Brass Compensators Research Articles

Influence of Compensator Thickness and Field Size on Buildup factor of a Brass Compensator for Intensity- Modulated Radiation Therapy

Introduction: Radiotherapy is one of the effective methods in the treatment of cancer. In the recent methods of radiotherapy, intensity modulated radiotherapy (IMRT) technique by compensators is used. when photon beam passes the compensator, scattered photons can be generated within the compensator. The presence of scattered photons may complicate the design of compensators as well as the subsequent calculations of dose distributions. The buildup factor, ‘B’, is one of these important properties of a material to be used for compensator construction. It directly affects the dose quantity. Materials and Methods: Radiotherapy is one of the effective methods in the treatment of cancer. In the recent methods of radiotherapy, intensity modulated radiotherapy (IMRT) technique by compensators is used. when photon beam passes the compensator, scattered photons can be generated within the compensator. The presence of scattered photons may complicate the design of compensators as well as the subsequent calculations of dose distributions. The buildup factor, ‘B’, is one of these important properties of a material to be used for compensator construction. It directly affects the dose quantity. Results: Our results indicated that the compensator thickness and field size have the most significant effect on the calculation of buildup factor for the 6-MV photon beam. Conclusion: Influence of compensator thickness and field size on calculation buildup factor can reduce the error due to delivered dose to target volume and organs at risk

The Influence of Brass Compensator Thickness and Field Size on Neutron Contamination Spectrum in 18MV Elekta SL 75/25 Medical Linear Accelerator with and without Flattening Filter: A Monte Carlo Study

One of the most significant Intensity Modulated Radiation Therapy treatment benefits is a high target to normal tissue dose ratio. To improve this advantage, an additional accessory such as a compensator is used to deliver doses. Compensator-based IMRT treatment is usually operated with an energy higher than 10 MV. Photoneutrons, which have high linear energy transfer and radiobiological effectiveness, are produced by colliding high-energy photon beams with linear accelerator structures, then they deliver the unwanted doses to patients and staff. Therefore, the neutron energy spectra should be determined in order to calculate and reduce the photoneutron risk. We have conducted a comprehensive and precise study on the influence of brass compensator thickness and field size on neutron contamination spectrum in an Elekta SL 75/25 medical linear accelerator with and without the flattening filter by Monte Carlo method. MCNPX MC Code version 2.6.0 was utilized to simulate the detailed geometry of Elekta SL 75/25 head components based on Linac's manual. This code includes an important feature to simulate the photo-neutron interactions. Photoneutrons spectrum was calculated after the Linac output benchmarking based on tuning the primary electron beam. Based on the Friedman and Wilcoxon nonparametric tests results (P<0.05), photoneutron fluence directly depends on the field size and compensator thickness. Moreover, the unflattened beam provides lower photoneutron fluence than the flattened beam. Photoneutrons fluence is not negligible in compensator-based IMRT treatment. However, in order to optimize treatment plans, this additional and unwanted dose must be accounted for patientss.

THE EXPERIMENTAL ASSESSMENT OF BUILD UP FACTOR AND ATTENUATION COEFFICIENT OF BRASS COMPENSATOR APPLIED IN INTENSITY-MODULATED RADIATION THERAPY (IMRT) FOR 6MV PHOTON BEAM

Introduction: Recent compensators are commonly applied in IMRT. The precise properties of applied compensators such as thickness, attenuation coefficient and build up factor are intensively important for IMRT calculations. Method: The brass compensator used for 6 MV photon beam was studied to estimate the relative effect of thickness and field size on IMRT calculations. Various field size together with several compensator thicknesses were examined. Result: The average reduction of effective attenuation coefficient (EAC), for the fields of 10A—10 cm2 to 20A—20 cm2, was 9.94%. By increasing the field size, EAC was decreased. The major reduction of EAC due to increasing field size was found to be 9.62%. The build up factor was increased by 2% to 21.8% respect to field size and compensator thickness. Also, the build up factor was increased by adding up the thickness. The rate of changes ranged from 24% to 48 %. Conclusion: The compensator thickness and field size are significantly important to calculate the effective attenuation coefficient and build up factor.

Influence of Compensator Thickness and Field Size on the Effective Attenuation Coefficient of a Brass Compensator for Intensity-Modulated Radiation Therapy

Influence of Compensator Thickness and Field Size on the Effective Attenuation Coefficient of a Brass Compensator for Intensity-Modulated Radiation Therapy

Quantitative analysis of brass compensators for commissioning of the Pinnacle planning system for IMRT.

