Strahlentherapie und Onkologie
Original Article
Comparison of Wedge versus Segmented Techniques in Whole Breast Irradiation Effects on Dose Exposure Outside the Treatment Volume Veronika Ludwig, Franz Schwab, Matthias Guckenberger, Thomas Krieger, Michael Flentje1
Purpose: To compare two irradiation techniques for whole breast irradiation: tangential wedged beams (WT) versus “open” fields (without wedges) with forward planned segments (ST). Patients and Methods: For 20 patients two comparative 3-D plans were defined using Pinnacle P3D and analyzed with respect to dose, dose homogeneity in the target volume, and scattered dose to organs at risk. The plans of six patients were reproduced in an Alderson phantom. Measurements were performed in the planning target volume (PTV), contralateral breast, lungs, heart, thyroid gland and in mid-pelvis. Results: Dose distribution in the PTV was nearly identical for WT and ST with the exception of D1. Scattered doses were significantly smaller for ST. In the contralateral breast the doses per 2-Gy fraction were 7.3 cGy ± 2.1 cGy (WT), and 4.7 cGy ± 1.9 cGy (ST; p < 0.01). Similar doses were measured for lung and heart. In mid pelvis the largest difference was observed (WT: 1.0 cGy ± 0.2 cGy, ST: 0.2 cGy ± 0.1 cGy; p < 0.01). Conclusion: Partial volume segments can replace wedges for improved dose coverage and homogeneity in the PTV. The ST causes significantly less scattered dose to extra-target organs. This may have implications for long-term risks after exposure to low radiation doses. Key Words: Breast cancer · Breast irradiation · Wedge technique · Segmented technique · Organs at risk • Phantom Strahlenther Onkol 2008;184:307–12 DOI 10.1007/s00066-008-1793-7 Vergleich zwischen Keilfilter- und segmentierter Technik in der Restbrust-Bestrahlung Ziel: Vergleich zweier Bestrahlungstechniken für die Brustbestrahlung: tangentiale Keilfiltertechnik (WT) versus „offene“ Felder (ohne Keil) mit vorwärts geplanten Segmenten (ST). Patienten und Methodik: Bei 20 Patientinnen wurden je ein Bestrahlungsplan mit Keilfiltern und ein Plan mit segmentierten Feldern erstellt und bezüglich Dosis, Dosishomogenität im Zielvolumen und Streuanteil in Risikoorganen analysiert. Die Pläne von sechs Patientinnen wurden am Alderson-Phantom reproduziert. Messungen wurden im Zielvolumen (PTV), der kontralateralen Brust, den Lungen, dem Herzen, der Schilddrüse und im kleinen Becken durchgeführt. Ergebnisse: Die Dosisverteilung im Planungszielvolumen (PTV) war für WT und ST nahezu identisch mit Ausnahme von D1. Der Streuanteil für Risikoorgane war für die ST signifikant geringer. In der kontralateralen Mamma betrug die Dosis pro 2-Gy-Fraktion mit der WT 7,3 cGy ± 2,1 cGy und mit der ST 4,7 cGy ± 1,9 cGy (p < 0,01). Ähnliche Ergebnisse wurden für die Dosen in Lunge und Herz gemessen. Der größte Unterschied zwischen WT und ST zeigte sich im kleinen Becken (WT 1,0 cGy ± 0,2 cGy, ST 0,2 cGy ± 0,1 cGy; p < 0,01). Schlussfolgerung: Teilvolumensegmente können Keile in Bezug auf Zielvolumenabdeckung und Dosishomogenität im PTV ersetzen. Die ST verursacht signifikant weniger Streuanteil außerhalb des Zielvolumens. Dies kann Einfluss auf Langzeitrisiken nach Exposition niedriger Strahlendosen haben. Schlüsselwörter: Mammakarzinom · Brustbestrahlung · Keilfiltertechnik · Segmenttechnik · Risikoorgane · Phantom
1
Department of Radiation Oncology, University Hospital Wuerzburg, Germany.
