A technique for treating local breast cancer using a single set-up point and asymmetric collimation

Verification of the junctional dose for irradiation of the chest wall and supraclavicular regions under the circumstances of advanced technologies

A single-isocenter half-beam technique is commonly used when irradiating the chest wall and supraclavicular regions in patients with high-risk breast cancer. However, several studies have reported that underdosage can occur at the junction of the chest wall and supraclavicular regions due to a "tongue-and-groove" effect. This study verified the efficacy of an open leaf technique (OL-tech) that involves placing a multileaf collimator 5 mm outside from the beam central axis to remove the effect of the multileaf collimator in a single-isocenter half-beam technique. We compared the junction doses of the OL-tech with those of a conventional technique (C-tech) in square and clinical plans, using 4 and 10 MV x-rays in the Clinac iX and 6 and 10 MV x-rays in the Trilogy accelerators (Varian Medical Systems, Palo Alto, CA). EBT3 radiochromic films were used for measurements. Measurements were performed at a depth of 3 cm when verifying field matching. The EBT3 films in the square plan indicated junction doses for the C-tech of 78.3% with the Clinac iX accelerator and 73.6% with the Trilogy accelerator. By contrast, the corresponding doses for the OL-tech were 107.2% and 99.8%, respectively. In the clinical plan, the junction doses for the C-tech were 76.5% with the Clinac iX accelerator and 72.6% with the Trilogy accelerator; the corresponding doses for the OL-tech were 108.3% and 101.7%, respectively. As with the square plan, variations in the junction dose were much smaller using the OL-tech than using the C-tech. Our results suggest that the OL-tech can be useful for improving dose homogeneity at the junction of the chest wall and supraclavicular regions.

Model-based cardiac dose estimation in radiation treatment of left breast cancer

OBJECTIVE:

To develop a mathematical model for cardiac dose estimation for patients who have been treated for left-sided breast cancer without CT data.

METHODS:

After obtaining institutional review board approvals, 147 patients with left-sided breast cancer were selected that were treated supine with opposed tangents. The heart blocks from the tangential fields were removed and dose calculations were performed with 6 MV beams using an advanced algorithm. This study was performed with CT data with DRR to represent a radiographic simulator image of yesteryear treatments. The beam's eye-view images showing delineated breast, lung and heart were created to represent views on radiograph. The maximum heart distance (MHD) was recorded and irradiated heart area (AREA) was computed by combination of triangles and rectangles. Based on accurate 3D dose calculation, mean dose (D_mean) and V₁₀ to V₅₀ of heart were analyzed with respect to MHD and AREA for dosimetric parameters using linear and quadratic fit.

RESULTS:

The treatment parameters calculated by MHD and segments using 2D radiographs were within 2% of the actual dosimetric parameters computed from the 3D planning system. The MHD and AREA vs D_mean, V₁₀, V₂₀, V₃₀ and V₅₀ showed very good correlation with linear model (R² > 0.91); however the correlation was significantly better with quadratic model (R² > 0.92). The analysis of the dosimetric error with our linear and quadratic model is remarkable within <3% error for most cases.

CONCLUSION:

The proposed mathematical model for the cardiac dose estimation is accurate within ±3% using a radiograph without CT data. This provides avenues for patient pooling in future studies related to radiation dose and cardiac toxicity. These results will help in estimating cardiac dose analysis accurately from previous studies as well as in centers still using 2D planning.

ADVANCES IN KNOWLEDGE:

The evidence of cardiac risk following radiotherapy continues to be one of the important considerations for the management of left-sided breast cancer patients. One of the problem in the estimation of dose-risk effects is the reconstruction of heart dose for pre-CT treatments. In this study, a simple mathematical model is presented that could estimate cardiac dose within ±3% in left breast cancer treatment from 2D radiograph where CT data do not exist.

