- Email: [email protected]
Field-in-field plan does not improve the dosimetric outcome compared with the wedged beams plan for breast cancer radiotherapy
Medical Dosimetry 39 (2014) 79–82
Medical Dosimetry journal homepage: www.meddos.org
Field-in-field plan does not improve the dosimetric outcome compared with the wedged beams plan for breast cancer radiotherapy Li-Min Sun, M.D., M.P.H.,* Fan-Yun Meng, M.D.,† Tsung-Han Yang, B.S.,* and Min-Jen Tsao, M.D.† *Department of Radiation Oncology, Zuoying Branch of Kaohsiung Armed Forces General Hospital, Kaohsiung City, Taiwan, ROC; and †Department of General Surgery, Zuoying Branch of Kaohsiung Armed Forces General Hospital, Kaohsiung City, Taiwan, ROC
A R T I C L E I N F O
A B S T R A C T
Article history: Received 26 November 2012 Accepted 18 October 2013
To evaluate and compare the dosimetry of field-in-field (FIF) and wedged beams (WB) techniques for patients with breast cancer receiving adjuvant radiotherapy after conservative surgery. A total of 89 patients with breast cancer participated in this study. Each patient received a computed tomography– based treatment plan with opposed tangential fields. Two planning techniques (FIF and WB) were generated for each patient by using the Pinnacle treatment-planning system. Three indices, the homogeneity index (HI), conformity index (CI), and uniformity index (UI), as well as maximum dose (Dmax), median dose (D50), number of portals, monitor unit (MU), and lung volume at 20 Gy (lung20) were used for comparison. The mean values tested using a t-test indicated that the WB technique had a significantly lower HI (p o 0.0001), a significantly higher CI (p o 0.0001), and a significantly higher D50 (p ¼ 0.0002) than did the FIF technique. The FIF technique had a significantly higher Dmax compared with the WB technique, but lung20 did not exhibit a significant difference. By contrast, the FIF technique had a significantly higher UI and a significantly lower MU compared with the WB technique, but a significantly higher number of portals were found in the FIF technique. The FIF technique did not demonstrate superior dosimetric results. The WB technique had a significantly lower HI, higher CI, lower Dmax, and lower number of portals; but the FIF technique had a significantly higher UI and lower MU. & 2014 American Association of Medical Dosimetrists.
Keywords: Breast cancer Radiotherapy Wedged beams
Introduction According to statistics of the Department of Health, Executive Yuan, ROC, breast cancer has been the most common malignancy among women in Taiwan since 1996, and its incidence rate has increased by 22.2% from 2001 to 2005.1 Postoperative adjuvant radiotherapy (RT) plays an essential role in the management of breast cancer. Breast-conserving surgery followed by adjuvant RT is preferred for T1, T2, and selected T3 tumors.2,3 Threedimensional conformal RT (3-D CRT) with opposed tangential fields is a well-known technique for planning breast cancer RT. Either the field-in-field (FIF) or wedged beams (WB) technique is typically used to conduct treatment plans and achieve suitable target volume coverage as well as to spare normal tissue in the vicinity of the target volume.4,5 The FIF technique, also known as forward intensity-modulated RT, has been evaluated and compared with the WB technique in dosimetry and also in clinical outcomes.4-6 The FIF technique
Reprint requests to: Li-Min Sun, M.D., Department of Radiation Oncology, Zuoying Branch of Kaohsiung Armed Forces General Hospital, 553 Junxiao Rd, Zuoying District, Kaohsiung City, Taiwan, ROC. E-mail: [email protected]
appears to provide an improved dose distribution because it enhances homogeneity and geometric conformity.4,5 However, we performed a preliminary investigation and found the reverse results in dosimetry. In this study, we extended our research by collecting more patient data and performed a comprehensive study by using dosimetric indices.
Methods and Materials Patients Between October 2011 and October 2012, 89 consecutive patients with breast cancer who had received breast-conserving surgery and were referred to our department for postoperative adjuvant RT were evaluated. Female patients who had pathologically proven primary breast carcinoma, a curative intent for RT (no Stage IV disease), and had the ability to raise their arms steadily when immobilized during daily RT were eligible to participate. There was no age limitation.
