Cerrobend Blocks Research Articles

Analysis of tumor control probability (TCP) and normal tissue complication probability (NTCP) in 40 prostate cancer patients treated with 2 different techniques of 3D-conformal radiation therapy (3D-CRT)

Purpose: To compare 2 regimens of 3D-CRT (3-field vs 6-field technique) in the treatment of prostate cancer, using TCP and NTCP data. Materials & Methods: Between October and December 1997, 20 consecutive patients with clinically localized adenocarcinoma of the prostate (4 T1, 9 T2, 7 T3) were treated with 3D-CRT 3-field technique. For the purpose of this study the records of 20 patients matched for tumor stage (5 T1, 9 T2, 6 T3) and treated with 3D-CRT 6-field technique, were analyzed and served as a basis for comparison. With the 3-field technique, the patients were simulated with rectum and bladder empty in a supine position using custom-made polyurethane foam or thermoplastic casts as immobilization devices. An anterior field (0) and two wedged posterior oblique fields (110 and 250 °) were devised. In order to minimize the dose delivered to the anterior rectal wall and despite a less homogenous dose distribution within the target, the wedges were inserted with the thick end toward the midline. Non-uniform margins were applied around the outline of the target volume, that is, 1 cm towards the rectal wall and 1.5 cm towards the bladder anteriorly and also for the upper and lower portal margins. The shaping of the beam apertures was realized by conventional cerrobend blocks. The total target dose was prescribed at the ICRU point and was 76 Gy for the prostate delivered by a 15 MV linear accelerator in 38 fractions and 56 days (mean). The seminal vesicles were excluded at 70 Gy. The 6-field technique was different only in positioning of the patient (prone) and arrangement of portals (45 °, 90, 135 °, 225*, 270*, 315*). For the calculation of the NTCP values regarding the bladder and rectum we have employed the Lyman's empirical model. The cumulative dose-volume histograms were reduced according to the effective volume model introduced by Kutcher and Burman. Radiobiological data published by Emami was used in this analysis. The TCP calculation was made according to Goitein's model. The mean TCP and NTCP values over the two groups were compared using a pooled two sample t-test. The log percentages were used to ensure that the normality assumptions were valid. Results: TCP values obtained with the two techniques were similar: mean value equals 98.7 for the 3-field technique (range: 97.6-99.4), 98.6 for the 6-field technique (range: 97.5-99.5), meaning a good target coverage with both techniques; p= 0.27, that means no evidence of difference between the two techniques for target coverage. NTCP values were: bladder NTCP 0.96 for 3-field (range: 0.02-4.99) and 0.91 for 6-field (range: 0.02-4.48) (p=0.67), rectum NTCP 7.6 for 3field (range: 2.7-16.9) and 5.7 for 6-field (range: 1.6-11.9) (p=0.051). There is no evidence of difference between the two techniques for bladder complication, but there may be some evidence of difference between the two groups for rectum complications. Conclusions: The two techniques yielded similar results in terms of TCP and bladder NTCP. In terms of rectum NTCP, there was a slight difference in favour of the 6-field technique. Acute toxicity was similar in either groups, but longer follow-up is needed to evaluate the relationship between the mathematical modelling and the rate of late toxicity. The difficulty in extrapolating data from other institutions when calculating TCP and NTCP prompted us to create our own database to evaluate the validity and the predictibility of these mathematical models, considering that different DVH calculation algorithms are usually applied in each center.