Angular Dependence of the MOSFET Dosimeter and its Impact on In vivo Surface Dose Measurement in Breast Cancer Treatment

The focus of this study is the angular dependence of two types of Metal Oxide Semiconductor Field Effect Transistor (MOSFET) dosimeters (MOSFET20 and OneDose/OneDosePlus) when used for surface dose measurements. External beam radiationat different gantry angles were delivered to a cubic solid water phantom with a MOSFET placed on the top surface at CAX. The long axis of the MOSFET was oriented along the gantry axis of rotation, with the dosimeter (bubble side) facing the radiation source. MOSFET-measured surface doses were compared against calibrated radiochromic film readings. It was found that both types of MOSFET dosimeters exhibited larger than previously reported angular dependence when measuring surface dose in beams at large oblique angles. For the MOSFET20 dosimeter the measured surface dose deviation against film readings was as high as 17% when the incident angle was 72 degrees to the norm of the phantom surface. It is concluded that some MOSFET dosimeters may have a strong angular dependence when placed on the surface of water-equivalent material, even though they may have an isotropic angular response when surrounded by uniform medium. Extra on-surface calibration maybe necessary before using MOSFET dosimeters for skin dose measurement in tangential fields.

A method for accurate zero calibration of asymmetric jaws in single-isocenter half-beam techniques

PURPOSE

To present a practical method for calibrating the zero position of asymmetric jaws that provides higher accuracy at the central axis and improves dose homogeneity in the abutting region of half-beams.

METHODS

Junction doses were measured for each asymmetric jaw using the double-exposure technique and electronic portal imaging devices. The junction dose was determined as a function of jaw position. The shift in the zero jaw position (or in its corresponding potentiometer readout) required to correct for the measured junction dose could thus be obtained. The jaw calibration was then modified to introduce the calculated shift and therefore achieve an accurate zero position in order to provide a relative junction dose that was as close to zero as possible.

RESULTS

All the asymmetric jaws from four medical linear accelerators were calibrated with the new calibration procedure. Measured relative junction doses at gantry 0° were reduced from a maximum of ±40% to a maximum of ±8% for all the jaws in the four considered accelerators. These results were valid for 6 MV and 18 MV photon beams and for any combination of asymmetric jaws set to zero. The calibration was stable over a long period of time; therefore, the need for recalibrating is seldom necessary.

CONCLUSIONS

Accurate calibration of the zero position of the jaws is feasible in current medical linear accelerators. The proposed procedure is fast and it improves dose homogeneity at the junction of half-beams, thus, allowing a more accurate and safer use of these techniques.

The influence of the field setup on the dosimetry of abutted fields in single-isocenter half-beam techniques

PURPOSE

To study the influence of the field setup on the dosimetry at the junction in single-isocenter half-beam techniques.

METHODS

The dosimetry at the junction for a two-field setup with the gantry at zero was first evaluated with radiochromic films. A three-field setup, with an anterior field and two opposed lateral fields, was also analyzed for two different relative positions of the fields involved. In all cases, the dose increase at the central axis, called the junction dose, was measured.

RESULTS

Junction doses varied greatly with the setup. For the three-field setup, the junction dose differed from that obtained with the two-field setup, and it greatly depended on the relative position of the fields. When the anterior field was closer to the gantry than the lateral fields, a field gap occurred and the junction dose was negative. When the anterior field was farther from the gantry than the lateral fields, a field overlap was obtained and the junction dose was positive. The difference in the junction dose between the three-field setups was around 18% for the three accelerators evaluated.

CONCLUSIONS

Having a uniform dose distribution for two fields at gantry 0 degrees does not guarantee a uniform distribution at other gantry angles. Junction doses are largely affected by the relative position of the radiation fields, which may have an impact in clinical practice. Therefore, any method aiming to assess or to optimize the dose homogeneity at the junction should take this effect into account.

Clinical applications of geometrical field matching in radiotherapy based on a new analytical solution

A new analytical formalism has been published recently that provides all the parameters necessary for geometrical field matching in radiotherapy. The present work applies the general expressions for craniospinal irradiation, breast irradiation with a supraclavicular half-field, and breast irradiation with a supraclavicular full-field. We also explore the formalism as a tool to analyze and compare different techniques. Field matching is achieved by imposing both parallelism and coincidence between the side planes of adjacent fields. The rotation angles and either the field aperture for a certain isocenter position or the isocenter coordinates for a given field aperture are supplied. All of the already known exact solutions are reproduced. New expressions for the field aperture and for the isocenter coordinates, which were not previously available, are also computed. If tangential fields at a fixed source-to-skin distance are used together with a supraclavicular full-field, different apertures for each tangential field are required to achieve a correct match. If an isocentric technique for the tangential fields or a supraclavicular half-field is used, this complication is avoided. The breast technique with the supraclavicular half-field is recommended, because it presents several advantages with respect to the supraclavicular full-field. This formalism provides a useful tool in cases where matching of adjacent fields is necessary.