Immobilization and treatment planning Each patient was immobilized using a customized Aquaplast thermoplastic chest mold for setup and reproducibility during the computed tomography (CT) simulation and treatment process.7 A 3-D CT planning scan with 5-mm slice spacing was performed using a Philips MX8000 AcQSim CT simulator. The radiation oncologist then contoured the target on the planning CT slices according to the guidelines of the International Commission of Radiation Units and Measurements
0958-3947/$ – see front matter Copyright Ó 2014 American Association of Medical Dosimetrists http://dx.doi.org/10.1016/j.meddos.2013.10.002
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L.M. Sun et al. / Medical Dosimetry 39 (2014) 79–82
(ICRU) Reports 50 and 62.8,9 Organs at risk (OARs, including ipsilateral lung, heart, and contralateral breast) were delineated as well. Subsequently, the radiation physicist performed the plan by using the Pinnacle treatment-planning system. The clinical target volume included the ipsilateral breast and possible axillary lymph nodes (LNs). The RT for the internal mammary chain LNs or ipsilateral supraclavicular LNs (SCLNs) or both was reserved for patients who had a higher risk of tumor recurrence in those areas. The planning target volume (PTV) was the clinical target volume with an extension of 0.5- to 1-cm margins. We applied opposed tangential fields on megavoltage linear accelerators using the 3-D CRT technique to cover the breast and possible ipsilateral axillary LNs and internal mammary LNs. The gantry angles were optimized in the beam's eye view to achieve maximum PTV coverage and a minimum lung volume.10 Each patient was planned for FIF and WB techniques. Both the techniques had opposing tangential fields that covered the entire PTV. Several less-weighted fields with a small treatment portal size were selected to optimize dose distributions. For the WB technique, dynamic wedges were used. This technique is a composite method, proportionally using open and 601 WB to generate an intended wedged-angled beam and compensate for variable tissue thickness in the female breast.10-12 The FIF technique is generally regarded as manual-based forward intensity-modulated RT. Through a trial-and-error process, the optimized FIF plans were determined by evaluating the 3-D dose distribution and dose-volume histograms. One or more subfields were merged into the main field, including several multileaf collimator segments for sequential irradiation.13 In addition to the tangential fields, 1 anterior-posterior field with a tilt of 101 to 151 was used to spare the larynx and proximal esophagus. Its purpose was to cover the supraclavicular fossa when the ipsilateral SCLNs were the target. The treatment plans were designed to deliver the prescribed dose to the target volumes considering normal tissue constraints. The prescribed dose for the PTV was 50.4 Gy in 28 fractions with 6 MV x-rays; and for selected patients with high-risk factors,14 we added a surgical scar boost for an additional 10.8 Gy in 6 fractions by using an electron beam. The linear accelerator used was the Elekta Precise Treatment System.
Indices and parameters used in the dosimetric comparison We used the treatment-planning system to measure the selected indices and parameters and then compared the 2 techniques. The homogeneity index (HI) is defined as the ratio of the maximum dose (Dmax) in the PTV to the prescribed dose.15 The conformity index (CI) represents the ratio of volume enclosed by the prescription isodose to the target volume.16 It is used to evaluate dose conformity. The uniformity index (UI) was evaluated according to the percentage of PTV included in the interval 97% to 103% of the prescribed dose,13 and it was used to evaluate the effect of dose improvement. ICRU Reports 50 and 62 recommend reporting the median dose of the PTV (D50),17 and we also evaluated the median dose. A previous study found a statistically significant correlation between the alteration of lung perfusion at 3 years and the lung volume receiving a radiation dose between 10 and 20 Gy,18 and we used lung20 to indicate the percentage of lung volume that received a dose of 20 Gy. Monitor unit (MU) and the number of portals are related to the planning complexity and treatment time. A longer treatment time may be a challenge for some older patients who cannot maintain the treatment position for a long period.
Statistical analysis A paired t-test was used to compare the mean values of the indices and parameters of the 2 techniques. The mean values are presented with 95% CIs. A p o 0.05 from a 2-tailed test was considered statistically significant. All biostatistics were performed using the software Stata 8 (StataCorp LP, College Station, TX).
Table 1 Patient's characteristics Factors
Case number (%)
Age (33 to 78 y, median ¼ 51) r 50 4 50
42 (47%) 47 (53%)
BMI (median ¼ 23.2) o 24 Z 24
48 (54%) 41 (46%)
Lesion side Left Right
50 (56%) 39 (44%)
Stage 0 IA IB IIA IIB IIIA IIIB
26 35 2 8 7 8 3
SCLNs coverage Yes No
13 (15%) 76 (85%)
(29%) (39%) (2%) (9%) (8%) (9%) (3%)
BMI ¼ body mass index.