Brass compensators for beam modulation present an alternative method for IMRT treatment planning to traditional dynamic leaf modulation. In this work, we present a detailed method to commission the Pinnacle treatment planning system for IMRT using brass compensators. Beam attenuation from various brass thicknesses were measured using an ion chamber, as well as a MapCHECK device, for a representative seven‐field IMRT plan. We show that Pinnacle's parameters for compensators can be optimized to match the measured results while still maintaining the original open‐beam model. Only Pinnacle's modified scatter factor and brass density value were adjusted. Beam attenuation in several brass slabs were optimized by minimizing a χ2 function of measured and modeled data for several depths in solid water. Using MapCHECK, Pinnacle's parameters were optimized by minimizing the number of points that fail during a gamma analysis of seven fields in a typical IMRT plan. Our results show that Pinnacle's 10X beam is best modeled using a brass density of 7.8 g/cm3 and a modified scatter factor of 0.1 cm−1.PACS numbers: 87.56.N, 87.56.ng, 87.55.kh, 87.55.de

Evaluation of inhomogeneity correction factors for 6 MV flattening filter‐free beams with brass compensators

The 6 MV flattening filter‐free (FFF) beam has been commissioned for use with compensators at our institution. This novel combination promises advantages in mitigating tumor motion due to the reduced treatment time made possible by the greatly increased dose rate of the FFF beam. Given the different energy spectrum of the FFF beam and the beam hardening effect of the compensator, the accuracy of the treatment planning system (TPS) model in the presence of low‐density heterogeneities cannot be assumed. Therefore, inhomogeneity correction factors (ICF) for an FFF beam attenuated by brass slabs were measured and compared to the TPS calculations in this work. The ICF is the ratio of the point dose in the presence of inhomogeneity to the dose in the same point in a homogeneous medium. The ICFs were measured with an ion chamber at a number of points in a flat water‐equivalent slab phantom containing a 7.5 cm deep heterogeneity (air or 0.27 g/cm3 wood). Comparisons for the FFF beam were carried out for the field sizes from 5×5 to 20×20 cm2 with the brass slabs ranging from 0 to 5 cm in thickness. For a low‐density wood heterogeneity in a slab phantom, with the exception of the point 1 cm beyond the proximal buildup interface, the TPS handles the inhomogeneity correction with the brass‐filtered 6 MV FFF beam at the requisite 2% error level. The combinations of field sizes and compensator thicknesses when the error exceeds 2% (2.6% maximum) are not likely to be experienced in clinical practice. In terms of heterogeneity corrections, the beam model is adequate for clinical use.PACS number: 87.56.ng

Comparing dose in the build‐up region between compensator‐ and MLC‐based IMRT

The build‐up dose in the megavoltage photon beams can be a limiting factor in intensity‐modulated radiation therapy (IMRT) treatments. Excessive surface dose can cause patient discomfort and treatment interruptions, while underdosing may lead to tumor repopulation and local failure. Dose in the build‐up region was investigated for IMRT delivery with solid brass compensator technique (compensator‐based IMRT) and compared with that of multileaf collimator (MLC)‐based IMRT. A Varian Trilogy linear accelerator equipped with an MLC was used for beam delivery. A special solid brass step‐wise compensator was designed and built for testing purposes. Two step‐and‐shoot MLC fields were programmed to produce a similar modulated step‐wise dose profile. The MLC and compensator dose profiles were measured and adjusted to match at the isocenter depth of 10 cm. Build‐up dose in the 1–5 mm depth range was measured with an ultrathin window, fixed volume parallel plate ionization chamber. Monte Carlo simulations were used to model the brass compensator and step‐and‐shoot MLC fields. The measured and simulated profiles for the two IMRT techniques were matched at the isocenter depth of 10 cm. Different component contributions to the shallow dose, including the MLC scatter, were quantified. Mean spectral energies for the open and filtered beams were calculated. The compensator and MLC profiles at 10 cm depth were matched better than ±1.5%. The build‐up dose was up to 7% lower for compensator IMRT compared to MLC IMRT due to beam hardening in the brass. Low‐energy electrons contribute 22% and 15% dose at 1 mm depth for compensator and MLC modalities, respectively. Compensator‐based IMRT delivers less dose in the build‐up region than MLC‐based IMRT does, even though a compensator is closer to the skin than the MLC.PACS number: 87.55.dk, 87.56.ng

Evaluation of beam hardening and photon scatter by brass compensator for IMRT.