Cost benefit of emerging technology in localized carcinoma of the prostate

In a health care environment strongly concerned with cost containment, cost-benefit studies of new technology must include analyses of loco-regional tumor control, morbidity, impact on quality of life, and financial considerations. This nonrandomized study analyzes 124 patients treated with three-dimensional conformal radiation therapy (3D CRT) and 153 with standard irradiation (SRT) between January 1992 and December 1995, for histologically proven adenocarcinoma of prostate, clinical Stage T1 or T2. Mean follow-up is 1.4 years. Three-dimensional CRT consisted of six or seven coplanar oblique and lateral and, in some patients, AP fields designed to treat the prostate with a 1 to 1.7 cm margin. SRT consisted of 120 degrees bilateral arc rotation. Total doses to prostate were 67 to 70 Gy when pelvic lymph nodes were irradiated or 68.4 to 73.8 Gy when prostatic volume only was treated; dose per fraction was 1.8 Gy. Patients were interviewed weekly for severity of 12 acute intestinal and urinary pelvic irradiation side effects (0 to 4+ grading). Time and effort for 3D RTP and daily treatment with 3D CRT and SRT were recorded. Dose-volume histograms (DVHs) were calculated for gross tumor volume, planning target volume, bladder, and rectum. Actual reimbursement to the hospital and university was determined for 41 3D CRT, 43 SRT, and 40 radical prostatectomy patients treated during the same period. Average treatment planning times (in minutes) were: 101 for 3D conformal therapy simulation, 66 for contouring of target volume and sensitive structures, 55 for virtual simulation, 39 for plan preparation and documentation, 65 for physical simulation, and 20 for approval of treatment plan. Daily mean treatment times were 19 min for 3D CRT with Cerrobend blocking, 16 with multileaf collimation, and 10 with bilateral arc rotation. Dosimetric analysis (DVHs) showed a reduction of 50% in volume of bladder or rectum receiving doses higher than 65 Gy. Acute side effects included dysuria, moderate difficulty in urinating, and nocturia in 25-39% of both SRT and CRT patients; loose stools or diarrhea in 5-12% of 3D CRT and 16-22% of SRT patients; moderate proctitis in 3% of 3D CRT and 12% of SRT patients (p = 0.01). Chemical disease-free survival (prostate-specific antigen < or =2 ng/ml) at 3 years was 90% with 3D CRT and 80% with SRT (p = 0.01). Average initial treatment reimbursements were $13,823 (3D CRT), $10,864 (SRT), and $12,250 (radical prostatectomy). Average total treatment reimbursement and projected cost of management of initial therapy failures per patients were $15,173, $16,264, and $16,405, respectively. Three-dimensional CRT irradiated less bladder and rectum volume than SRT; CRT initial reimbursement was 28% higher than SRT and 12% higher than radical prostatectomy. Because of projected better local tumor control, average total cost of treating a patient with 3D CRT or radical prostatectomy is equivalent to cost of SRT. Treatment morbidity was lower with 3D CRT. Our findings reflect an overall benefit with 3D CRT as a new promising technology in treatment of localized prostate cancer. Dose-escalation studies may enhance its efficacy and cost benefit.

Execution of a single-isocenter three-field technique, using a multileaf collimator or tray-mounted cerrobend blocks: Effect on treatment time

Purpose : In this article, we studied the total treatment time of a single-isocenter three-field irradiation of breast and axilla, using either tray-mounted cerrobend blocks, or a multileaf collimator (MLC) for field shaping. Methods had Materials : A total of 20 female, unselected patients were given 50 Gy (2 Gy/fraction) on breast and 46 Gy on axilla and supraclavicular region (2 Gy/fraction). Patients were randomized between two different treatment groups. The first group ( n = 10) was treated on a Philips SL-75 linear accelerator (SL-75), using 5 MV photons with tray-mounted cerrobend blocks. The second group ( n = 10) was treated on a Philips SL-25 linear accelerator, using 6 MV photons and a MLC (SL-25-MLC). Results : Although the beam-on time on the SL-25-MLC was significantly higher ( p < 0.0001) compared to the SE-75-overall treatment time was significantly shorter using a MLC instead of tray-mounted cerrobend blocks ( p < 0.0001). The difference in total treatment time was in the range of 100 s per patient per day. The main difference between the two accelerators was observed when setup of the second and third field was done using the automatic setup facility of the SL-25-MLC (avoids entering the treatment room). A mean time gain of 124 s per treatment session was observed using automatic setup. Considering the yearly number of patients receiving this treatment, a total time gain equivalent to 16.15 8-h workdays was calculated. Conclusions : Compared to a technique using tray-mounted cerrobend blocks in the single-isocenter three-field irradiation of a breast and axilla, a MLC combined with automatic field setup provides a significant time advantage, by reducing the number of manipulations inside the treatment room.

