and energy fluence, which is required by a new dose calculation model that does not require the measurement of beam profiles. The Monte Carlo generated data; the photon energy spectrum, the fluence, and the energy fluence are presented.

In this paper a method of computer-aided optimization of 3-D conformal treatment plans is presented which incorporates models to predict the clinical consequences of resulting dose distributions. Even though these models are simplistic, it is submitted that their intelligent use leads to treatment plans which indicate lower normal tissue complications and higher tumor control. Dose distribution data, biological models, and observed normal tissue and tumor response data are used to compute tumor control and normal tissue complication probabilities for each of the critical normal structures encountered in a treatment plan. These quantities are combined into a single score using an objective function which incorporates the importance of each end point as assessed by the physician. Using the "simulated annealing" method of optimization, the beam weights are adjusted to maximize the score. Additional constraints are applied to ensure consistency of the results of optimization with the judgment of the physician. These optimization methods have been applied to conformal treatment plans consisting of multiple fixed fields with conformal field shaping. The results indicate that the methods presented have considerable potential.

New tools are needed to help in evaluating 3-D treatment plans because of the large volume of data. One technique which may prove useful is the application of complication probability calculations. A method of calculating complication probabilities for inhomogeneously irradiated normal tissues is presented in this paper. The method uses clinical estimates of tolerance doses for a few discreet conditions of uniform partial organ irradiation, an empirical fit of a continuous function to these data, and a technique (the effective volume method) for transforming nonuniform dose-volume histograms into equivalent uniform histograms. The behavior of the effective volume histogram reduction method for various boundary conditions is reviewed. The use of complication probabilities in evaluating treatment plans is presented, using examples from an NCI 3-D treatment planning contract.

This is a report on numerical scoring techniques developed for the evaluation of treatment plans as part of a four-institution study of the role of 3-D planning in high energy external beam photon therapy. A formal evaluation process was developed in which plans were assessed by a clinician who displayed dose distributions in transverse, sagittal, coronal, and arbitrary oblique planes, viewed dose-volume histograms which summarized dose distributions to target volumes and the normal tissues of interest, and reviewed dose statistics which characterized the volume dose distribution for each plan. In addition, tumor control probabilities were calculated for each biological target volume and normal tissue complication probabilities were calculated for each normal tissue defined in the agreed-upon protocols. To score a plan, the physician assigned a score for each normal tissue to reflect possible complications; for each target volume two separate scores were assigned, one representing the adequacy of tumor coverage, the second the likelihood of a complication. After scoring each target and normal tissue individually, two summary scores were given, one for target coverage, the second reflecting the impact on all normal tissues. Finally, each plan was given an overall rating (which could include a downgrading of the plan if the treatment was judged to be overly complex).

In May 1979, Memorial Sloan-Kettering embarked on a programme of hyperfractionated TBI (HFTBI), 1320 cGy in 11 fractions over 4 days with partial lung shielding (1 HVL), followed by cyclophosphamide (60 mg/kg/d x 2d) for cytoreduction prior to allogeneic bone marrow transplantation (BMT). Anterior and posterior chest wall electron "boosts" were given to the areas blocked (600 cGy in 2 fractions) on the last two days of treatment. Since then, we have treated over 600 patients with HFTBI, the majority for allogeneic BMT. Several modifications have occurred over the years. We have added a "boost" electron dose of 400 cGy to the testes in all male leukemic patients; this reduced testicular relapses from a rate of 14% (4/28) to 0%. In an attempt to increase engraftment of T-depleted BMTs, we added one additional fraction; since our present dose/fraction was also increased to 125 cGy, we now deliver a total dose of 1500 cGy in 12 fractions over 4 days for allogeneic transplants. Tolerance to HFTBI has been excellent relative to the single dose (SD) regimen utilised prior to May, 1979. The incidence of fatal interstitial pneumonitis (IP) decreased from 50% in the SD regimen to 18% after the introduction of HFTBI. In children, the incidence of IP was only 4% with HFTBI. With the introduction of T-depleted marrows, fatal IP in adults has decreased also, e.g. to less than 10% in CML patients. With conventional BMT after HFTBI, relapse at 5 years has been exceedingly low (e.g. in children, 13% for ALL, 2nd remission and 0% for AML, 1st remission) and engraftment has been 100%. With matched T-depleted BMT, rejections have occurred in 15% overall; the incidence of graft failure has not been reduced by the higher dose of HFTBI. Relapses in this setting are equivalent to relapses with conventional BMT for AML, but appear to be increased for ALL. Radiobiological findings related to HFTBI will also be discussed.