Results The patient characteristics are listed in Table 1. The age range was 33 to 78 years, and the median age was 51 years. Because of a relatively young population, the median body mass index was 23.2. Left-sided breast cancers were more common. Most patients were in Stage I or earlier (70%) stages. Only 13 (15%) patients had the treatment field extended to cover the SCLNs. Table 2 displays the dosimetric comparison between techniques according to indices and parameters. The mean values are presented with the 95% CI. The FIF technique had a significantly higher Dmax than did the WB technique. A lower HI indicated a more uniform target dose across the target volume.6 The WB technique had a significantly lower HI than the FIF technique did. By contrast, the WB technique had a significantly higher CI than did the FIF technique, and a high CI represented greater dose conformity within the target volume. We usually use UI to evaluate the effect of dose improvement, and a higher UI indicated a larger improvement. The FIF technique had a higher UI, and the difference was statistically significant. The WB technique had a significantly higher D50 than the FIF technique did. Although the OARs for breast cancer treatment planning included the heart, contralateral breast, and lungs, we selected only lung20 to evaluate the dose/volume because it was more clinically relevant than the other organs were.17 Nevertheless, a slightly higher lung20 in the WB technique was not statistically significant. The WB technique
Table 2 Dosimetric comparison between FIF and WB techniques Parameter
Dmax (Gy) Homogeneity index Conformity index Uniformity index D50 (Gy) Lung20 Monitor unit Number of portals
FIF
WB
p-Value
Mean
95% CI
Mean
95% CI
59.88 1.188 0.796 26.33 53.23 15.24 263.3 4.562, range: 3 to 7
59.79 to 59.97 1.186 to 1.190 0.787 to 0.805 26.08 to 26.57 53.17 to 53.28 14.44 to 16.03 246.3 to 280.2 4.391 to 4.733
59.60 1.182 0.819 24.43 53.36 15.30 439.0 3.798, range: 3 to 5
59.50 to 59.71 1.180 to 1.185 0.809 to 0.828 23.98 to 24.88 53.31 to 53.42 14.45 to 16.16 422.4 to 455.6 3.670 to 3.925
o o o o
0.0001 0.0001 0.0001 0.0001 0.0002 0.5836 o 0.0001 o 0.0001
L.M. Sun et al. / Medical Dosimetry 39 (2014) 79–82
Fig. 1. The dose-volume histogram of a patient with right breast cancer. It illustrates the PTV (red) and right lung (blue) receiving absolute doses by the corresponding volumes. The solid lines represent the WB plan and the dash lines are the FIF plan. (Color version of figure is available online.)
had a significantly higher mean MU but a lower mean number of portals than the FIF technique did. Figure 1 demonstrates the dose-volume histogram of a patient with right breast cancer. It illustrates the absolute doses of the PTV (red) and right lung (blue) according to the corresponding volumes. The solid lines represent the WB plan and the dashed lines represent the FIF plan. The WB plan had a relatively higher PTV and lung dose than did the FIF plan. Figure 2 demonstrates the isodose distribution of the 2 techniques at the field margin of a patient with right breast cancer. The areas covered by the 105% and 110% prescribed dose (52.92 and 55.44 Gy, respectively) were larger for the WB plan than they were for the FIF plan.
81
Because of breast contour irregularities and underlying normal organs (the lung and heart), improving the dose distribution for breast cancer RT is challenging.12,19 To spare the underlying lung tissue, a tangential parallel-opposed pair technique is typically used for whole-breast RT. A 3-D analysis of this technique demonstrated that there could be large dose inhomogeneity inside the target volume.20,21 The introduction of CT scanning and the availability of sophisticated treatment-planning methods have improved the delivery efficiency of radiation to the breast.22 3-D CRT techniques that improve dose distribution have been proposed. The WB method usually compensates for breast contour variation.10,23 The FIF technique involves using conventional uniform intensity fields to deliver most of the dose to breast tissue, and then the FIF technique is used to minimize hot spots in 3 dimensions. Several studies have found that using the FIF technique can improve dose homogeneity as well as PTV conformity.5,11,13,24 Onal et al.5 used the HI, PTV dose improvement index, and dose to the OARs to conduct a dosimetric comparison between the FIF and WB techniques. They suggested that the FIF method provided significantly improved dose homogeneity in the PTV and a trend of delivering a lower dose to nearby healthy tissue. This study followed a similar approach to evaluate and compare the 2 techniques dosimetrically. However, our results indicated several differences compared with earlier studies. The WB technique demonstrated a significantly lower Dmax, lower HI, higher CI, and higher D50. D50 is an index recommended in dosevolume reporting by ICRU Reports 50 and 62. It is the median dose of the PTV and also represents the “typical dose” of the PTV.17 Bratengeier et al.25 advocated reporting the PTV median dose when using 3-D CRT. The MU was significantly higher when using the WB technique, which is consistent with the results of previous studies. A significantly higher UI, but no difference in lung20, was noted when using the FIF technique, which is partially in agreement with the results of previous studies. The WB technique was suggested to have a higher dose at the superior and inferior aspects near the wedge filters.5 The results of this study supported this finding (Fig. 2).