Effect of beam blocking on linac head scatter factors

Purpose : This study aimed to investigate the influence of beam blocking on head scatter within the context of a two-component x-ray source model. Methods and Materials : Head scatter factors were measured for open rectangular fields defined by X and Y jaws and for fields blocked by a multileaf collimator (MLC) for a 6-MV photon beam from a Varian 2300CD accelerator. A simple formula based on consideration of linac head geometry and an x-ray source model was derived to determine when the head scatter factor for a shaped field depends only on jaw settings and when it is influenced by the blocking (cerrobend blocks or MLC) itself. Results : Experimental data demonstrate that the assumption that head scatter is unaffected by the introduction of beam blocking is not always acceptable, particularly for fields blocked by an MLC. The ratios of open to blocked field dimensions were compared with simple functions characterizing the accelerator head geometry to predict changes in the behavior of head scatter factors observed experimentally. Conclusion : A simple formula can be used to determine when head scatter factors are influenced by beam blocking (cerrobend blocks or MLC).

The use of diode dosimetry in quality improvement of patient care in radiation therapy

The purpose of this work is to improve the quality of patient care in radiation therapy by implementing a comprehensive quality assurance (QA) program aiming to enhance patient in vivo dosimetry on a routine basis. The characteristics of two commercially available semi-conductor diode dosimetry systems were evaluated. The diodes were calibrated relative to an ionization chamber-electrometer system with calibrations traceable to the National Institute of Standards and Technology (NIST). Correction factors of clinical relevance were quantified to convert the diode readings into patient dose. The results of dose measurements on 6 patients undergoing external beam radiation therapy for carcinoma of the prostate on three different therapy units are presented. Field shaping during treatments was accomplished either by multileaf collimation or by cerrobend blocking. A deviation of less than ±4% between the measured and prescribed patient doses was observed. The results indicate that the diodes exhibit excellent linearity, dose reproducibility, minimal anisotropy, and can be used with confidence for patient dose verification. Furthermore, diodes render real time verification of dose delivered to patients.

Analytical representation of head scatter factors for shaped photon beams using a two-component x-ray source model.

An analytic representation proposed for the relative intensity distribution of the extra-focal source in a two-component x-ray source model serves as the basis for calculation. From this representation, a closed-form expression for head scatter factors defined on the central beam axis is derived by integrating over the area of the extra-focal source plane visible from the measurement point. The resulting expression is applicable to photon beams from different Varian accelerators and different photon energies, and includes effects arising from beam shaping with cerrobend blocks or multileaf collimators (MLCs). For 6- and 15-MV photon beams from Varian 600C and 2300CD accelerators (Varian Oncology Systems, Palo Alto, CA), 361 measured head scatter factors for square, rectangular, asymmetric, and arbitrarily shaped fields, formed by either the X and Y jaws, the MLC and Y jaws, or by the MLC alone, were compared with model calculations. Results show that 93.4% of calculated values match corresponding experimental points to within 0.5%, the average deviation being 0.23% and the maximum deviation 0.9%. Thus, as a consequence of this work, the different influence of the X jaws, the Y jaws, and the MLC on head scatter factors is quantitatively described. In particular, it is demonstrated that in the case of radially symmetric scattering, the collimator exchange effect arises as a result of the different distances of the X and Y jaws from the focal spot.

Field edge smoothing for multileaf collimators

Purpose : To smooth the scalloped dose pattern that occurs for stepped leaves at a treatment field edge defined by a multileaf collimator. Methods and Materials : Fields with centers shifted slightly in space were superimposed to blur the staggered dose distribution at the field edge. Film dosimetry was used to monitor changes. The dose distribution for a single field position was compared to the distribution for one and three shifts. Three depths were examined and divergent alloy blocks were included in the comparison. Results : The structure that appears at an edge for a single field when leaves are staggered was nearly eliminated when the field was shifted three times to give a total of four different positions. However, shifting the field one time so that two fields were superimposed gave an intermediate result with only slight improvement in the undulating dose distribution. For the four superimposed fields, the 50 % isodose pattern converged to a smoothed line running along the center of the original undulating pattern. The 80 and 20 % isodoses did not converge to the center of their scalloped patterns. Instead, these isodose lines were spread leaving a larger penumbra width than a divergent alloy block. Conclusions : Shifting and adding fields is an effective method for smoothing the staggered dose distribution that results when the leaves of a multileaf collimator are stepped to form an irregular field pattern. However, the width of the penumbra for the combined fields is wider than the penumbra for a cerrobend block.