Dosimetry and field matching for radiotherapy to the breast and supraclavicular fossa

PURPOSE

Early breast cancer radiotherapy aims for local disease control and reduced recurrence. Treatment is directed to breast or chest wall alone using tangential fields, or includes regional lymph nodes with a separate anterior field. The complex geometry of this region necessitates matching adjacent radiation fields in three-dimensions. Potential exists for overdosage or underdosage and cosmetic results may be compromised if fields are not accurately aligned.

METHODS AND MATERIALS

A study of dosimetry across the match line region using different techniques, as reported in the multicentre START Trial Quality Assurance programme, was undertaken. A custom-made anthropomorphic phantom assessed dose distribution in three-dimensions using film dosimetry.

RESULTS

Methods with varying degrees of complexity were employed for field matching. Techniques combined half beam blocking and machine rotations to achieve geometric alignment. Asymmetric beam matching allowed use of a single isocentre technique. Where field matching was not undertaken a gap between tangential and nodal fields was employed. Results demonstrated differences between techniques and variations for similar techniques in different centres. Geometric alignment techniques produced more homogenous dose distributions in the match region than gap techniques or those techniques not correcting for field divergence.

CONCLUSIONS

Field matching techniques during the START trial varied between centres. Film dosimetry used in conjunction with a breast-shaped phantom provided relative dose information. The study highlighted difficulties in matching treatment fields to achieve homogenous dose distribution through the region of the match plane and the degree of inhomogeneity as a consequence of a gap between treatment fields.

Regional lymph node irradiation in breast cancer

A recent meta-analysis, published by the Early Breast Cancer Trialists' Collaborative Group, demonstrated a clear survival advantage of post-operative radiotherapy on the breast, chest wall and regional lymphatics in node-positive disease. The extensive target volume in locoregional irradiation of breast cancer, in close proximity to the heart and lungs, complicates treatment planning. The breast or chest wall fields need to match the supraclavicular/axillary and parasternal fields, at the subclavicular and parasternal matchline, respectively. Dose distribution near the junction area is often inhomogeneous, and under- and over-dosage can occur, which can lead to recurrences and complications. This paper describes briefly the indications, complications and target localization concerning regional lymph node radiotherapy and discusses more extensively the advantages and disadvantages of the most frequently used treatment techniques.

Moving to a High-tech Approach to the Irradiation of Early Breast Cancer: Is It Possible to Balance Efficacy, Morbidity and Resource Use?

There is substantial evidence documenting the potential morbidity associated with radiotherapy in early breast cancer. An appraisal of current standard radiation practice is therefore necessary, given that women are surviving longer, have an improved quality of life, and are overcoming subsequent side-effects caused by postoperative irradiation. New technology allows the application of more complex approaches. This discussion paper considers some of the benefits of the widespread use of new complex approaches, such as intensity-modulated radiotherapy (IMRT) in the light of staffing and equipment shortfalls, and possible consequences on waiting times for treatment. The discussion is considered under the following themes: (1) which women with breast cancer benefit from complex treatment approaches? (2) What is the role of treatment accuracy in limiting morbidity? And (3) what is the potential effect of complex breast irradiation approaches on service delivery? In the UK, and globally, many departments are struggling to meet waiting-time guidelines. The use of more complex approaches for breast irradiation may increase this difficulty. However, a number of simple technical changes can be used to enhance efficacy and reduce levels of normal tissue morbidity. A sub-set of women who are at greatest risk from normal tissue morbidity or reduced cosmesis should be accurately defined in order to allow departments to plan their treatment strategies with optimal use of resources.

