Comparison study of the partial-breast irradiation techniques: Dosimetric analysis of three-dimensional conformal radiation therapy, electron beam therapy, and helical tomotherapy depending on various tumor locations

Medical Dosimetry 38 (2013) 327–331

Medical Dosimetry journal homepage: www.meddos.org

Comparison study of the partial-breast irradiation techniques: Dosimetric analysis of three-dimensional conformal radiation therapy, electron beam therapy, and helical tomotherapy depending on various tumor locations Min-Joo Kim, M.S.,ny So-Hyun Park, M.S.,ny Seok-Hyun Son, M.D.,z Keum-Seong Cheon, B.S.,z Byung-Ock Choi, M.D.,z and Tae-Suk Suh, Ph.D.ny *Department of Biomedical Engineering, The Catholic University of Korea, Seoul, Republic of Korea yResearch Institute of Biomedical Engineering, The Catholic University of Korea, Seoul, Republic of Korea and zDepartment of Radiation Oncology, Seoul St. Mary’s Hospital, The Catholic University of Korea, Seoul, Republic of Korea

A R T I C L E I N F O

A B S T R A C T

Article history: Received 14 December 2011 Accepted 8 March 2013

The partial-breast irradiation (PBI) technique, an alternative to whole-breast irradiation, is a beam delivery method that uses a limited range of treatment volume. The present study was designed to determine the optimal PBI treatment modalities for 8 different tumor locations. Treatment planning was performed on computed tomography (CT) data sets of 6 patients who had received lumpectomy treatments. Tumor locations were classified into 8 subsections according to breast quadrant and depth. Three-dimensional conformal radiation therapy (3D-CRT), electron beam therapy (ET), and helical tomotherapy (H-TOMO) were utilized to evaluate the dosimetric effect for each tumor location. Conformation number (CN), radical dose homogeneity index (rDHI), and dose delivered to healthy tissue were estimated. The Kruskal-Wallis, Mann-Whitney U, and Bonferroni tests were used for statistical analysis. The ET approach showed good sparing effects and acceptable target coverage for the lower inner quadrant—superficial (LIQ-S) and lower inner quadrant—deep (LIQ-D) locations. The HTOMO method was the least effective technique as no evaluation index achieved superiority for all tumor locations except CN. The ET method is advisable for treating LIQ-S and LIQ-D tumors, as opposed to 3D-CRT or H-TOMO, because of acceptable target coverage and much lower dose applied to surrounding tissue. & 2013 American Association of Medical Dosimetrists.

Keywords: Partial-breast irradiation Tumor location 3D conformal radiation therapy Electron beam therapy

Introduction Breast cancer is the leading cause of cancer death in women worldwide.1 Over the past 20 years, the number of breast cancer patients diagnosed at an early stage [I and II] has increased, and the prognoses of early-stage patients have improved with a 25% reduction in mortality.2 The standard treatment for patients with early-stage breast cancer patient is postlumpectomy radiation therapy, which is often performed by the whole-breast radiation therapy (WBRT). Alternatively, partial-breast irradiation (PBI), which involves beam delivery using a limited range of treatment volume, is a newer treatment approach that should be compared with WBRT. Because of the decreased treatment volumes used with PBI, the radiation dose can be safely delivered to the target

Reprint requests to: Tae-Suk Suh, Ph.D., Department of Biomedical Engineering, The Catholic University of Korea, 505 Banpo-dong, Seocho-gu, Seoul 137-701, Republic of Korea. E-mails: [email protected], [email protected]

tissue without unwanted irradiation of the normal adjacent breast tissue and surrounding organs (such as the heart and lungs). These results in a better cosmetic outcome for the patient, minimizes the damage to healthy tissue, and allows for accelerated dose delivery thereby reducing treatment duration.3 Several methods using PBI to treat early-stage breast cancer patient have been studied such as intracavitary brachytherapy, interstitial brachytherapy, and external beam radiation therapy (EBRT). Brachytherapy, regardless of whether it is intracavitary or interstitial, has a lower level of coverage of the planning target volume (PTV) than EBRT. In addition, EBRT has potential advantages over brachytherapy that include being noninvasive, being less operator dependent, and having acceptable cosmetic outcome. Several recent studies have thoroughly investigated the use of the EBRT with PBI using different methods of beam delivery.4–8 Most of these studies indicated that PBI with variable EBRT modalities provides acceptable coverage of the PTV as well as good conformity, particularly with helical tomotherapy (H-TOMO) and intensity-modulated arc therapy.