Received: July 25, 2007; accepted: April 11, 2008
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Ludwig V, et al. WT versus ST in Breast Irradiation
Introduction Standard treatment for early-stage breast cancer (AJCC stage 0–II) is tumorectomy and adjuvant whole breast irradiation [1, 9, 25, 26, 28, 34, 35, 37, 39]. Hard wedges usually compensate the different diameter of the breast [19]. Alternative techniques have been developed, especially several intensitymodulated radiotherapy (IMRT) techniques [5, 6, 12, 14, 17, 24, 30, 38].
(MU) weight for this first segment is iteratively raised for better homogeneity, but hot spots have to be avoided. Remaining underdosed areas are compensated by an additional smaller segment in the opposing tangential field (see Figure 2). The weight of these segments is typically in the range of 8–10%. Dose is prescribed to the ICRU reference point that represents the mean dose within the PTV.
Patients and Methods A retrospectively chosen cohort of 20 patients that underwent adjuvant breast irradiation at the Department of Radiation Oncology, University of Wuerzburg, Germany, between 11/2005 and 03/2006 was analyzed. All patients had been irradiated with a technique of opposed beams with segmentation of one or both beams. We used maximum two segments per beam. For comparison, a wedge technique (WT) and a segment technique (ST) 3-D plan for all 20 patients was generated. Plans from six patients (three women with right-sided and three women with left-sided breast cancer and small, medium or large breast volumes) were transferred to a Rando-Alderson phantom and dose measurements at various points were performed. Treatment Planning A 3-D CT scan with 5-mm slice spacing was performed. A radiation oncologist delineated the planning target volume (PTV). The PTV comprised the ipsilateral mamma and the chest wall. The volume of the contralateral breast, the lung and the heart were defined as organs at risk (OARs). The isocenter was positioned in the middle of the PTV. The gantry angle was optimized in the beam’s eye view (BEV) for a minimum lung area and beam divergence toward the lung was compensated by adjusting the gantry angle of the beams. All plans were calculated at the Pinnacle P3D planning system (V7.6) using the collapsed-cone algorithm with a voxel size of 3 × 3 × 3 mm3. Photon energy of 6 MV at a Siemens PRIMUS accelerator was used. Both techniques had two fields that covered the whole PTV (see Figure 1). The ipsilateral lung was spared using a multileaf collimator (MLC). The shape of the MLC was defined in the BEV with a distance of 7 mm to the PTV to compensate the penumbra in craniocaudal direction and toward the lung. The beams exceeded the patient surface by about 4 cm. For the WT, a 15° wedge typically compensated the dose inhomogeneity (ICRU 52). The ST uses open fields (without wedges). First, the tangential fields are adapted to the whole PTV using the MLC. The preliminary dose reference point is positioned so that the volume exceeding 107% of the prescribed dose is minimized. In the BEV of one tangential field the 95% isodose volumes are projected on the digitally reconstructed radiograph (DRR). Then, the underdosed areas toward the lung are compensated by an additional segment. The monitor unit
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a
b Figures 1a and 1b. Central CT slice with dose distribution (calculated at the treatment planning system Pinnacle P3D) for the wedge technique (a) and the segment technique (b). PTV: red contour. Abbildungen 1a und 1b. Zentrale CT-Schicht mit Dosisverteilung (berechnet mit dem Bestrahlungsplanungssystem Pinnacle P3D) für die Keil- (a) und die Segmenttechnik (b). PTV: rote Kontur.