Discussion This study demonstrated that, in general, the WB technique had a better dosimetric outcome concerning Dmax, HI, and CI than the FIF technique did. However, the lower UI and higher MU were substantial drawbacks.
Conclusions Other than the parameters of MU and number of portals, which demonstrated that the results were consistent with previous
Fig. 2. The isodose curves distribution of the 2 techniques on the field margin of a patient with right breast cancer. The color wash area represents PTV: (A) FIF plan and (B) WB plan. (Color version of figure is available online.)
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studies, our results did not support previous studies that indicated improved dosimetry outcomes when using the FIF technique. By contrast, our findings suggest that the WB technique may offer improved results concerning Dmax, the HI, and the CI. Further large-scale comprehensive research is required to provide a definitive conclusion.
Acknowledgments This work was supported by the Zuoying Branch of Kaohsiung Armed Forces General Hospital grant ZAFGH-101-20. References 1. Cancer Statistics Annual Report. Taiwan Cancer Registry. Available at: http:// www.tcr.cph.ntu.edu.tw/main.php?Page=N2. Accessed November 11, 2012. 2. Early Breast Cancer Trialists' Collaborative Group. Effects of radiotherapy and surgery in early breast cancer: An overview of the randomized trials. N. Engl. J. Med. 333:1444–55; 1995. 3. Rapiti, E.; Fioretta, G.; Vlastos, G.; et al. Breast-conserving surgery has equivalent effect as mastectomy on stage I breast cancer prognosis only when followed by radiotherapy. Radiother. Oncol. 69:277–84; 2003. 4. Prabhakar, R.; Julka, P.K.; Rath, G.K. Can field-in-field technique replace wedge filter in radiotherapy treatment planning: A comparative analysis in various treatment sites. Australas. Phys. Eng. Sci. Med. 31:317–24; 2008. 5. Onal, C.; Sonmez, A.; Arslan, G.; et al. Dosimetric comparison of the field-infield technique and tangential wedged beams for breast irradiation. Jpn. J. Radiol. 30:218–26; 2012. 6. Tu, C.P.; Chuang, H.D.; Chang, N.C.; et al. Clinical outcomes of the field-in-field technique for breast cancer patients with breast conservative therapy. Ther. Radiol. Oncol. 16:63–7; 2009. 7. Huang, K.M.; Hsu, C.H.; Jeng, S.C.; et al. The application of Aquaplast Thermoplastic as a bolus material in the radiotherapy of a patient with classic Kaposi's sarcoma at the lower extremity. Anticancer Res. 26:759–62; 2006. 8. International Commission on Radiation Units and Measurements. ICRU Report 50: Prescribing, Recording, and Reporting Photon Beam Therapy.Bethesda, MD, 1993. 9. International Commission on Radiation Units and Measurements. ICRU Report 62: Prescribing, Recording, and Reporting Photon Beam Therapy (supplement to ICRU Report 50). Bethesda, MD, 1999.