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Treatment results of radiation therapy are known to vary depending on tumor location, which affects the delivered dose to the organ at risk (OAR). In particular, because tumors of the left breast may be located close to the heart and the inner surface of the skin, the dosimetric effects on these OARs may be influenced by tumor location. The primary objective of this study was to determine the most suitable PBI therapy modality among three-dimensional conformal radiation therapy (3D-CRT), electron beam therapy (ET), and H-TOMO for tumors at different locations. We examined 8 groupings based on breast quadrant and depth of the tumor. The quadrant is a standard description for classifying breast tumors; and tumor depth, which is measured from the skin, is used to compare superficial and deep-seated tumors. Conformation number (CN), the radical dose homogeneity index (rDHI), and the dose delivered to normal tissues were reported to evaluate the treatment planning results.

Methods and Materials Planning computed tomography (CT) scans and tumor locations Treatment planning was performed on the CT data sets of 6 patients who had received lumpectomy treatments for early-stage T1 N0 left breast cancer, with tumor size o 2 cm and no observable regional lymph node metastasis. The CT (Somatom Sensation 64, Siemens Medical Solutions, Forchheim, Germany) data set was acquired with 3-mm slice intervals from the lungs to the mandible while patients were in the treatment position (i.e., supine with both arms above the head). A key step in this study was to group the tumor into 1 of 8 sections according to the quadrant of the breast and depth level (superficial or deep). Superficial and deep locations were classified based on the distance from the skin surface to the gross tumor volume (GTV) and the proximity of the GTV to the chest wall. Figure 1 shows a diagram of the specified nomenclature for each of the regions of the left breast. The CT data set of a patient was used as a reference to estimate the general effects on the 8 tumor locations for the 3D-CRT, ET, and H-TOMO treatments. We used only 1 planning result as a reference so that we could determine the effects of each modality on the target coverage of each classified tumor and OAR using the same CT data set with constant anatomical structure. Furthermore, extra planning for 3D-CRT and ET was performed using the CT data sets of 5 patients to confirm the results from the reference CT data set.

Treatment planning All contouring in this study was performed with reference to the National Surgical Adjuvant Breast and Bowel Project (NSABP) B-39/Radiation Therapy Oncology Group (RTOG) 0413 guidance9 and defined in the Pinnacle treatment planning system. The following structures were outlined for evaluation of the prescribed dose coverage: GTV, clinical target volume (CTV), PTV, and PTV for evaluation (PTV_eval). The PTV contours were outlined with a 5 mm expansion of the CTV to prevent overlap of the PTV regions between each tumor location, even though the NSABP B-39/RTOG 0413 recommends 10 mm. The copied contouring of the PTV was modified to create PTV_eval, which included the PTV but excluded

Fig. 1. A simple diagram showing the names and locations of the 8 classified sections in the left breast. UIQ-S ¼ upper inner quadrant—superficial; UIQ-D ¼ upper inner quadrant—deep; UOQ-S ¼ upper outer quadrant—superficial; UOQ-D ¼ upper outer quadrant—deep; LOQ-D ¼ lower outer quadrant—deep.