Intensity-modulated radiotherapy technique for three-field breast treatment

PURPOSE

To develop a simplified intensity-modulated radiotherapy (IMRT) algorithm for three-field breast treatment using a single isocenter setup. The algorithm aims to deliver a uniform dose throughout the breast volume. Special attention was paid to the highly divergent nature of the beam configuration.

METHODS AND MATERIALS

Computed tomography (CT) image setup of the patient was acquired. On each CT slice, the computer automatically generated lines parallel to the posterior edge of the tangent field. The mid-point of each line segment that intersected the breast was determined and the dose from an open field calculated. The intensity of the divergent pencil beam corresponding to the mid-point was set to be inversely proportional to the open field dose to the mid-point. Forward dose calculation was then performed using this intensity distribution.

RESULTS

A total of 15 breast cancer patients undergoing three-field IMRT who underwent planning and treatment with this algorithm were included in this study. Compared with standard wedged pair tangents, the IMRT plan produced statistically significant better dose distributions in terms of target coverage and target dose uniformity, as well as reduced dose to the contralateral breast and reduced hot spots to the ipsilateral lung.

CONCLUSION

Since March 2004, the new IMRT algorithm has been used for planning and treatment of > 20 patients undergoing three-field treatment, as well as >200 patients undergoing regular two-field tangent treatment, all with excellent dose distributions throughout the breast volume.

Real-time 3D surface-image-guided beam setup in radiotherapy of breast cancer

We describe an approach for external beam radiotherapy of breast cancer that utilizes the three-dimensional (3D) surface information of the breast. The surface data of the breast are obtained from a 3D optical camera that is rigidly mounted on the ceiling of the treatment vault. This 3D camera utilizes light in the visible range therefore it introduces no ionization radiation to the patient. In addition to the surface topographical information of the treated area, the camera also captures gray-scale information that is overlaid on the 3D surface image. This allows us to visualize the skin markers and automatically determine the isocenter position and the beam angles in the breast tangential fields. The field sizes and shapes of the tangential, supraclavicular, and internal mammary gland fields can all be determined according to the 3D surface image of the target. A least-squares method is first introduced for the tangential-field setup that is useful for compensation of the target shape changes. The entire process of capturing the 3D surface data and subsequent calculation of beam parameters typically requires less than 1 min. Our tests on phantom experiments and patient images have achieved the accuracy of 1 mm in shift and 0.5 degrees in rotation. Importantly, the target shape and position changes in each treatment session can both be corrected through this real-time image-guided system.

A simplified intensity modulated radiation therapy technique for the breast

A simplified intensity modulated radiation therapy (sIMRT) technique for the breast is presented. The technique aims to produce a uniform dose distribution in the entire breast volume. Using the standard tangential beam arrangement, we first determine for each pencil beam the midpoint of the segment that intersects the treatment volume. The dose to the midpoint from the open field is then calculated. The intensity of the pencil beam is determined as proportional to the inverse of the open field dose. With this intensity modulated beam, the dose delivered to the midpoint of each pencil beam segment that intersects the treatment volume is now equalized. The dose distribution in the entire treatment volume is nearly as uniform as can be achieved under the given beam arrangement. Fifteen left breast patients were planned with the sIMRT technique. For comparison, the same group of patients was also planned with the standard wedged pair technique and the full-fledged volume-based IMRT (vIMRT) technique. Both the sIMRT and the vIMRT techniques achieved more homogeneous dose in the treatment volume than the standard plan. Doses to the heart, the ipsilateral lung, and the contralateral breast were also reduced. The planning time and the treatment time for the sIMRT technique were comparable to that of the standard technique, and significantly less than that required by the vIMRT technique. The sIMRT technique is practical for large-scale implementation in a busy clinic without requiring significant increase of resources.

An improved technique for breast cancer irradiation including the locoregional lymph nodes

PURPOSE

To find an irradiation technique for locoregional irradiation of breast cancer patients which, compared with a standard technique, improves the dose distribution to the internal mammary-medial supraclavicular (IM-MS) lymph nodes. The improved technique is intended to minimize the lung dose and reduce the dose to the heart.