10. Ludwig, V.; Schwab, F.; Guckenberger, M.; et al. Comparison of wedge versus segmented techniques in whole breast irradiation: Effects on dose exposure outside the treatment volume. Strahlenther. Onkol. 184:307–12; 2008. 11. Pili, G.; Grimaldi, L.; Fidanza, C.; et al. Geometric and dosimetric approach to determine probability of late cardiac mortality in left tangential breast irradiation: Comparison between wedged beams and field-in-field technique. Int. J. Radiat. Oncol. Biol. Phys. 81:894–900; 2011. 12. Venables, K.; Miles, E.A.; Aird, E.G.; et al. What is the optimum breast plan: A study based on the START trial plans. Br. J. Radiol. 79:734–9; 2006. 13. Lee, J.W.; Hong, S.; Choi, K.S.; et al. Performance evaluation of field-in-field technique for tangential breast irradiation. Jpn. J. Clin. Oncol. 38:158–63; 2008. 14. Graham, P.; Fourquet, A. Placing the boost in breast-conservation radiotherapy: A review of the role, indications and techniques for breast-boost radiotherapy. Clin. Oncol. (R. Coll. Radiol.) 18:210–9; 2006 ([Review]). 15. Yoon, M.; Shin, D.; Park, J.; et al. A new evaluation method of target volume coverage and homogeneity for IMRT treatment planning. Phys. Med. 22:43–51; 2006. 16. Dhabaan, A.; Elder, E.; Schreibmann, E.; et al. Dosimetric performance of the new high-definition multileaf collimator for intracranial stereotactic radiosurgery. J. Appl. Clin. Med. Phys. 11:3040; 2010. 17. Mackie, T.R.; Gregoire, V. ICRU recommendations. Available at: http://www. aapm.org/meetings/2011SS/documents/MackieUncertainty.pdf. Accessed May 18, 2013. 18. Jaen, J.; Vazquez, G.; Alonso, E.; et al. Changes in pulmonary function after incidental lung irradiation for breast cancer: A prospective study. Int. J. Radiat. Oncol. Biol. Phys. 65:1381–8; 2006. 19. McCloskey, S.A.; Lee, S.P.; Steinberg, M.L. Roles and types of radiation in breast cancer treatment: Early breast cancer, locoregionally advanced, and metastatic disease. Curr. Opin. Obstet. Gynecol. 23:51–7; 2011. 20. Aref, A.; Thornton, D.; Youssef, E.; et al. Dosimetric improvements following 3D planning of tangential breast irradiation. Int. J. Radiat. Oncol. Biol. Phys. 48:1569–74; 2000. 21. Buchholz, T.A.; Gurogze, E.; Bice, W.S. Dosimetric analysis of intact breast irradiation in off-axis planes. Int. J. Radiat. Oncol. Biol. Phys. 39:361–7; 1997. 22. Prabhakar, R.; Rath, G.K.; Julka, P.K.; et al. Breast dose heterogeneity in CT-based radiotherapy treatment planning. J. Med. Phys. 33:43–8; 2008. 23. Kutcher, G.J.; Smith, A.R.; Fowble, B.L.; et al. Treatment planning for primary breast cancer: A patterns of care study. Int. J. Radiat. Oncol. Biol. Phys. 36:731–7; 1996. 24. Donovan, E.M.; Johnson, U.; Shentall, G.; et al. Evaluation of compensation in breast radiotherapy: A planning study using multiple static field. Int. J. Radiat. Oncol. Biol. Phys. 46:671–9; 2000. 25. Bratengeier, K.; Oechsner, M.; Gainey, M.; et al. Remarks on reporting and recording consistent with the ICRU reference dose. Radiat. Oncol. 4:44; 2009.
Medical Dosimetry journal homepage: www.meddos.org
Field-in-field plan does not improve the dosimetric outcome compared with the wedged beams plan for breast cancer radiotherapy Li-Min Sun, M.D., M.P.H.,* Fan-Yun Meng, M.D.,† Tsung-Han Yang, B.S.,* and Min-Jen Tsao, M.D.† *Department of Radiation Oncology, Zuoying Branch of Kaohsiung Armed Forces General Hospital, Kaohsiung City, Taiwan, ROC; and †Department of General Surgery, Zuoying Branch of Kaohsiung Armed Forces General Hospital, Kaohsiung City, Taiwan, ROC
A R T I C L E I N F O
A B S T R A C T
Article history: Received 26 November 2012 Accepted 18 October 2013
To evaluate and compare the dosimetry of field-in-field (FIF) and wedged beams (WB) techniques for patients with breast cancer receiving adjuvant radiotherapy after conservative surgery. A total of 89 patients with breast cancer participated in this study. Each patient received a computed tomography– based treatment plan with opposed tangential fields. Two planning techniques (FIF and WB) were generated for each patient by using the Pinnacle treatment-planning system. Three indices, the homogeneity index (HI), conformity index (CI), and uniformity index (UI), as well as maximum dose (Dmax), median dose (D50), number of portals, monitor unit (MU), and lung volume at 20 Gy (lung20) were used for comparison. The mean values tested using a t-test indicated that the WB technique had a significantly lower HI (p o 0.0001), a significantly higher CI (p o 0.0001), and a significantly higher D50 (p ¼ 0.0002) than did the FIF technique. The FIF technique had a significantly higher Dmax compared with the WB technique, but lung20 did not exhibit a significant difference. By contrast, the FIF technique had a significantly higher UI and a significantly lower MU compared with the WB technique, but a significantly higher number of portals were found in the FIF technique. The FIF technique did not demonstrate superior dosimetric results. The WB technique had a significantly lower HI, higher CI, lower Dmax, and lower number of portals; but the FIF technique had a significantly higher UI and lower MU. & 2014 American Association of Medical Dosimetrists.