the first 5 mm below the skin surface, outside of the breast, chest wall, and pectoralis muscles. The ipsilateral breast, ipsilateral lung, contralateral breast, contralateral lung, and heart were contoured for the evaluation of OAR (Fig. 2). Prescription and constraints In accordance with NSABP B-39/RTOG 0413 guidelines, treatment planning for all the techniques was performed with as close to equivalent dosimetry as possible. The prescribed dose for 3D-CRT, ET, and H-TOMO was 50 Gy divided into 25 daily doses for safe coverage of the PTV. The comparison of the planning results between the 3 modalities was therefore based on equivalent dosimetry, but it should be noted that NSABP B-39/RTOG 0413 recommends 38.5 Gy for accelerated PBI. The dose distributions in all the plans were required to fulfill the International Commission on Radiation Units and Measurements specifications that include 100% coverage of the CTV with Z 95% of the prescribed dose and that the volume inside the CTV receiving 4 107% of the prescribed dose should be minimized. 3D-CRT For all 3D-CRT cases, treatment planning was performed using the Pinnacle planning system with 4 noncoplanar 6-MV beams applied with the isocenter in the center of the PTV. The beam was autoblocked to cover the PTV with either 0.3cm or 0.5-cm penumbra margins depending on the planner, anatomical structures, and proximity to the skin surface. No beams were directed to the surrounding healthy organs (e.g., the heart and lungs) or the contralateral breast. Beam modifiers were used and manually manipulated to maximize CTV coverage and dose conformity. Angles of gantry, couch, wedge, and weighting were manually selected by the planner. ET For all ET cases, treatment planning was performed with the Pinnacle planning system, using a single-portal electron beam. Depending on the depth of the tumor, an electron beam energy of either 6 or 10 MeV was used to comply with the International Commission on Radiation Units and Measurements dosimetric conditions discussed before. H-TOMO The tomotherapy planning station Hi-ART system (version4.0.4, TomoTherapy Incorporated, Madison, WI, USA) was used for H-TOMO treatment planning. The main parameters of H-TOMO treatment planning were field width, pitch, modulation factor, and dose calculation grid size, and these factors could affect the treatment planning results such as dose conformity and treatment time. The field width and the pitch are similar to those settings in conventional CT systems. In this study, the field width and pitch were 2.5 cm and 0.25, respectively. The modulation factor which was 2.5 in this study, defined as the ratio of the maximum beam intensity to the average beam intensity, was determined to measure the duration of the longest leaf opening and the average leaf opening.4,10 Dose calculation grid size was set to ‘‘normal.’’ No bolus was applied in these plans. H-TOMO treatment planning always meets the prescribed condition because of constraint for target dose-volume prescription. For 95% of PTV, 50 Gy in 25 fractions was prescribed. The heart volume receiving 15 Gy was limited to less than 10%. The delivered dose to the left lung was limited to 12 Gy. The normal organ such as the right breast and lung was limited to receive o 5 Gy.

Fig. 2. Contouring guideline. (Color version of figure is available online.)

M.-J. Kim et al. / Medical Dosimetry 38 (2013) 327–331 Evaluation For quantitative evaluation, the planning results were analyzed using a dosevolume histogram (DVH). Doses for 99% (D99%) and 1% (D1%) of the volume displayed on the cumulative DVH and the volume that received 95% of the prescribed dose (V95%) were calculated for the CTV. D99% and D1% are considered the maximum and minimum doses for each targeted volume for calculating rDHI. For the evaluation of OARs, the appropriate Vx and Dy values were acquired for the left breast, the left lung, the right breast, the right lung, and the heart.7 When examining the planning results of the left breast, the volume of the PTV_eval was excluded to achieve an accurate assessment of the normal tissue. The CN, proposed by Van’t,17 is defined as TVRI TVRI  CN ¼ TV VRI

Table 2 Dosimetric characteristics for left breast and left lung as a function of tumor locations UIQ-S UOQ-S LIQ-S LOQ-S UIQ-D UOQ-D LIQ-D LOQ-D Left breast V45 Gy (%) 3D-CRT 7.7 ET 10.6 H-TOMO 16.5

1.0 9.9 15.5

10.0 9.8 16.0

16.2 6.0 13.1

21.3 18.7 16.8

9.1 13.3 17.1

5.5 4.4 10.6

6.3 39.0 14.9

V50 Gy (%) 3D-CRT 1.5 ET 1.9 H-TOMO 12.1

0.0 3.7 11.6

3.2 0.3 10.0

11.2 0.4 9.3

5.2 12.0 12.2

4.1 10.1 13.6

0.0 11.4 7.9

0.0 22.0 10.1

Left lung V5 Gy (%) 3D-CRT 0.0 ET 15.9 H-TOMO 48.3

2.1 11.6 49.7

9.3 7.9 46.5

26.2 12.0 26.2

13.2 26.6 67.6

9.5 58.8 56.6

16.2 8.8 24.5

36.2 23.8 46.1

2.8 0.1

0.0 2.7 2.1

0.0 3.5 0.0

2.3 9.5 7.8

0.0 9.5 5.7

1.2 7.5 4.1

3.1 9.8 6.5

ð1Þ

In this equation, TVRI is the target volume covered by the reference dose, TV is the target volume, and VRI is the volume of the reference isodose. In our study, the rDHI, defined by rDHI ¼