When a brass compensator is set in a treatment beam, beam hardening may take place. This variation of the energy spectrum may affect the accuracy of dose calculation by a treatment planning system and the results of dose measurement of brass compensator intensity modulated radiation therapy (IMRT). In addition, when X-rays pass the compensator, scattered photons are generated within the compensator. Scattered photons may affect the monitor unit (MU) calculation. In this study, to evaluate the variation of dose distribution by the compensator, dose distribution was measured and energy spectrum was simulated using the Monte Carlo method. To investigate the influence of beam hardening for dose measurement using an ionization chamber, the beam quality correction factor was determined. Moreover, to clarify the effect of scattered photons generated within the compensator for the MU calculation, the head scatter factor was measured and energy spectrum analyses were performed. As a result, when X-rays passed the brass compensator, beam hardening occurred and dose distribution was varied. The variation of dose distribution and energy spectrum was larger with decreasing field size. This means that energy spectrum should be reproduced correctly to obtain high accuracy of dose calculation for the compensator IMRT. On the other hand, the influence of beam hardening on kQ was insignificant. Furthermore, scattered photons were generated within the compensator, and scattered photons affect the head scatter factor. These results show that scattered photons must be taken into account for MU calculation for brass compensator IMRT.

Monte Carlo simulation of spectrum changes in a photon beam due to a brass compensator

Monte Carlo simulations were used to study the changes in the incident spectrum when a poly-energetic photon beam passes through a static brass compensator. The simulated photon beam spectrum was evaluated by comparing it against the incident spectra. We also discriminated the changes in the transmitted spectrum produced by each of the microscopic processes. (i.e. Rayleigh scattering, photoelectric effect, Compton scattering, and pair production). The results show that the relevant process in the energy range considered is the Compton Effect, as expected for composite materials of intermediate atomic number and energy range considered.

Commissioning compensator‐based IMRT on the Pinnacle treatment planning system

We present a systematic approach to commissioning of the compensator‐based IMRT in Pinnacle treatment planning system for commercially manufactured brass compensators. Some model parameters for the beams modulated by the variable‐thickness compensators can only be associated with a single compensator thickness. To intelligently choose that thickness for beam modeling, we empirically determined the most probable filter thickness occurring within the modulated portion of the compensators typically used in clinics. We demonstrated that a set of relative output factors measured with the brass slab of most probable thickness (2 cm) differs from the traditionally used open field set, and leads to improved agreement between measurements and calculations, particularly for the larger field sizes. By iteratively adjusting the modifier scatter factor and filter density, the calculated effective attenuation of the flat filters was brought to within 2% of the ion chamber measurement for the clinically‐relevant range of filter thicknesses, depths and filed sizes. Beam hardening representation in Pinnacle provides for adequate depth dose modeling beyond the depth of about 5 cm. Disagreement at shallower depth for the large field sizes is likely due to the algorithm's inability to account for the low‐energy scattered photons generated in the filter. The average ion chamber point dose error at isocenter for ten clinical compensator‐based IMRT plans was under 1%. A biplanar 3D diode dosimeter was calibrated and validated for use with the compensators. The average gamma analysis (3%/3 mm) passing rate for ten IMRT plans was 98.9%± 1.0%. The device is particularly attractive because it easily generates dose comparisons at both the fraction and beam levels. Overlaying dose profiles for individual beams would easily uncover any errors in compensator orientation.PACS number: 87.55Qr

The inter‐ and intrafraction reproducibilities of three common IMRT delivery techniques