METHODS AND MATERIALS

The standard technique consists of an anterior mixed electron/photon IM-MS field. In the improved technique, an oblique electron and an oblique asymmetric photon field are combined to irradiate the IM lymph nodes. To irradiate the MS lymph nodes, a combination of an anterior electron and an anterior asymmetric photon field is used. For both the standard and the improved technique, tangential photon fields are used to irradiate the breast. Three-dimensional (3D) treatment planning was performed for 8 patients with various breast sizes for these two techniques. Dose-volume histograms (DVHs) and normal tissue complication probabilities (NTCPs) were compared for both techniques. The field dimensions and energy of the standard technique were determined at simulation, whereas for the improved technique the fields were designed by CT-based treatment planning.

RESULTS

The dose in the breast planning target volume was essentially the same for both techniques. For the improved technique, combined with 3D localization information, an improvement in the IM-MS planning target coverage is seen. The volume within the 95% isodose surface was on average 25% (range, 0-64%) and 74% (range, 43-90%) for the standard and improved technique, respectively. The heart generally receives less dose with the improved technique. However, sometimes a small but acceptable increase in lung dose is found.

CONCLUSION

The improved technique, combined with localization information of the IM-MS lymph nodes, greatly improves the dose distribution in the planning target volume for a large group of patients without significantly increasing the dose to organs at risk.

Dosimetric assessment of nonperfectly abutted fields using asymmetric collimators

The abutment of adjacent fields has been facilitated through the use of asymmetric collimators. Conceptually, the abutment yields a perfectly uniform dose distribution across the junction, provided the asymmetric jaw is set precisely at the beam central axis. However, the asymmetric jaw has an associated tolerance, which can cause the abutment to be misaligned. This study examined the dose distribution at the junction of nonperfectly abutted fields. The abutment of fields was carried out using an asymmetric collimation of 5 x 10 cm, with an asymmetric jaw positioned at the beam central axis. A film was initially exposed using this field with the collimator set at 90 degrees. The collimator was then rotated 180 degrees and the same film was exposed for the second time to create the field abutment. Positioning the asymmetric jaw with respect to the beam central axis set the amount of gap and overlap between the abutted fields. The dose distribution was measured for asymmetric jaw positioning of -2, -1, 0, + 1, and +2 mm from the beam central axis. In addition, the dose distribution was also computed mathematically by summing the 2 dose profiles with defined gap or overlap. A field mismatch of +/-1 mm would result in a dose nonuniformity of 17%, and a +/-2 mm mismatch would produce a 35% dose nonuniformity.

Effect of set-up uncertainties on the dose distribution in the match region of supraclavicular and tangential breast fields

BACKGROUND

The objective of this paper is to illustrate the effect of a gap of 5 mm, an overlap of 5 mm and a perfect match on the dose distribution across the junction of tangential breast fields and adjacent supraclavicular and axillary fields.

MATERIALS AND METHODS

For this purpose film dosimetry was applied to measure relative dose distributions in two sagittal planes in an anthropomorphic breast phantom having cork lungs, simulating a radiation therapy treatment of the breast and adjacent supraclavicular lymph nodes. Two different treatment techniques, an SSD match technique and a geometrically exact isocentric match technique, as routinely applied in the two institutes were examined. The three-dimensional treatment planning system of each institute was used to calculate the dose distribution in the match region of the supraclavicular fields and the two opposing tangential fields. The measured and calculated dose distributions were evaluated and compared along lines in two sagittal planes from the supraclavicular fields down to the tangential fields crossing the match planes. These dose distributions in the match region were extremely dependent on the set-up of the fields.

RESULTS

Although the reproducibility of the film measurements was within 2%, it became clear that the set-up of the fields to achieve a gap of 5 mm, a perfect match or an overlap of 5 mm required a lot of attention, even when using a phantom.

CONCLUSIONS

It can be concluded that in clinical practice, these set-up difficulties do influence the dose distribution in the match region much more than the systematic uncertainties in the dose calculation algorithms of the treatment planning systems and the type of treatment technique.

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 AND 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 SL-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.