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Ludwig V, et al. WT versus ST in Breast Irradiation
Fractions of 2.0 Gy (prescribed to the mean dose in the and the ST plan. Irradiation was performed with the MUs calPTV) were prescribed to a cumulative dose of 50.0 Gy. We culated for a treatment fraction having a dose of 2 Gy. compared the dose-volume histograms (DVHs) of both plans (see Figure 3) and recorded the D95, D5 and D1 as well as the Results MUs for both techniques. D95 specifies the dose that is reached Comparison of the DVHs of the plans of the 20 patients or surpassed in 95% of the volume. The high dose level in the showed that the dose in the PTV (D95) was nearly identical for the ST and the WT. The mean D95 was 46.4 Gy ± 0.5 Gy and volume is characterized by D5. D1 is a measure for the maximum dose values. For typical breasts it comprises a volume of 46.8 Gy ± 0.5 Gy for the ST and WT plans, respectively. The about 5–15 cm3. Both techniques were optimized to be commean D5 was 53.1 Gy ± 0.4 Gy for the ST plans and 52.8 Gy ± parable and to fulfill the ICRU homogeneity criterion for the 0.4 Gy for the WT plans. The mean D1 was 53.7 Gy ± 0.4 Gy for the ST plans and 54.3 Gy ± 0.5 Gy for the WT plans. The PTV (dose variation between –5% to +7% of the prescribed difference of D1 (0.5 Gy) between WT and ST was statistically dose). significant (p < 0.01). The calculated MUs were 232 MU ± 33 Treatment plans of six selected patients were verified in an MU for the ST plans and 304 MU ± 38 MU for the WT plans. Alderson phantom with mamma attachments (see Figure 4). These findings were reproduced in the measurements A MOSFET 20 dose verification system (Thomson/Nielsen) with the Alderson phantom (six patients): the dose to the PTV and Unidos/Multidos dosimeters (PTW Freiburg, Germany) was identical for the WT and the ST (see Table 1 for the measwith 0.125-cm3 thimble chambers (type TM31013) were used to measure the dose. The first MOSFET detector was placed ured doses [for a single irradiation with 2 Gy] in the ipsilateral between two caudal slices of the Alderson phantom at midmamma [PTV]). Extrapolating this to 25 fractions, 54.3 Gy ± pelvis. A second detector registered the dose at the heart and 1.2 Gy and 55.0 Gy ± 1.6 Gy were delivered for the ST and WT was placed ventrally on the left side. The third was used for plans, respectively. The dose in the ipsilateral mamma was the left lung and was placed dorsal and two slices cranial to the (10%) higher in the phantom than calculated for the patients. heart. The fourth was placed opposite and laterally in the right lung. The fifth represented the thyroid gland. Two thimble chambers (allowing buildup and backscattering) of the Multidos were used for the right and left breast (mid-bore of the mamma attachments) and at the level of the MOSFET detector. Both dosimetry systems had been calibrated under reference conditions (T = 293.2 K, p = 1.01 × 105 Pa). The mean values of the calibration factors have been implemented into the dose readout system so that the unit of the measurement is given in cGy. The energy dependence of the MOSFET dosimeters is < 2% [4]. The dose reproducibility reported ranges between 1% and 3% for the high-sensitivity setup. For the lowest scatter doses (at mid-pelvis) [27], an additional thimble chamber was positioned at the phantom surface. Patients with different breast sizes were studied: small (< 1,000 cm3), middle (1,000–1,500 cm3) and large (> 1,500 cm3). The final position of the central beam in the Alderson phantom was adapted to the field shape, in order to simulate geometry of the patient Figure 2. BEV with DRRs for the tangential fields and the segments – ST. treatment. The position of the Alderson phantom was identical for the WT Abbildung 2. BEV mit DRRs für die tangentialen Felder und ihre Segmente – ST.
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This was attributed to the smaller size of the mamma attachment compared to the real patient anatomy. The dose to the contralateral mamma was in the order of 2% of the isocenter dose. The dose measured for the ST was 4.7 cGy ± 1.9 cGy, the dose for the WT was about 50% higher (WT plan 7.3 cGy ± 2.1 cGy). Similar effects were observed for the lung and the heart: the ST resulted in significantly lower scatter doses compared to the WT (contralateral lung: ST 3.9 cGy ± 1.0 cGy, WT 6.0 cGy ± 1.3 cGy; heart if irradiated on the right side: ST Dose-volume histogram 1.0 0.9
3.8 cGy ± 1.0 cGy, WT 5.6 cGy ± 0.8 cGy). The ipsilateral lung received higher doses than the contralateral lung, and the heart received higher doses if the left breast was irradiated. The doses in the mid-pelvis were below the detection limit of the MOSFET system. The ionization chamber measurements resulted in mean doses of 0.2 cGy ± 0.1 cGy and 1.0 cGy ± 0.2 cGy for ST and WT irradiations, respectively (p < 0.01). The dose values given above have been recalculated for 2 Gy per fraction, the measured doses are given in Table 1. A correlation was found between the distance from the PTV and the measured difference between WT and ST: at measurement points close to the PTV, the dose for the ST was about 20% lower than for the WT; this ratio increased to more than 80% for mid-pelvis and the contralateral lung.