Keywords: Breast cancer Radiotherapy Wedged beams
Introduction According to statistics of the Department of Health, Executive Yuan, ROC, breast cancer has been the most common malignancy among women in Taiwan since 1996, and its incidence rate has increased by 22.2% from 2001 to 2005.1 Postoperative adjuvant radiotherapy (RT) plays an essential role in the management of breast cancer. Breast-conserving surgery followed by adjuvant RT is preferred for T1, T2, and selected T3 tumors.2,3 Threedimensional conformal RT (3-D CRT) with opposed tangential fields is a well-known technique for planning breast cancer RT. Either the field-in-field (FIF) or wedged beams (WB) technique is typically used to conduct treatment plans and achieve suitable target volume coverage as well as to spare normal tissue in the vicinity of the target volume.4,5 The FIF technique, also known as forward intensity-modulated RT, has been evaluated and compared with the WB technique in dosimetry and also in clinical outcomes.4-6 The FIF technique
Reprint requests to: Li-Min Sun, M.D., Department of Radiation Oncology, Zuoying Branch of Kaohsiung Armed Forces General Hospital, 553 Junxiao Rd, Zuoying District, Kaohsiung City, Taiwan, ROC. E-mail: [email protected]
appears to provide an improved dose distribution because it enhances homogeneity and geometric conformity.4,5 However, we performed a preliminary investigation and found the reverse results in dosimetry. In this study, we extended our research by collecting more patient data and performed a comprehensive study by using dosimetric indices.
Methods and Materials Patients Between October 2011 and October 2012, 89 consecutive patients with breast cancer who had received breast-conserving surgery and were referred to our department for postoperative adjuvant RT were evaluated. Female patients who had pathologically proven primary breast carcinoma, a curative intent for RT (no Stage IV disease), and had the ability to raise their arms steadily when immobilized during daily RT were eligible to participate. There was no age limitation.
Immobilization and treatment planning Each patient was immobilized using a customized Aquaplast thermoplastic chest mold for setup and reproducibility during the computed tomography (CT) simulation and treatment process.7 A 3-D CT planning scan with 5-mm slice spacing was performed using a Philips MX8000 AcQSim CT simulator. The radiation oncologist then contoured the target on the planning CT slices according to the guidelines of the International Commission of Radiation Units and Measurements
0958-3947/$ – see front matter Copyright Ó 2014 American Association of Medical Dosimetrists http://dx.doi.org/10.1016/j.meddos.2013.10.002
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L.M. Sun et al. / Medical Dosimetry 39 (2014) 79–82
(ICRU) Reports 50 and 62.8,9 Organs at risk (OARs, including ipsilateral lung, heart, and contralateral breast) were delineated as well. Subsequently, the radiation physicist performed the plan by using the Pinnacle treatment-planning system. The clinical target volume included the ipsilateral breast and possible axillary lymph nodes (LNs). The RT for the internal mammary chain LNs or ipsilateral supraclavicular LNs (SCLNs) or both was reserved for patients who had a higher risk of tumor recurrence in those areas. The planning target volume (PTV) was the clinical target volume with an extension of 0.5- to 1-cm margins. We applied opposed tangential fields on megavoltage linear accelerators using the 3-D CRT technique to cover the breast and possible ipsilateral axillary LNs and internal mammary LNs. The gantry angles were optimized in the beam's eye view to achieve maximum PTV coverage and a minimum lung volume.10 Each patient was planned for FIF and WB techniques. Both the techniques had opposing tangential fields that covered the entire PTV. Several less-weighted fields with a small treatment portal size were selected to optimize dose distributions. For the WB technique, dynamic wedges were used. This technique is a composite method, proportionally using open and 601 WB to generate an intended wedged-angled beam and compensate for variable tissue thickness in the female breast.10-12 The FIF technique is generally regarded as manual-based forward intensity-modulated RT. Through a trial-and-error process, the optimized FIF plans were determined by evaluating the 3-D dose distribution and dose-volume histograms. One or more subfields were merged into the main field, including several multileaf collimator segments for sequential irradiation.13 In addition to the tangential fields, 1 anterior-posterior field with a tilt of 101 to 151 was used to spare the larynx and proximal esophagus. Its purpose was to cover the supraclavicular fossa when the ipsilateral SCLNs were the target. The treatment plans were designed to deliver the prescribed dose to the target volumes considering normal tissue constraints. The prescribed dose for the PTV was 50.4 Gy in 28 fractions with 6 MV x-rays; and for selected patients with high-risk factors,14 we added a surgical scar boost for an additional 10.8 Gy in 6 fractions by using an electron beam. The linear accelerator used was the Elekta Precise Treatment System.