Patterns of dose variability in radiation prescription of breast cancer

PURPOSE

Comparison of radiation outcome of various treatment protocols is difficult due to the variability of dose prescription. A retrospective analysis of the pattern and intercomparison of dose prescriptions is presented for the treatment of breast cancer.

MATERIALS AND METHODS

To represent the clinical practice for breast irradiation with tangential fields, commonly used prescription points were chosen that lie on the perpendicular bisector of the chest wall separation (s) that represents the breast apex height (h). These points are located at 1.5 cm from the posterior beam edge, at the chest wall-lung interface (2-3 cm), at distances of h/3 and h/2, and at the isocenter. One hundred consecutive patients treated with intact breast following excisional biopsy were used in this study. For analysis, treatment planning was carried out without lung correction with a 6 MV beam for all patients, even though some of the patients were treated with high energy beams for dose uniformity. Dose distributions were optimized with wedges and beam weights to provide a symmetrical dose distribution on the central axis plane. The statistical analyses of the different parameters, s, h, maximum dose, and doses at various prescription points were carried out.

RESULTS

The maximum dose (hot spot) in breast varied from +5% to +27% above the prescribed dose among the patient population. The hot spot was directly related to s, and appeared to be independent of h and the ratio h/s. Among 55%, 40%, and 5% of the patients, the magnitude of the hot spot was 5-10%, 10-15%, and >15%, respectively. Except for the magnitude of the hot spot, the doses at various prescription points were independent of the breast size. For a prescription point at h/3 or at the lung-chest wall interface, the dose variation within +/- 1% is observed for 90% of the patient population. On the other hand, the average dose variation is about +/- 3% among other protocols with dose prescription point varying up to the h/2 point. With the prescription point at the isocenter, an average and maximum variation of 4-5% and 11% were observed, respectively. The maximum dose inhomogeneity for some patients was significantly higher, i.e. up to +27% even without the lung correction.

CONCLUSIONS

A wide variation in prescription dose is observed among the different treatment protocols commonly used in breast treatment. For a total dose of 46-50 Gy delivered at 2 Gy/fraction to the breast, the prescribed dose may vary between 50 and 55 Gy and the hot spot dose per fraction may range between 2.3 and 2.5 Gy depending on the protocol and breast size. Thus dose normalization at hot spot and the isocenter should be discouraged unless the total dose to the breast is modified. A uniform definition of dose prescription for breast treatment is greatly required for intercomparison of clinical data.

Clinical use of asymmetric collimators

PURPOSE

To illustrate some of the radiation treatment techniques with asymmetric collimators in one field dimension.

METHODS AND MATERIALS

Treatment planning for various sites is done with an in-house developed treatment planning system. Dose distributions in the central plane are illustrated.

RESULTS

The use of asymmetric collimation, in addition to being a replacement for cerrobend and lead blocks, can facilitate treatment setup with boost fields and with half-beam asymmetric fields as in matching two adjacent fields, in avoiding nearby critical organ or tissue, and in tangential breast treatment. The use of asymmetric collimators would alter the dose distribution across the radiation field and should be accounted for during treatment planning. In conjunction with arc rotation or multiple asymmetric fields, two-dimensional conformal radiotherapy is possible.

CONCLUSION

The full potential of asymmetric collimation requires the use of a proper treatment planning algorithm. Some of the treatment techniques with asymmetric collimation in one field dimension are shown here.

Radiation therapy of breast carcinoma: confirmation of prescription dose using diodes

PURPOSE

To quantitate the dose delivered during tangential breast radiation therapy and measure the scatter dose to the contralateral breast for three different breast setup techniques.

METHODS AND MATERIALS

A commercial semiconductor diode system is used for dose measurements. The diode characteristics were studied by comparing the diode response against a standard ionization chamber response in a reference configuration. In vivo dose measurements on 11 patients undergoing tangential breast radiation therapy with 6 MV photons were performed. Medial and lateral field entrance and exit doses were measured and compared with the expected values from the treatment planning system. Scatter doses to the contralateral breast for three breast setup techniques were measured and documented as a function of distance from the field edge and various beam modifiers commonly used in breast radiation therapy.