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Figure 3. Dose-volume histogram for the wedge technique (dashed line) and the segment technique (solid line) presented in Figure 1. Red lines: PTV; blue lines: ipsilateral lung. Abbildung 3. Dosis-Volumen-Histogramm für die Keil- (gestrichelte Linie) und die Segmenttechnik (durchgezogene Linie), die in Abbildung 1 dargestellt sind. Rote Linien: PTV; blaue Linien: ipsilaterale Lunge.
Figure 4. Experimental setup for the dose measurements using the Alderson phantom. Abbildung 4. Experimenteller Aufbau für die Dosismessungen mit einem Alderson-Phantom.
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Discussion This study demonstrates that treatment plans for whole breast irradiation with only two additional segments per field, and created by a simple forward planning procedure are equivalent in target coverage produced with wedges. Furthermore, the scattered dose in the patient is significantly reduced. The ST is not more time-consuming than the WT, especially as far as treatment planning and application are concerned. Parallel opposing beams with hard wedges compensate different tissue thickness in the female breast. MLCs and inverse treatment planning have raised interest in replacing hard wedges. Use of additional partial volume segments without changing the beam entry give better coverage and homogeneity within the breast and spare adjacent lung [30, 38, 42]. A major advantage of the ST plans over hard wedges is intensity modulation in craniocaudal direction [30] improving dose homogeneity in cranial and caudal breast areas. Techniques using two [30], four [38] and multiple [6, 17, 23, 24] additional segments have been reported with similar results. In this study the question of dose exposure outside the target volume is additionally addressed. Several authors investigated the risk of second nonhematologic malignancies [3, 7, 10, 31] and contralateral breast cancer (CBC) among patients with breast-conserving therapy [2, 10, 11, 13, 18, 36]. Hill-Kayser et al. demonstrated that the risk of CBC persists for at least 20 years after the treatment for early-stage breast cancer [13]. There are emerging data on lung cancer after whole breast irradiation showing an excess risk in the ipsilateral versus contralateral lung starting after 12–14 years [8]. Studies that deal with hematologic second malignancies after breast cancer treatment [15, 16, 20–22, 29, 40] examined radiotherapy either as single modality or in combination with chemotherapy [16, 20, 22, 40]. Le Deley et al. [20] observed that the risk for leukemia increased with the extent of radiation fields and with the mean radiation dose absorbed by active bone marrow. An increment of 1 Gy in patients treated with irradiation was associated with a risk ratio of 1.14 [20]. Howard et al. [15] found a significant excess of leukemia for at least 25 years after breast cancer radiotherapy. However, they
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Ludwig V, et al. WT versus ST in Breast Irradiation
Table 1. Measured dose values (in cGy) at the heart, lung, thyroid, mamma, and in mid-pelvis for the wedge technique (WT) and the segment technique (ST). The mean values for the six plans and the standard deviation (SD) are given in the last column. PTV: planning target volume. Tabelle 1. Gemessene Dosiswerte (in cGy) an Herz, Lunge, Schilddrüse, Brustdrüse und im kleinen Becken für Keil- (WT) und Segmenttechnik (ST). Die Mittelwerte und die Standardabweichung (SD) der sechs Patienten sind in der letzten Spalte angegeben. PTV: Planungszielvolumen. Patient # Localization mamma/breast size PTV (cm3) Monitor units Dose (cGy) Lung ipsilateral Dose (cGy) Lung contralateral Dose (cGy) Thyroid Dose (cGy) Mamma ipsilateral (PTV) Dose (cGy) Mamma contralateral Dose (cGy) Mid-pelvis Dose (cGy)
ST WT WT ST WT ST WT ST WT ST WT ST WT ST WT
1 Left/ small
2 Left/ medium
3 Left/ large
4 Right/ small
5 Right/ medium
6 Right/ large
Mean (SD)
706 230 328 8.3 15.0 16.7 2.3 3.7 0.7 1.7 235.1 229.0 3.1 5.1 0.2 0.6
1,260 234 328 15.3 16.5 17.1 4.0 5.7 1.3 3.0 223.4 225.0 4.3 7.1 0.2 1.0