Indices and parameters used in the dosimetric comparison We used the treatment-planning system to measure the selected indices and parameters and then compared the 2 techniques. The homogeneity index (HI) is defined as the ratio of the maximum dose (Dmax) in the PTV to the prescribed dose.15 The conformity index (CI) represents the ratio of volume enclosed by the prescription isodose to the target volume.16 It is used to evaluate dose conformity. The uniformity index (UI) was evaluated according to the percentage of PTV included in the interval 97% to 103% of the prescribed dose,13 and it was used to evaluate the effect of dose improvement. ICRU Reports 50 and 62 recommend reporting the median dose of the PTV (D50),17 and we also evaluated the median dose. A previous study found a statistically significant correlation between the alteration of lung perfusion at 3 years and the lung volume receiving a radiation dose between 10 and 20 Gy,18 and we used lung20 to indicate the percentage of lung volume that received a dose of 20 Gy. Monitor unit (MU) and the number of portals are related to the planning complexity and treatment time. A longer treatment time may be a challenge for some older patients who cannot maintain the treatment position for a long period.
Statistical analysis A paired t-test was used to compare the mean values of the indices and parameters of the 2 techniques. The mean values are presented with 95% CIs. A p o 0.05 from a 2-tailed test was considered statistically significant. All biostatistics were performed using the software Stata 8 (StataCorp LP, College Station, TX).
Table 1 Patient's characteristics Factors
Case number (%)
Age (33 to 78 y, median ¼ 51) r 50 4 50
42 (47%) 47 (53%)
BMI (median ¼ 23.2) o 24 Z 24
48 (54%) 41 (46%)
Lesion side Left Right
50 (56%) 39 (44%)
Stage 0 IA IB IIA IIB IIIA IIIB
26 35 2 8 7 8 3
SCLNs coverage Yes No
13 (15%) 76 (85%)
(29%) (39%) (2%) (9%) (8%) (9%) (3%)
BMI ¼ body mass index.
Results The patient characteristics are listed in Table 1. The age range was 33 to 78 years, and the median age was 51 years. Because of a relatively young population, the median body mass index was 23.2. Left-sided breast cancers were more common. Most patients were in Stage I or earlier (70%) stages. Only 13 (15%) patients had the treatment field extended to cover the SCLNs. Table 2 displays the dosimetric comparison between techniques according to indices and parameters. The mean values are presented with the 95% CI. The FIF technique had a significantly higher Dmax than did the WB technique. A lower HI indicated a more uniform target dose across the target volume.6 The WB technique had a significantly lower HI than the FIF technique did. By contrast, the WB technique had a significantly higher CI than did the FIF technique, and a high CI represented greater dose conformity within the target volume. We usually use UI to evaluate the effect of dose improvement, and a higher UI indicated a larger improvement. The FIF technique had a higher UI, and the difference was statistically significant. The WB technique had a significantly higher D50 than the FIF technique did. Although the OARs for breast cancer treatment planning included the heart, contralateral breast, and lungs, we selected only lung20 to evaluate the dose/volume because it was more clinically relevant than the other organs were.17 Nevertheless, a slightly higher lung20 in the WB technique was not statistically significant. The WB technique
Table 2 Dosimetric comparison between FIF and WB techniques Parameter
Dmax (Gy) Homogeneity index Conformity index Uniformity index D50 (Gy) Lung20 Monitor unit Number of portals
FIF
WB
p-Value
Mean
95% CI
Mean
95% CI
59.88 1.188 0.796 26.33 53.23 15.24 263.3 4.562, range: 3 to 7
59.79 to 59.97 1.186 to 1.190 0.787 to 0.805 26.08 to 26.57 53.17 to 53.28 14.44 to 16.03 246.3 to 280.2 4.391 to 4.733
59.60 1.182 0.819 24.43 53.36 15.30 439.0 3.798, range: 3 to 5
59.50 to 59.71 1.180 to 1.185 0.809 to 0.828 23.98 to 24.88 53.31 to 53.42 14.45 to 16.16 422.4 to 455.6 3.670 to 3.925
o o o o
0.0001 0.0001 0.0001 0.0001 0.0002 0.5836 o 0.0001 o 0.0001
L.M. Sun et al. / Medical Dosimetry 39 (2014) 79–82
Fig. 1. The dose-volume histogram of a patient with right breast cancer. It illustrates the PTV (red) and right lung (blue) receiving absolute doses by the corresponding volumes. The solid lines represent the WB plan and the dash lines are the FIF plan. (Color version of figure is available online.)