Dmin ðwithin PTV  evalÞ Dmax ðwithin PTV  evalÞ

ð2Þ

was used to assess dose homogeneity in the PTV.5,11,12 Either the nonparametric Kruskal-Wallis test or the Mann-Whitney U test was used for statistical analysis of the 3 treatment modalities (3D-CRT, ET, and H-TOMO) for each evaluation index, with significance set at an adjusted possibility value (p-value) of r 0.05. In addition, a posteriori Bonferroni test was used with an adjusted p-value of r 0.0167. All statistical analyses were performed using SPSS (release 17.0.0, SPSS Inc., Chicago, IL, USA).

Results The results for CTV coverage, CN, and rDHI for the various tumor locations with the 3 treatment modalities (3D-CRT, ET, and H-TOMO) are summarized in Table 1. The V95% of the CTV showed acceptable coverage in all plans, and no specific dosimetric differences were observed for the CTV coverage based on tumor location and treatment modality. The CN for H-TOMO was much higher than that for 3D-CRT and ET at all tumor locations. However, the rDHI values were higher for 3D-CRT and ET compared with H-TOMO. Table 2 outlines the dosimetric characteristics for the left breast and left lung based on DVH analysis. The ET method had much lower delivered doses to OARs including the right breast, right lung, and heart for lower inner quadrant—superficial (LIQ-S) and lower inner quadrant—deep (LIQ-D) sections, while still achieving acceptable target coverage. The low dose volume (percentage of volume receiving Z 5 Gy) to the left lung was greater for H-TOMO in all locations except upper outer quadrant—deep section. The irradiated volume for the low dose to the right breast, right lung, and heart was highest with

329

V15 Gy (%) 3D-CRT ET H-TOMO

0.0 8.7 0.1

UIQ-S ¼ upper inner quadrant—superficial; UOQ-S ¼ upper outer quadrant—superficial; LOQ-S ¼ lower outer quadrant—superficial; UIQ-D ¼ upper inner quadrant—deep; UOQ-D ¼ upper outer quadrant—deep; LIQ-D ¼ lower inner quadrant—deep; LOQ-D ¼ lower outer quadrant—deep.

H-TOMO (Table 3). In particular, the volume of the heart that received more than 5 Gy (10% of the prescription dose) with HTOMO ranged from 25.5% to 81.7%, depending on location. The extra research results are shown in Table 4. Significant differences were observed in the delivered dose to normal organs according to beam delivery modality for LIQ-S and LIQ-D sections. The CTV coverage for 3D-CRT and ET showed acceptable levels of 98.27% and 99.13%, respectively.

Discussion Most studies on PBI have focused on comparing it with WBRT and determining the dosimetric effects that depend on different treatment modalities, such as 3D-CRT, ET, H-TOMO, intensitymodulated radiation therapy, and proton therapy.4–6,10,13 Despite the rapidly growing interest in PBI techniques, previous studies have only shown their potential advantages.2,14 Toscas et al.,7 Bouchardy et al.,15 and Cox et al.16 described the dosimetric and side effects of radiation therapy of the breast for different tumor

Table 1 Dosimetric characteristics of CTV coverage and CN, rDHI, and mDHI of PTV_eval as a function of tumor locations. Significance levels of the probability values (p-value) as follows: p o 0.05. UIQ-S