Intensity modulated radiation therapy (IMRT) treatment delivery requires higher precision than conventional 3D treatment delivery because of the sensitivity of the resulting dose to small geometric misalignment of the modulated beamlets. The chosen treatment delivery technique will affect the treatment precision in different ways, based on the characteristics of the delivery method. Delivery using a multileaf collimator (MLC) can reduce treatment time and therapist workload, but typically requires a greater number of monitor units and the fields are prone to both systematic and random leaf positioning errors. An alternative to MLC-based fields, patient specific brass compensators, do not suffer from these leaf positioning errors. In our study, we set out to investigate which delivery method will provide the highest levels of dosimetric reproducibility and the minimum amount of interfraction variability. In our study, a seven field IMRT plan for a head and neck treatment was created using the Pinnacle3 treatment planning system and the intensity maps for each field were obtained. The intensity maps of the fields were delivered with a Varian 2100C/D linear accelerator, using solid compensators and sliding window (SW) and step-and-shoot (SS) MLC segments. Three fields were selected from the seven-beam IMRT plan for comparison. Analysis was carried out using the MatriXX ion chamber array, radiochromic film, and Varian dynalog files. Our results show that the error in MLC leaf positioning has no gantry angle dependence. The compensator and SW deliveries showed excellent agreement, even when stricter than usual gamma criteria were applied. However, we noted that under these strict conditions, the SS field had at least ten times more pixels out of range than did the compensators. When using step-and-shoot MLC fields, it was observed that the increase in dose rate or the increase of MU/segment degrades the quality of the plan. Analysis of the dynalog files showed that while each individual field had its own propensity for error, all fields showed the same trend: a greater percentage of time the leaves are out of position as dose rate increases, MUs decrease, or both. The compensator-based field and both types of MLC-based fields have MatriXX results that are within the clinically acceptable tolerance of 3% dose difference and 2 mm DTA. However, when the criteria are tightened, it becomes evident that the compensators have a definite advantage over their comparable MLC-based competitors in terms of interfraction reproducibility. Fewer monitor units are required to deliver each portal, potentially improving patient outcomes and reducing unwanted side effects to both patients and therapists. In centers without MLC, compensators represent a simple and cost effective way to offer patients state of the art treatment. Based on the results of this study, compensator-based IMRT is a reliable, viable option for use in clinics both with and without MLC-equipped linacs.

SU-GG-T-118: Investigation of Intensity Modulated Compensator-Based TBI for Small Children

Purpose: A study was performed to determine the feasibility of using inversely optimized solid modulators designed on a commercial treatment planning system for the purpose of total body irradiation (TBI) of small children. It is suggested that solid modulators will provide improved dose distributions relative to traditional TBI techniques as well as reduce errors in the setup of blocks since only a single device needs to be placed per field. Method and Materials: A brass compensator model for a clinical 18 MV photon therapy beam was constructed for the purposes of intensity modulated radiation therapy. Using this model, two TBI treatment plans were constructed to deliver a single, AP beam to a small anthropomorphic phantom, representative of a six to twelve month old child. A third treatment plan was constructed using opposed AP/PA fields. Phantom irradiations proceeded according to these plans. TLD measurements were obtained at several anatomical positions for comparison to planned values, and traditional TBI plans were created for comparison to the intensity modulated plans. All plans used extended treatment distances. Results: The TLD results indicate that dose delivery within +/− 5% of the planned value is achievable to all anatomical sites with the exception of the mediastinum and neck. This is attributed to the difficulty of providing full dose to the mediastinum, while reducing lungdose to acceptable levels. However, the compensator‐based results are still clinically superior to the expected dose of standard TBI techniques. Conclusion: Solid modulator‐based TBI in small children can be successfully implemented using commercially available planning systems in either a single or dual field technique. Compared to traditional techniques, modulator‐based TBI results in greater dose homogeneity, reduced hot spots, and the ability to more accurately reduce dose to selected anatomical areas while reducing the number of blocks required to uniformly deliver dose.

SU‐FF‐T‐447: A Monte Carlo Study for Characterization of Solid Brass Compensators Used for IMRT

Purpose: A Monte Carlo study for characterization of solid brass compensators used for IMRT is performed, to investigate their dosimetric properties. Method and Materials: A group various solid brass compensators with different shapes were constructed and simulated by Monte Carlo code EGSnrc (BEAMnrc/DOSXYZnrc). A Varian 2100Ex linear accelerator was commissioned prior to the simulation of the compensators. The simulation included five solid slab brass compensators with thicknesses ranging from 1 to 7cm and solid brass compensators of varying thickness within the field. The same fluences as the ones produced by the compensators were created using Varian Millennium MLC collimator. Phase‐space files were scored after the compensators and after the MLC collimator. The phase space files were used as sources in DOSXYZnrc and the dose was calculated in a solid 30×30×30cm3 water phantom. Dose distribution profiles and PDD were obtained from the dose simulations. Particle fluence, energy fluence, energy spectrum and angular distributions were obtained from the phase space files. Results: Dosimetric data analysis of the PDDs and profiles showed that beam hardening does not have a significant effect on the PDD as it does on the energy spectrum. The PDDs are more penetrating than their open‐field counterparts. Comparing results from MLC‐based IMRT fields and compensators, we find them to be consistently dosimetrically equivalent for all the fields we studied. Conclusion: This study shows that the compensator is an inexpensive and reliable dose delivery device for IMRT and produces an optimized dose profile similar to the ones obtained with the MLC based IMRT delivery.Research sponsored by .decimal Incorporation.