had a significantly higher mean MU but a lower mean number of portals than the FIF technique did. Figure 1 demonstrates the dose-volume histogram of a patient with right breast cancer. It illustrates the absolute doses of the PTV (red) and right lung (blue) according to the corresponding volumes. The solid lines represent the WB plan and the dashed lines represent the FIF plan. The WB plan had a relatively higher PTV and lung dose than did the FIF plan. Figure 2 demonstrates the isodose distribution of the 2 techniques at the field margin of a patient with right breast cancer. The areas covered by the 105% and 110% prescribed dose (52.92 and 55.44 Gy, respectively) were larger for the WB plan than they were for the FIF plan.
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Because of breast contour irregularities and underlying normal organs (the lung and heart), improving the dose distribution for breast cancer RT is challenging.12,19 To spare the underlying lung tissue, a tangential parallel-opposed pair technique is typically used for whole-breast RT. A 3-D analysis of this technique demonstrated that there could be large dose inhomogeneity inside the target volume.20,21 The introduction of CT scanning and the availability of sophisticated treatment-planning methods have improved the delivery efficiency of radiation to the breast.22 3-D CRT techniques that improve dose distribution have been proposed. The WB method usually compensates for breast contour variation.10,23 The FIF technique involves using conventional uniform intensity fields to deliver most of the dose to breast tissue, and then the FIF technique is used to minimize hot spots in 3 dimensions. Several studies have found that using the FIF technique can improve dose homogeneity as well as PTV conformity.5,11,13,24 Onal et al.5 used the HI, PTV dose improvement index, and dose to the OARs to conduct a dosimetric comparison between the FIF and WB techniques. They suggested that the FIF method provided significantly improved dose homogeneity in the PTV and a trend of delivering a lower dose to nearby healthy tissue. This study followed a similar approach to evaluate and compare the 2 techniques dosimetrically. However, our results indicated several differences compared with earlier studies. The WB technique demonstrated a significantly lower Dmax, lower HI, higher CI, and higher D50. D50 is an index recommended in dosevolume reporting by ICRU Reports 50 and 62. It is the median dose of the PTV and also represents the “typical dose” of the PTV.17 Bratengeier et al.25 advocated reporting the PTV median dose when using 3-D CRT. The MU was significantly higher when using the WB technique, which is consistent with the results of previous studies. A significantly higher UI, but no difference in lung20, was noted when using the FIF technique, which is partially in agreement with the results of previous studies. The WB technique was suggested to have a higher dose at the superior and inferior aspects near the wedge filters.5 The results of this study supported this finding (Fig. 2).
Discussion This study demonstrated that, in general, the WB technique had a better dosimetric outcome concerning Dmax, HI, and CI than the FIF technique did. However, the lower UI and higher MU were substantial drawbacks.
Conclusions Other than the parameters of MU and number of portals, which demonstrated that the results were consistent with previous
Fig. 2. The isodose curves distribution of the 2 techniques on the field margin of a patient with right breast cancer. The color wash area represents PTV: (A) FIF plan and (B) WB plan. (Color version of figure is available online.)
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studies, our results did not support previous studies that indicated improved dosimetry outcomes when using the FIF technique. By contrast, our findings suggest that the WB technique may offer improved results concerning Dmax, the HI, and the CI. Further large-scale comprehensive research is required to provide a definitive conclusion.
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