UOQ-S

LIQ-S

LOQ-S

UIQ-D

UOQ-D

LIQ-D

LOQ-D

Average

p-value

V47.5 Gy of CTV (%) 3D-CRT ET H-TOMO

100.0 100.0 100.0

99.5 100.0 100.0

99.9 100.0 100.0

100.0 99.9 100.0

99.7 99.9 100.0

99.1 99.4 100.0

100.0 99.7 99.9

99.4 100.0 100.0

CN 3D-CRT ET H-TOMO

0.681 0.727 1.022

0.683 0.714 0.914

0.682 0.727 0.897

0.716 0.659 0.878

0.646 0.557 0.988

0.762 0.607 1.083

0.477 0.417 0.849

0.748 0.574 0.886

0.674 ⫾ 0.088 0.623 ⫾ 0.107 0.940 ⫾ 0.820

3D-CRT vs EBRT (0.293) EBRT vs H-TOMO (0.001) H-TOMO vs 3D-CRT (0.001)

rDHI 3D-CRT ET H-TOMO

0.753 0.789 0.590

0.687 0.824 0.671

0.740 0.787 0.606

0.745 0.632 0.548

0.771 0.801 0.598

0.684 0.641 0.598

0.59 0.765 0.470

0.815 0.836 0.633

0.793 ⫾ 0.068 0.759 ⫾ 0.079 0.589 ⫾ 0.059

3D-CRT vs EBRT (0.0172) EBRT vs H-TOMO (0.002) H-TOMO vs 3D-CRT (0.005)

UIQ-S ¼ upper inner quadrant—superficial; UOQ-S ¼ upper outer quadrant—superficial; LOQ-S ¼ lower outer quadrant—superficial; UIQ-D ¼ upper inner quadrant—deep; UOQ-D ¼ upper outer quadrant—deep; LIQ-D ¼ lower inner quadrant—deep; LOQ-D ¼ lower outer quadrant—deep.

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Table 3 Dosimetric characteristics for the right breast, right lung, and heart as a function of tumor locations UIQ-S UOQ-S Right breast V2.5 Gy (%) 3D-CRT 0.00 0.00 ET 0.00 0.00 H-TOMO 91.60 60.00

LIQ-S

LOQ-S UIQ-D UOQ-D LIQ-D LOQ-D

1.20 0.00 82.90

0.00 0.00 44.20

0.20 0.00 4.10

1.20 0.00 5.40

0.20 6.60 5.60

22.04 0.00 0.00 0.00 77.60 10.50

7.50 0.00 69.90

0.00 0.00 23.50

0.10 1.20 3.00

6.60 0.00 5.70

0.20 1.90 4.20

Heart V2.5 Gy (%) 3D-CRT 0.00 2.10 ET 21.40 5.00 H-TOMO 96.50 92.30

25.20 2.00 15.50 9.00 100.00 78.00

1.00 27.40 99.30

0.00 30.90 91.90

40.20 21.30 31.90 17.60 93.20 99.40

V5 Gy (%) 3D-CRT 0.00 0.00 ET 8.20 0.80 H-TOMO 39.80 31.20

21.30 1.20 8.70 5.40 81.70 25.50

3.80 17.20 61.40

0.00 13.90 34.20

32.10 15.40 14.50 13.20 42.60 64.80

D3% (Gy) 3D-CRT ET H-TOMO

9.10 11.10 12.70

2.10 13.80 9.90

0.80 14.30 7.70

11.50 7.00 17.90 23.80 9.40 12.50

D3% (Gy) 3D-CRT ET H-TOMO

0.19 0.00 5.60

2.60 1.20 4.70

Right lung V2.5 Gy (%) 3D-CRT 0.00 1.30 ET 0.00 0.00 H-TOMO 50.00 28.00 D3% (Gy) 3D-CRT ET H-TOMO

0.20 0.00 6.20

0.60 8.20 7.30

0.00 0.00 0.00 0.00 76.90 37.60 0.40 0.00 4.60

2.10 1.20 4.10

2.00 3.00 6.90

5.60 0.00 5.30

1.20 8.10 7.80

0.00 0.00 0.00 0.00 77.20 48.50 0.50 0.00 6.50

0.20 9.00 5.20

29.20 0.00 0.00 0.00 17.40 55.40 7.90 2.10 4.60

0.20 1.90 4.90

UIQ-S ¼ upper inner quadrant—superficial; UOQ-S ¼ upper outer quadrant—superficial; LOQ-S ¼ lower outer quadrant—superficial; UIQ-D ¼ upper inner quadrant—deep; UOQ-D ¼ upper outer quadrant—deep; LIQ-D ¼ lower inner quadrant—deep; LOQ-D ¼ lower outer quadrant—deep.