Purpose: Three-dimensional conformal radiation therapy (3D-CRT) is a technique designed to deliver prescribed radiation doses to localized tumors with high precision, while effectively excluding the surrounding normal tissues. It facilitates tumor dose escalation which should overcome the relative resistance of tumor clonogens to conventional radiation dose levels. The present study was undertaken to test this hypothesis in patients with clinically localized prostate cancer. Methods and Materials: A total of 743 patients with clinically localized prostate cancer were treated with 3D-CRT. As part of a phase I study, the tumor target dose was increased from 64.8 to 81 Gy in increments of 5.4 Gy. Tumor response was evaluated by post-treatment decrease of serum prostate-specific antigen (PSA) to levels of 2.5 years after completion of 3D-CRT. PSA relapse-free survival was used to evaluate long-term outcome. The median follow-up was 3 years (range: 1‐7.6 years). Results: Induction of an initial clinical response was dose-dependent, with 90% of patients receiving 75.6 or 81.0 Gy achieving a PSA nadir 75.6 Gy (p 2.5 years after 3D-CRT was observed in only 1/15 (7%) of patients receiving 81.0 Gy, compared with 12/25 (48%) after 75.6 Gy, 19/42 (45%) after 70.2 Gy, and 13/23 (57%) after 64.8 Gy (p < 0.05). Conclusions: The data provide evidence for a significant effect of dose escalation on the response of human prostate cancer to irradiation and defines new standards for curative radiotherapy in this disease. © 1998 Elsevier Science Inc.

The search for new radiation treatment techniques to improve local tumor control continues to represent a major challenge in the management of localized human cancer. Before CT scans became available for planning radiation treatments, tumor target volumes and the boundaries between tumor and normal organs were poorly defined. To reduce the risk of local relapse, radiation target volumes classically included a wide “safety” margin of surrounding normal tissue. Restrictions imposed by normal tissue tolerance often limited the radiation doses administered to less than optimal levels. CT-assisted treatment planning improved the ability to define tumor target volumes for radiotherapy. However, the limitations imposed by computer technology have confined the use of CT information in conventional 2D treatment planning to one or at most a few CT slices at or near the central axis of the radiation beam, whereas treatment planning at levels beyond the central axis is not based on detailed CT anatomical information. The introduction of three-dimensional conformai radiation therapy (3D-CRT) has heralded a new era in radiotherapy. Sophisticated computer-aided techniques are used to plan and deliver prescribed radiation doses, conforming the desired dose distribution to the entire 3D configuration of the tumor. The use of 3D imaging technology for treat -ment planning has reduced the risk of underdosing parts of the tumor. In addition, the effective exclusion of normal tissues from the volume carried to high radiation dose levels has allowed an escalation in tumor dose to levels beyond those feasible with conventional 2D radiotherapy. The aim is to improve local tumor control, as failure to control the primary tumors is associated with increased rates of both metastatic disease and mortality. Because of the increasing complexity of 3D treatment planning and the need to evaluate large numbers of competing plans, a critical development for realizing the full potential of 3D-CRT is the introduction of computer-aided optimization of treatment planning. In addition, the extraordinary increase in the complexity of treatment delivery requires the introduction of immobilization devices, computer-controlled treatment delivery machines with automated multileaf collimatore for beam shaping, on-line verification systems with a potential for feedback control loops to automatically correct set-up errors during treatment, and patient safety devices. Dosimetrie methods are being developed to compensate for the inaccuracy in 3D treatment caused by organ motion. Many of these components have been implemented at the Memorial Sloan-Kettering Cancer Center.

The wall of an ionization chamber is commonly assumed to have a negligible effect on chamber response in electron beams. For cylindrical chambers with thin walls this assumption is valid. However, parallel‐plate chambers commonly possess large mechanical supports which may affect chamber response in a manner not accounted for in current dosimetry protocols. This is due to changes with energy in the relative backscattered electron fluence between chamber support and phantom materials. To investigate this effect, electron backscatter from low atomic number materials has been measured with electrons from 6 to 20 MeV. The effect of the diameter and thickness of the backscatteringmaterial has also been studied. Based on these data, Lucite and polystyrene chambers in water phantoms are expected to underrespond by 1% and 2% at 6 MeV. The expected underresponse decreases to 0.8% and 0.4% for polystyrene and Lucite at 12 MeV and is insignificant above 16 MeV. Two commercially available parallel‐plate chambers were compared with a cylindrical chamber in electron beams from 6 to 20 MeV. Using the 20‐MeV intercomparison, the expected chamber responses at the lower energies were calculated and compared with measurements. Both parallel‐plate chambers underresponded by approximately 1% at 6 MeV and 0.5% at 9 MeV which is qualitatively consistent with the electron backscatter data. Recommendations for minimizing electron backscatter effects through chamber design are discussed.