RESULTS

The diodes used in this study exhibited excellent linearity, dose reproducibility, and minimal anisotropy. The in-phantom measurements resulted in dose accuracy within +/- 1.5%. Dose measurements on patients resulted in standard deviations of 1.2 and 2.3% for the medial entrance and exit doses and 1.7 and 2.2% for the lateral entrance and exit doses, respectively. In patients, the scatter doses to the opposite breast at a 5 cm perpendicular distance from the medial field edge resulted in cumulative scatter doses of 2.47 to 5.30 Gy from the tangential fields and an additional 0.50 Gy from the supraclavicular or axillary field, if included.

CONCLUSION

Quantitative verification of the prescribed daily dose is important in breast radiation therapy to ensure precision in patient setup and accuracy in dose delivery. Diodes provide a convenient way of real-time patient dose verification and are easy to use by the therapists.

Applications of asymmetric collimation on linear accelerators

Frequency of use of asymmetric collimation (AC) at an academic radiation oncology center equipped with AC-capable linear accelerators was determined, and the type of use was cataloged. Records of patients beginning radiation treatment at U.C. Davis Cancer Center within a 3-month period (3/1/92 to 5/31/92) were reviewed. Forty-seven percent of 102 patients and 56% of 123 courses of treatment involved AC. Six common uses of AC were identified: beam-split field matching, planned boosts, other field size changes, adjustments to match divergent fields, matchline feathering, and opposed tangential fields. This study demonstrates that asymmetric collimation is a useful and powerful clinical treatment tool with widespread applications to radiation therapy.

In vivo determination of the accuracy of field matching in breast cancer irradiation using an electronic portal imaging device

The purpose of this study was to investigate the accuracy of field matching in patients treated by irradiation of the breast and adjacent lymph nodes. Field matching is performed by the radiographers during each session on a match line drawn on the patient's skin. Field edge positions were assessed in the cranial match plane of tangential breast fields and supraclavicular-axillary fields using an electronic portal imaging device and match line markers placed on the skin of the patients. The mean gap/overlap of the four fields for individual patients during each treatment session, derived from 374 marker projections, was +0.5 mm indicating that no systematic gap or overlap was observed. The uncertainty in the position of the four fields with respect to the match plane ranges from 3.1 to 5.1 mm (1 SD) for the individual patients. Gaps and overlaps between fields were also related to an absolute match line position, found by comparison of simulator and portal images, showing a small systematic uncertainty of 2.4 mm and a standard deviation of 3.3 mm. It can be concluded that the use of an electronic portal imaging device in combination with match line markers is a good method to quantify the accuracy of field matching in vivo. The results showed good stability and reproducibility in the field matching region for this treatment technique of breast cancer irradiation.

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.

A single isocenter three-field breast irradiation technique using an empiric simulation and asymmetric collimator

The treatment of abutting fields presents multiple difficulties, including problems of field overlaps or gaps, complexity of simulation, and the difficulties of daily setup and variation. Multiple techniques have been described for the treatment of the breast/chest wall and supraclavicular nodes using tangents and a matched supraclavicular field. The techniques described have used collimator angles, couch angles, and/or corner blocks in an attempt to match these fields with no overlap or gap. Some of these techniques required complex calculations or treatment devices to achieve a geometric match between fields. We describe a technique for treatment of breast and supraclavicular nodes that uses a single isocenter and requires asymmetric collimator jaws to give half-blocked fields. The simulation and setup are done empirically, with no complex calculations required. The daily setup and treatment can be done rapidly and reliably, with no extra equipment required. Custom blocks may be used to conform to the chest wall contour and decrease the amount of lung in the treatment fields.

Three-field isocentric breast irradiation using asymmetric jaws and a tilt board

Perfect abutment of medial and lateral tangential breast portals with the adjacent supraclavicular field may be achieved with ease. A simple and safe approach was developed using a tilt board and new technology that is standard on a popular linear accelerator. The patient is secured on a tilt board as a means to level the chest wall. Isocenter is placed at depth on the matchline, where asymmetric jaws are used to produce non-divergent field edges and a perfect abutment. This is done without the need for table or collimator rotations, beam-splitters, or vertical cephalad blocks. The dorsal beam edge of the tangents is made coplanar by rotating the gantry more than 180 degrees. This procedure produces a dosimetrically sharp field edge and eliminates concern about block transmission and excess dose to the contralateral breast. Set-up is fast, and the steps involved are simple and few. Advantages and limitations of this technique are presented.