Calibration of semiconductor detectors for dose assessment in total body irradiation

The aim of this paper is to discuss the measurements carried out to implement ‘in vivo dosimetry’ with EDP-30 diodes in total body irradiation (TBI) techniques. Exit calibrations and calibrations behind cerrobend protection blocks showed the importance of calibrating diodes in all relevant clinical conditions. Special attention was given to calibration of diodes behind cerrobend blocks. Dependence of the calibration factors on the thickness of the shielding blocks was, therefore, studied. This dependence was again studied after adding a wax cap to the diode and when the ionisation chamber was placed at the same depth as the measuring point of the diode. Temperature dependence in diode sensitivity and dependence on accumulated dose for diodes response and for temperature correction factors were also examined.

2229 Opposite breast dose from tangential fields: Comparison of asymmetric jaws, cerrobend blocking, multileaf collimator, machine wedges, and dynamic wedges

2229 Opposite breast dose from tangential fields: Comparison of asymmetric jaws, cerrobend blocking, multileaf collimator, machine wedges, and dynamic wedges

Effects of treatment setup variation on beam's eye view dosimetry for radiation therapy using the multileaf collimator vs. the cerrobend block

The purpose of this study is to quantify and compare retrospectively the effects of treatment setup variation on beam's eye view (BEV) dosimetry for radiation therapy using a multileaf collimator (MLC) vs. cerrobend block. A study was performed on a group of 18 patients with cancer of the head and neck, lung, and pelvis who were treated with irregularly shaped fields. The BEV dosimetry of the fields shaped with cerrobend blocks and the MLC was measured with films at the depth of dose prescription in a solid water phantom. A "one-half-leaf" insertion convention was used to shape the MLC. In addition, an average of 15 sequential daily port films was taken per patient during the course of radiotherapy. The port films were aligned with the prescription film for each patient. Systematic error and random error of treatment setup for each patient were calculated. The effects of setup variation were incorporated by convolving the patient portal imaging data with the corresponding BEV film dosimetry. Two parameters were used to quantify the BEV dosimetry. First, the field penumbra width was calculated, which represented the average of the normal separations between 20 and 80% isodose lines along the prescription outline. Second, the ratio of areas covered by the 90 and 20% isodose lines, A90/20, was determined. The BEV dosimetry was then characterized with and without the effects of treatment setup variation. In addition, the difference in BEV dosimetry between the cerrobend block and the MLC was used to estimate the corresponding changes in tumor control probability (TCP). These changes were also compared to the changes in TCP for the treatment with or without the effects of random setup variation. With or without daily setup variation, the use of cerrobend block was more favorable than the MLC in terms of the field penumbra width and A90/20 for all treatment sites. In the absence of daily variation, the MLC field penumbra width was on average 1.3 mm larger than that of the cerrobend block, and 0.9 mm larger in the presence of daily setup variation. Similarly, the ratio A90/20 of the cerrobend block was on average 0.03 larger than that of MLC without daily setup variation, and 0.02 with daily setup variation. The difference in field penumbra width and A90/20 between the MLC and the cerrobend block was slightly reduced due to the effects of daily setup variation. For both the cerrobend block and the MLC, daily setup variation produced a significant increase in the field penumbra width, 2.3 mm for the cerrobend block and 1.9 mm for the MLC, and a decrease in the A90/20, 0.06 for the former and 0.05 for the latter. The change due to the daily setup variation was about a factor of 2 larger than the changes due to replacing the cerrobend block with the MLC. Using the TCP model, the change in TCP due to the daily setup variation was more than a factor of 3 larger than the change in TCP due to replacing the cerrobend block with the MLC. It was noted that the average changes in the penumbra, the A90/20 and the TCP calculated for the patient population did not adequately describe the changes for the individual patient. Our results do not show significant dosimetric differences between the MLC and the cerrobend block in conventional radiation treatment, whether or not daily setup variation was taken into consideration. The effects of daily setup variation alone produced a larger dosimetric change. The same results were obtained when the data were applied to calculate changes in TCP. For optimal radiation therapy, efforts should be concentrated on reducing daily setup variation. Our results also demonstrate the importance of frequent evaluation of MLC treatment using electronic portal imaging devices.