Forty patients with locally extensive, unresectable adenocarcinoma of the pancreas received precision high dose (PHD) radiation therapy with a 45 MeV betatron. A histologic diagnosis of cancer was established at laparotomy in every case. The gross margins of the tumor were outlined with radio-opaque clips in all but one case. The clipped tumor volume plus a 1 to 3 cm margin was irradiated to a minimum dose of 5900 to 7000 rad in 180 rad fractions over 7 to 9 weeks. For slender patients, a "mixed beam" technique was employed: opposed lateral 45-MeV photon beams mated to an anterior "mixed beam" consisting of 50 % 45-MeV photons and 50% 15–35 MeV electrons. The choice of electron energy depended upon the depth of the posterior margin of the target volume. For non-slender patients, a "box" technique consisting of 3 or 4 fields of 45-MeV photons was used. Where indicated, fields were shaped to conform the isodose distribution to the shape of the target volume. PHD radiotherapy was generally well tolerated. During !treatment, only 7 patients experienced significant nausea, vomiting, diarrhea or anorexia. Late gastrointestinal radiation reactions were observed in 7 patients (severe in 3 patients). Twelve patients received adjuvant chemotherapy. Relief of pain occurred in 2232 patients and anorexia improved in 815 patients following PHD radiotherapy. The projected survival of patients with unresectable pancreatic cancer treated with PHD radiotherapy is comparable to that of patients with resectable disease operated on for cure. The projected one year survival rate is 49%.

A breast applicator has been designed to optimize 3 field breast treatments for 60 Co. The device has a six half value layer beam splitting block constructed in two sections. The larger permanently mounted section is sufficient for treating 90% of the patients. Slots are available for mounting cerroband blocks, and any of five brass half field wedges. A magnetically attached front and back-pointer assembly readily breaks away in the event of a collision between pointer and patient. With this design the breast applicator with wedges and blocks has in-field surface doses reduced to that of an open field without accessory devices. The 50–90% dose decrement of the radiation penumbra for the half field block is comparable to that for the field edge of a typical 6 MV X ray unit, although the 10%–50 decrement is larger. The average out-of-field dose at the surface is 8% and is 5% at the depth of dose maximum. The combination for this applicator of sharp penumbra and low out-of-field dose leads to reduced lung and opposite breast doses. The latter was confirmed with TLD measurements on 10 patients, and yielded an average opposite breast dose of 230 cGy for a 4600 cGy prescription. Thus, half-field blocking devices do not preclude, as has been stated in the literature, acceptable opposite breast doses. In addition, proper design of these devices can significantly improve the radiation characteristics for primary breast treatments.

A technique is described for the use of lymphoscintigraphy in treatment planning of primary breast patients. During simulation of the treatment fields, the positions of the internal mammary nodes are projected back toward the source onto the patient skin surface and are marked by radio-opaque markers for visualization on films. Exact solutions for the coordinates of these surface projection points are derived. Approximate solutions are also given which are independent of the isocenter location and primarily dependent on the treatment field gantry angle. If a typical couch angle and field size are assumed, the projection points can be calculated for various gantry angles prior to simulation. Generally, a decision can then be made beforehand whether it would be better to use deep tangents or a separate field to treat the internal mammary nodes. During simulation, the surface projection points serve as visual and fluoroscopic guides to field design and optimization. A method is also presented for projecting the internal mammary node positions onto a single transverse patient contour for conventional 2-dimensional treatment planning. By accurately showing the projected location of the node with respect to the field edge, adequate treatment margin can be assured.

The current method for determining absorbed dose from high-energy photons is to use in water a Cobalt-60 exposure-calibrated ionization chamber and Clambda. Recently questions have arisen about the correctness of currently recommended Clambda values and whether or not the Cobalt-60 buildup cap should be used for in-water dose measurements. The effect of the buidup cap on Clambda was obtained by measuring Farmer chamber response in water with and without the buildup cap. Results show no measurable change at Cobalt-60 or 4 MV but an increase of 1%-2% above 20 MV. The effects of the wall materials and the displacement factors on Clambda are also considered. It is shown that at high energies errors of 3%-5% can be made in determining dose in water using commercially available Farmer or Farmer-like chambers and currently recommended Clambda. The exact size of the error depends upon the materials and thicknesses of the wall and cap, the chamber cavity volume, and whether or not the buildup cap is used.