1,950 227 315 21.3 17.3 17.4 4.7 7.0 1.3 3.3 218.3 226.0 8.3 11.2 0.4 1.1
702 226 310 4.7 17.3 18.7 3.3 6.0 0.7 2.3 213.4 216.4 3.9 6.2 0.2 0.8
1,260 231 302 5.7 13.8 14.7 4.0 6.3 1.3 2.7 212.9 212.6 4.4 7.2 0.3 1.0
1,825 233 314 6.3 18.0 19.1 5.3 7.3 2.0 3.3 215.7 214.9 4.0 6.7 0.3 1.2
1,542.5 (399.5) 232.0 (1.4) 308.0 (8.5) 6.0 (0.5) 16.2 (1.7) 17.0 (1.6) 4.0 (0.5) 6.3 (0.6) 1.2 (0.3) 2.8 (0.4) 217.0 (4.9) 220.0 (6.5) 5.2 (2.1) 7.9 (2.2) 0.2 (0.1) 1.0 (0.1)
described a significant decrease of the excess absolute risk with calendar year of breast cancer diagnosis [15]. This may result from changes and optimization of treatment schemes during the past decades and indirectly supports the aim for further efforts toward reduced radiation exposure. Different types of scatter have to be taken into account: internal scatter, leakage irradiation, and the scatter dose due to material between target and patient. Internal scatter is the dose from the treated volume to more distant areas within the patient. Comparison of different breast sizes (range 700–1,950 cm3) confirms that internal scatter increases with the volume of the irradiated PTV by up to a factor of 2. This factor is patient-specific and not easily addressed by treatment technique. Leakage irradiation from the accelerator and collimator is dependent on beam-on time. In this respect the resulting “fourfield” technique may have advantages over approaches using multiple segments and beam entries as in typical IMRT [32, 33]. The main impact on patient dose is achieved by abolishing scatter from hard wedges. Wedges are close to the photon target and induce Compton scatter with considerable divergence toward the patient. MUs have to be increased to compensate absorption within the wedge. The relative contribution from the hard wedge to patient exposure rises with distance from the target volume. We measured differences of about a factor of 1.5 for the contralateral breast and the heart. This increased to a factor of 3 for distant lung and mid-pelvis. Similar results (factor 2.4–3.3) have been reported by Woo et al. [41], who compared extra-target dose from IMRT,
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dynamic wedges, and hard wedges using thermoluminescence detectors (TLDs) on the body surface. There is good agreement for absolute doses of the contralateral breast. After a treatment series of 50 Gy the mean dose absorbed to the midpoint of the opposite breast (comparing wedges to segments) was 2 Gy versus 1.3 Gy in our series and 2.4 Gy versus 0.96 Gy in the study of Woo et al. [41] This is also consistent with the extensive phantom study of Chang et al. [5]: 10 cm distant to the field edge the dose in the contralateral breast was 1.4 Gy for hard wedges versus 0.75 Gy for MLC-virtual wedges. Lower doses were measured at mid-pelvis in our phantom verification (0.22 Gy vs. 0.05 Gy) compared to the surface measurements from Woo et al. [41] on abdomen and lower back of patients (1.2 Gy vs. 0.4 Gy). This difference is explained by a different distance of the measuring points from the field edge (15 cm in the study by Woo et al. [41] and 30 cm in our approach). The data, however, establish that whole body exposure from scatter in irradiated breast cancer patients is in a range relevant to stochastic risks such as second malignancies, and can be substantially reduced by choice of treatment technique and avoidance of hard wedges. Conclusion The ST clearly reduces the dose to the contralateral breast and OARs without loss of plan quality. Therefore, the ST abates the risk of second malignancies. Meanwhile, in our clinic, the ST has become routine. Furthermore, other conventional and new techniques should be analyzed with respect to profit and damage.
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Address for Correspondence Dr. Veronika Ludwig Department of Radiation Oncology University of Wuerzburg Josef-Schneider-Straße 11 97080 Würzburg Germany Phone (+49/931) 201-28891, Fax -28221 e-mail:
[email protected]
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