Analytical representation of head scatter factors for shaped photon beams using a two-component x-ray source model.

An analytic representation proposed for the relative intensity distribution of the extra-focal source in a two-component x-ray source model serves as the basis for calculation. From this representation, a closed-form expression for head scatter factors defined on the central beam axis is derived by integrating over the area of the extra-focal source plane visible from the measurement point. The resulting expression is applicable to photon beams from different Varian accelerators and different photon energies, and includes effects arising from beam shaping with cerrobend blocks or multileaf collimators (MLCs). For 6- and 15-MV photon beams from Varian 600 C and 2300 CD accelerators (Varian Oncology Systems, Palo Alto, CA), 361 measured head scatter factors for square, rectangular, asymmetric, and arbitrarily shaped fields, formed by either the X and Y jaws, the MLC and Y jaws, or by the MLC alone, were compared with model calculations. Results show that 93.4% of calculated values match corresponding experimental points to within 0.5%, the average deviation being 0.23% and the maximum deviation 0.9%. Thus, as a consequence of this work, the different influence of the X jaws, the Y jaws, and the MLC on head scatter factors is quantitatively described. In particular, it is demonstrated that in the case of radially symmetric scattering, the collimator exchange effect arises as a result of the different distances of the X and Y jaws from the focal spot.

432 Matching half beams in head and neck radiotherapy. planning and dosimetry

<h3>Introduction</h3> In the treatment planning of the head and neck tumours, usually matching fields are necessary. There are inherent hazards with this technique in that overlapping of these portals may result in hot and cold spots at the field edges. The use of linacs with independent collimators enables to eliminate the beam divergency, matching then the central axes of adjacent fields where there is no divergence. On the other hand, the 3D radiation treatment planning systems allow us to calculate the dose variation through the field's junction. <h3>Technical description</h3> We describe a technique consistent in three isocentric half-fields (two upper laterals and one lower anterior). Patients are immobilized with a thermoplastic mask. The upper limit of the lateral fields and the midline are outlined in the mask. It's not necessary to move either the patient or the treatment couch for the different fields of treatment. The SSD for each field is calculated at simulation and/or planihcation time. The lateral fields are shielded with individual focalized Cerrobend's blocks, leaving the half field limit free. <h3>Conclusion</h3> In this technique there isn't beam divergency at the junction's fields, so the homogeneity is maximal. A dosimetric study with a 3D planning system shows a good distribution of dose. The dose contribution of each half field in the other half field is <2%. The new linacs working with asymmetric collimators enable us to use the split matching technique in a simple, reproducible and accurate form.

207 Extension of Sherouse's GRATIS™ three-dimensional treatment planning system for use with a multileaf collimator

In February 1995, a linear accelerator (Philips SL25) was retrofitted with a Philips multileaf collimator (MLC). MLC reduces workload (decreases the numbers of cerrobend blocks) and increases throughput (avoids tray placements). However, programming the leaf settings is a labur intensive procedure on the Philips MLC, as settings have to be derived manually from simulator films. We use the GRATIS 3D planning system by George W. Sherouse and have written an extension to plan the leaf settings in beam's eye view mode. From a tool implemented in GRATIS' last release, which draws a conformal block around the target with a predefined margin, we developed a feature for automatic or interactive leaf setting. After virtual simulation, a file with the leaf settings is transferred to the MLC computer over a local network. The advantages of this system are efficiency, automatization and elimination of transfer errors. Within the philosophy of GRATIS, we will make our tool freely available to the GRATIS users.