Beam modifying devices in the treatment of early breast cancer: 3-D stepped compensating technique

A 3-D compensating technique utilizing stepped brass filters to ensure an adequate dose distribution in the breast after conservative surgery is described. No compensation for lung inhomogeneity is applied. These compensators are part of a procedure which also involves the use of a rotatable half-beam block (HBB) and of an individualized immobilization cast. Results of patient dosimetry show a good agreement with dose prescription in the target volume (measured dose/prescribed dose = 100.9% +/- 1.8% at the reference point). Phantom dosimetry is discussed.

Early breast cancer irradiation after conservative surgery: quality control by portal localization films

A review of geometrical errors on portal localization films in 20 patients undergoing breast irradiation after conservative surgery is presented. Patients were immobilized using a cellulose acetate individual cast. Good precision in daily set-up has been noted with a mean ventrodorsal shift of 2.7 mm on medial films and 3.0 mm on lateral films; a mean craniocaudal shift of 1.8 mm on medial films and of 2.0 mm on lateral films have been also observed. The mean half-beam block rotation error has been minimal, ranging 0.8 degrees on medial and 1.1 degrees on lateral films. A new cast was made in 3/20 (15%) patients depending on variation of breast morphology during treatment and on geometrical errors noted by portal localization film check.

Accuracy in tangential breast treatment set-up: a portal imaging study

To test the accuracy and reproducibility of the tangential breast treatment set-up used in The Netherlands Cancer Institute, a portal imaging study was performed in 12 patients treated for early stage breast cancer. With an on-line electronic portal imaging device (EPID) images were obtained of each patient in several fractions and compared with simulator films and with each other. In five patients, multiple images (on the average 7) per fraction were obtained to evaluate set-up variations due to respiratory movement. The central lung distance (CLD) and other set-up parameters varied within one fraction about 1 mm (1 SD). The average variation of these parameters between various fractions was about 2 mm (1 SD). The differences between simulator and treatment set-up over all patients and all fractions was on the average 2-3 mm for the central beam edge to skin distance and the central lung distance. It can be concluded that the tangential breast treatment set-up is very stable and reproducible and that respiration does not have a significant influence on treatment volume. The EPID appears to be an adequate tool for studies of treatment set-up accuracy like this.

Quality assurance of the EORTC trial 22881/10882: "assessment of the role of the booster dose in breast conserving therapy": the Dummy Run. EORTC Radiotherapy Cooperative Group

The EORTC trial 22881/10882 is a randomised trial with the aim to assess the role of the boost dose in breast conserving therapy in stage I and II breast cancer. In order to detect potential protocol deviations concerning irradiation technique and in the dose specification procedure of participating institutions before actual patient accrual, a Dummy Run was performed. Three transverse sections of a patient were sent to 16 participating institutions with a request to make a three-plane treatment plan according to the protocol prescriptions. A treatment chart and beam data were also requested for recalculation of the dose. Additional information was asked in a questionnaire. On evaluation, the techniques differed considerably with respect to photon beam energy, varying between 60Co gamma-rays and 8 MV X-rays, and the use of wedge filters. Two institutions did not apply wedges, whereas wedge angles in the other institutions varied between 6 degrees and 45 degrees. Twelve institutions used collimator rotation and/or a table wedge to diminish the amount of irradiated lung volume. The dose was specified in a point according to the protocol prescription in 11 institutions and to the 90, 95 or 100% isodose curve in four. Twelve institutions applied lung density corrections during treatment planning, while nine reported problems with their planning system in off-axis dose distribution calculation and/or the simulation of collimator rotation. Recalculation of the dose at the isocentre showed agreement within 2% compared with the stated dose. The dose reported in the tumour excision area varied between 93 and 100%.(ABSTRACT TRUNCATED AT 250 WORDS)