TH-D-BRB-07: Skin Dose Determination for Various IMRT Delivery Methods

Purpose: To examine the effect of various methods of IMRT delivery on skin dose. Method and Materials:MLC based optimized IMRT plans were created using Pinnacle® TPS: two controlled cases — a 6‐step step‐wedge and a 4‐field forward planned IMRT plans — and one clinical case for abdomen treatment. For the same cases, optimized fluence maps were used to define the corresponding thickness distribution of brass compensators using the p.d software (.decimal) The planned compensator apertures were imported in Pinnacle® TPS for dose calculation using the same geometry with the MLC plans (isocentric method on a 30cm diameter cylindrical phantom). Equivalence between the MLC and compensator calculated plans were verified through isodose lines and Dose Volume Histogram comparisons. All measurements were performed using a 23Ex Varian accelerator with 120MLC and the compensators were fitted at the wedge tray location. Absolute dose measurements using ionization chambers were also performed at the center of the phantom geometry for plan verification purposes. Entrance, exit and total skin dose measurements were done using ultra thin TLDs and gafchromic EBT film. Results: Measurements of the entrance skin dose was shown to be reduced for the compensator based plan for the controlled cases up to 45% (step‐wedge plan) and the effect was even more prominent for thicker compensator material. However, the exit and total skin dose measurements were higher for the compensator IMRT plans than the respective MLC ones for the most part of the surface area. Conclusion: Preliminary results showed that the hardening of the beam when using compensator based IMRT delivery affects the patient's skin dose reducing the entrance skin dose but leading to higher skin doses when accounting for the total plan delivery. Current and future work includes Monte Carlo simulations for verification purposes. Conflict of Interest: Research sponsored by .decimal.

Compensator‐based intensity‐modulated radiation therapy for malignant pleural mesothelioma post extrapleural pneumonectomy

The present work investigated the potential of compensator‐based intensity‐modulated radiation therapy (CB‐IMRT) as an alternative to multileaf collimator (MLC)–based intensity‐modulated radiation therapy (IMRT) to treat malignant pleural mesothelioma (MPM) post extrapleural pneumonectomy.Treatment plans for 4 right‐sided and 1 left‐sided MPM post‐surgery cases were generated using a commercial treatment planning system, XIO/CMS (Computerized Medical Systems, St. Louis, MO). We used a 7‐gantry‐angle arrangement with 6 MV beams to generate these plans. The maximum required field size was 30×40 cm. We evaluated IMRT plans with brass compensators (•Decimal, Sanford, FL) by examining isodose distributions, dose–volume histograms, metrics to quantify conformal plan quality, and homogeneity. Quality assurance was performed for one of the compensator plans.Conformal dose distributions were achieved with CB‐IMRT for all 5 cases, the average planning target volume (PTV) coverage being 95.1% of the PTV volume receiving the full prescription dose. The average lung V20 (volume of lung receiving 20 Gy) was 1.8%, the mean lung dose was 6.7 Gy, and the average contralateral kidney V15 was 0.6%. The average liver dose V30 was 34.0% for the right‐sided cases and 10% for the left‐sided case. The average monitor units (MUs) per fraction were 980 MUs for the 45‐Gy prescriptions (mean: 50 Gy) and 1083 MUs for the 50‐Gy prescriptions (mean: 54 Gy).Post surgery, CB‐IMRT for MPM is a feasible IMRT technique for treatment with a single isocenter. Compensator plans achieved dose objectives and were safely delivered on a Siemens Oncor machine (Siemens Medical Solutions, Malvern, PA). These plans showed acceptably conformal dose distributions as confirmed by multiple measurement techniques. Not all linear accelerators can deliver large‐field MLC‐based IMRT, but most can deliver a maximum conformal field of 40×40 cm. It is possible and reasonable to deliver IMRT with compensators for fields this size with most conventional linear accelerators.PACS numbers: 87.56.ng, 87.56.N, 87.55.D, 87.55.dk

SU‐GG‐T‐139: Evaluation of a Biplanar Diode Array Detector for MLC‐ and Compensator‐Based IMRT QA