Three-dimensional treatment planning and conformal radiation therapy: preliminary evaluation

Preliminary clinical results are presented for 209 patients with cancer who had treatment planned on our three-dimensional radiation treatment planning (3-D RTP) system and were treated with external beam conformal radiation therapy. Average times (min) for CT volumetric simulation were: 74 without or 84 with contrast material; 36 for contouring of tumor/target volume and 44 for normal anatomy; 78 for treatment planning; 53 for plan evaluation/optimization; and 58 for verification simulation. Average time of daily treatment sessions with 3-D conformal therapy or standard techniques was comparable for brain, head and neck, thoracic, and hepatobiliary tumors (11.8–14 min and 11.5–12.1, respectively). For prostate cancer patients treated with 3-D conformal technique and Cerrobend blocks, mean treatment time was 19 min; with multileaf collimation it was 14 min and with bilateral arc rotation, 9.8 min. Acute toxicity was comparable to or lower than with standard techniques. Sophisticated 3-D RTP and conformal irradiation can be performed in a significant number of patients at a reasonable cost. Further efforts, including dose-escalation studies, are necessary to develop more versatile and efficient 3-D RTP systems and to enhance the cost benefit of this technology in treatment of patients with cancer.

A multifeaf collimator field prescription preparation system for conventional radiotherapy

The purpose of this work is to develop a prescription preparation system for efficient field shaping using a multileaf collimator that can be used in community settings as well as research institutions. The efficiency advantage of the computer-controlled multileaf collimator, over cerrobend blocks, to shape radiation fields has been shown in conformal treatments, which typically require complete volumetric computerized tomographic data for three-dimensional radiation treatment planning--a utility not readily available to the general community. As a result, most patients today are treated with conventional radiation therapy. Therefore, we believe that it is very important to fully use the same efficiency advantage of multileaf collimator as a block replacement in conventional practice. The multileaf collimator prescription preparation system developed by us acquires prescription images from different sources, including film scanner and radiation treatment planning systems. The multileaf collimator angle and leaf positions are set from the desired field contour defined on the prescription image, by minimizing the area discrepancies. Interactive graphical tools include manual adjustment of collimator angle and leaf positions, and definition of portions of the field edges that require maximal conformation. Data files of the final leaf positions are transferred to the multileaf collimator controller via a dedicated communication link. We have implemented the field prescription preparation system and a network model for integrating the multileaf collimator and other radiotherapy modalities for routine treatments. For routine plan evaluation, isodose contours measured with film in solid water phantom at prescription depth are overlaid on the prescription image. Preliminary study indicates that the efficiency advantage of the MLC over cerrobend blocks in conformal therapy also holds true for conventional treatments. Our model of computer-controlled prescription, evaluation, and treatment using multileaf collimators can be effectively implemented in both community settings and research institutions. The resultant increase in treatment efficiency and accuracy is now available for conventional radiotherapy.

A multileaf collimator field prescription preparation system for conventional radiotherapy

Purpose: The purpose of this work is to develop a prescription preparation system for efficient field shaping using a multileaf collimator that can be used in community settings as well as research institutions. The efficiency advantage of the computer-controlled multileaf collimator, over cerrobend blocks, to shape radiation fields has been shown in conformal treatments, which typically require complete volumetric computerized tomographic data for three-dimensional radiation treatment planning—a utility not readily available to the general community. As a result, most patients today are treated with conventional radiation therapy. Therefore, we believe that it is very important to fully use the same efficiency advantage of multileaf collimator as a block replacement in conventional practice. Methods and Material: The multileaf collimator prescription preparation systems developed by us acquires prescription images from different sources, including film scanner, and radiation treatment planning systems. The multileaf collimator angle and leaf positions are set from the desired field contour defined on the prescription image, by minimizing the area discrepancies. Interactive graphical tools include manual adjustment of collimator angle and leaf positions, and definition of portions of the field edges that require maximal conformation. Data files of the final leaf positions are transferred to the multileaf collimator controller via a dedicated communication link. Results: We have implemented the field prescription preparation system and a network model for integrating the multileaf collimator and other radiotherapy modalities for routine treatments. For routine plan evaluation, isodose contours measured with film in solid water phantom at prescription depth are overlaid on the prescription image. Preliminary study indicates that the efficiency advantage of the MLC over cerrobend blocks in conformal therapy also holds true for conventional treatments. Conclusion: Our model of computer-controlled prescription, evaluation and treatment using multileaf collimators can be effectively implemented in both community settings and research institutions. The resultant increase in treatment efficiency and accuracy is now available for conventional radiotherapy.

Clinical use of a concomitant boost technique using a gypsum compensator

Purpose : To develop a clinical procedure to treat field within a field (concomitant boost) portals with a single compensated field. Methods and Materials : An ordinary manual cerrobend block former was used to produce styrofoam molds from simulator film data. A special gypsum compound was poured into the molds. The compensator block is independently mounted to the treatment machine via a custom-made compensator holder. Results : Measurements confirm that the inhomogeneous dose distribution has been reliably delivered via this technique. The accuracy of placement of the high dose region is sufficient for clinical use. Conclusion : The procedure enables the concomitant boost effect to be easily implemented in the clinic without increasing clinical setup time.

Effects of beam modifiers and immobilization devices on the dose in the build-up region

Purpose : To analyze the effect that immobilization devices used in conjunction with beam modifiers may have on the dose to the skin and build-up region. Methods and Materials : Central axis depth dose measurements were made in a polystyrene phantom in the build-up regions using the 6 and 15 MV photon beams, at two different source-to-phantom distances, and various field sizes. The effects of acrylic blocking trays, lead wedges, and cerrobend blocks were assessed in conjunction with the enhancement of dose in the build-up region due to immobilizing devices using plaster and thermoplastic casting materials of different thicknesses. Results : For the 6 MV photons, solid (3 mm) thermoplastic casting material was found to have the greatest effect on surface dose: for a 12 × 12 cm field we measured 79% of maximum dose when treating through the material versus 22% of maximum dose when no beam modifiers or immobilization devices are used. Measurements were also made to evaluate the effect of the immobilization of patients receiving three-dimensional conformal treatments using a 15 MV photon beam. Conclusions : The relevance of these results to treatments in the pelvis, breast, and head and neck regions is discussed. For 6 MV beams, special consideration should be given if the need arises to treat through the immobilization device, as unacceptable skin reactions may result.

Preclinical evaluation of the reliability of a 50 MeV racetrack microtron

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.

A mono isocentric technique for breast and regional nodal therapy using dual asymmetric jaws

Purpose : Definitive radiation therapy for breast cancer with regional nodal involvement often introduces treatment of adjacent abutted regions. Many methods describe techniques to achieve an effective transverse plane match. Our facility recently adopted a matching technique using asymmetric jaws to beam-split all portals along the central axis plane. Our technique uses one isocenter to treat the opposed tangential breast fields, the supraclavicular port and the posterior axillary field. Methods and Materials : Our linear accelerator has four collimator jaws capable of being set independently. The longitudinal (Y) jaws beam-split all the portals at the match plane, namely the upper border of the tangential beams and the bottom border of the nodal fields. The transverse (X) jaws define the lateral borders of the nodal fields, and in a near beam-split fashion in conjunction with customized Cerrobend, block the lung for the tangential beams. The unique isocenter is chosen along the mid-bridge through the tangential match plane. Dosimetric qualities and calculational techniques of the asymmetric beams were analyzed with ionimetric water scans, ion chamber studies, and film. The match-line is clinically confirmed with composite port films. Results : Our dosimetric studies show asymmetric jaws provide nearly equivalent field edge definition and superior absorption in comparison with Cerrobend blocks. The use of one isocenter results in a reduction of in-room treatment time by a factor of two. The burden of lifting heavy Cerrobend blocks has been removed. A composite port film, which includes the medial tangential and supraclavicular ports, shows a perfect match-line in all cases. Similar composite port films taken with our previous technique of geometric matching with collimator and table angulation exhibit slight overlap or underdose regions in many cases. Conclusion : Our treatment technique takes full advantage of dual asymmetric jaws to achieve a perfect match-line, necessitates only one isocenter and set-up point, and supplies more absorption in reference to lung and contralateral breast dose. The pure match-line is accompanied by the fact that the patient does not have to move in any direction.