Purpose: To evaluate a biplanar diode array detector, with two types of modulators used for IMRT — MLC (step‐and‐shoot) and brass compensators. All calibration procedures were evaluated. Method and Materials: An absolute calibration procedure in the rectangular PMMA phantom was evaluated and compared to water phantom measurements. IMRT plans were prepared on CMS XiO and modulated by either 120‐leaf MLC or brass compensators. 10 MLC based plans and 7 compensator plans were tested. Dose calculations were done on a homogeneous dataset and on an MVCT of the Delta4 phantom. Results: Effective point of measurement is at the midplane of the detector board. The PMMA to water conversion factor for absolute calibration with ion chamber is 0.988. An effective TMR calculation yields the same calibration dose. Angular response range is 0.977–1.033 (0.977–1.010 if the angles of ±5° around the detector planes are excluded, consistent with the intended use of the device). Point doses along the fan lines differ −0.6 to +0.3% from those calculated on a homogeneous dataset, and −0.9 to −2.2% from those on the MVCT dataset. For 7 compensator plans, the average percentage of detectors with γ(5%/3mm)⩽1.0 is 97% (range 89–100%). For 10 MLC plans the average is 93% (72–100%), but the statistics may be skewed because some detectors are frequently at the very edge of the beamlet aperture while others are always at least 5 mm away. Conclusion: For absolute dosimetry measurements the PMMA output to water conversion factor needs to be determined using an appropriate calibration technique. There was no advantage in utilizing an MVCT dataset. It is desirable to have a software modification (isocenter shift) to ensure that the reported dose statistics are always representative of the population. Conflict of Interest: G. Nilsson is President of ScandiDos, who loaned the unit for evaluation at no charge.

SU‐FF‐T‐323: MLC IMRT Vs. Solid Compensators Based IMRT — Comparison of Various Clinical Cases

Purpose: To compare multileaf collimator (MLC) intensity modulated radiation therapy (IMRT) with solid compensator based IMRT in order to demonstrate the feasibility and accuracy of solid compensator based IMRT. Method and Materials: Using the CMS radiotherapy planning system a sequence of two separate IMRT plans were prepared using superposition algorithm. First plan in each sequence delivered IMRT by means of the MLC, whereas the second one delivered IMRT by means of solid compensator. Quality Assurance (QA) IMRT was performed on a 2100CD Varian linear accelerator for both plans. For the QA measurements we utilized a 120‐leaf Millennium MLC with 0.5 cm resolution and brass compensators (milled by DECIMAL INC.). Cases for different sites, e.g. head and neck, chest, abdomen, pelvis were investigated using 6 or 16 MV photons. Number of delivered monitor units, intensity map shapes and the dose distributions for GTV, CTV and OAR were compared for both plans. Results: Preliminary results indicate that solid compensator plans use significantly less monitor units and deliver smaller dose to the OAR as compared to the MLC plans. Moreover, solid compensators offer generally better resolution of intensity maps than MLC. Finally, in the process of segmentation, MLC IMRT changes DVH. In contrast, solid compensators by avoiding segmentation process of intensity maps, leave DVH unchanged between original, optimized plan and deliverable plan. Conclusion: The presented comparison demonstrates that solid compensator based IMRT is feasible. In addition, solid compensator based IMRT can provide clinical advantage over MLC IMRT for some treatments.

Head and Neck Compensation for Total Body Irradiation Using Opposed Laterals

Using shaped brass compensators that follow the coronal profile of a patient’s head and neck, we confirm that adequate compensation can be made to prevent overdosing in these regions when delivering total body irradiation using opposed lateral fields. Initially, these compensators were custom made for each patient, but we have shown that the variation from patient to patient is sufficiently small that individual compensators can be used for a number of different patients without compromising the dose distribution. In a subgroup of 35 patients on whom diode measurements were made, 20 used compensators from the library of ∼40 compensators made for previous patients and 15 required new compensators to be fabricated. No significant difference (3.4% on average) was observed in the dose distribution. By examining the profiles accumulated from 81 patients, we have shown that the primary difference between patients is not in the shape of the head and neck, but in the distance from the top of the head to the suprasternal notch and in the slope of the shoulders; thus, shaped compensators are not necessary and the same quality of dose homogeneity can be achieved using simple flat brass plates. Further, if the arms are supported so that the slope of the shoulders is constant, a relatively small number (15) of square-ended plates of thickness ranging from 4.0 to 11.0 mm would be sufficient to treat all 81 patients in our sample.