locations. Jose et al. investigated the dosimetric results of PBI for deep-seated tumors in the right and left breasts of early-stage breast cancer patient and determined that treatments used for boosting the tumor bed may be benefited by application of ET. Bouchardy et al. showed that radiation therapy for cancer in the inner quadrant of the breast was associated with an increase in mortality owing to cardiovascular disease. Brett et al. investigated the effect of an increased margin of the PTV and suggested the need for further study of the dosimetric effects of PBI according to tumor location. These studies clearly demonstrate the need to investigate the dosimetric results as a function of tumor position to plan for efficient radiotherapy. The present study examined the efficacy of 3 different PBI treatment modalities for 8 tumor locations. The tumors were classified into 1 of 8 sections according to the quadrant of the

breast it was located in and whether it was superficial or deep. All locations had distinct characteristics. For example, the lower and inner sections were located closer to the heart, which could potentially be reached by the treatment beam. Therefore, a slight increase in the low dose to the heart is an unavoidable occurrence. The superficial vs deep classification was applied into the present study because tumors may be situated just above the chest wall or just below the skin, and both cases are clinically important. For quantitative evaluation of treatment planning results, we calculated CN, which was recently used as a focused index17,18 and takes into account the irradiated target volume and the irradiated adjacent healthy tissue. The CN index can be purposely adjusted with the utilized target volume, according to the research aim.17,18 In this study, the TV (Eq. (1)) was the PTV, and the TVRI was the PTV volume irradiated at 95% of the prescribed dose. The VRI was the whole ipsilateral breast volume irradiated at 95% of the prescribed dose so as to fully include the surrounding tissue OARs. Therefore, a high CN indicates a sparing of normal healthy tissue while maintaining suitable target coverage. With the exception of CN, our results showed that there were no distinguishable advantages with the H-TOMO technique (compared with 3D-CRT and ET) at any tumor location. Although the CN results for H-TOMO were approximately 0.3 higher than that of 3D-CRT and ET, the CN values of 3D-CRT and ET were at acceptable levels. As the CN parameter applied in this study takes into account both the target coverage and unnecessary doses to the surrounding healthy tissue, a low CN value cannot necessarily be interpreted as low coverage of the target volume, as discussed before. Indeed, the V95% of the CTV for 3D-CRT and ET was more than 99.0%. Regarding CN values, Caudrelier et al.21 conducted a dosimetric breast cancer comparison study using intensitymodulated radiation therapy and H-TOMO. The CN values in this study ranged from 0.32 ⫾ 0.18 to 0.56 ⫾ 0.10, whereas the values for 3D-CRT and ET in our present study ranged from 0.417 to 0.748. In addition, rDHI for H-TOMO was lower than for 3D-CRT and ET at all locations, and the dose delivered to the left lung, right breast, right lung, and the heart was noticeably higher with H-TOMO. More specifically, the dose delivered to the right breast, right lung, and heart with 3D-CRT and ET treatments were nearly 0, whereas the delivered dose to these organs with H-TOMO ranged from 17.4% (V2.5 Gy, right lung, LIQ-D) to 100% (V2.5 Gy, heart, LIQ-S) (Table 3). The ET modality may be the advisable approach for the treatment of tumors in LIQ-S and LIQ-D sections because the heart and right lung undergo lower irradiation compared with 3D-CRT. These results can be interpreted by the distance from the OAR to the target. Although an electron beam is limited by multiple scatterings, the delivered dose with ET is rapidly reduced because it involves a shorter range from the skin than with other modalities, and it can be attenuated after irradiation of the tumor. However, a photon beam can penetrate the breast tissue more than an electron beam. Additionally, the target coverage of the ET method for LIQ-S and LIQ-D cases was more than 99%.

Table 4 The average volume irradiated by each appropriate dose of the extra planning study in LIQ-S and LIQ-D cases Heart

3D-CRT (%) ET (%) p-Value n

CTV V47.5 Gy

V2.5 Gy

98.270 ⫾ 2.100 99.130 ⫾ 1.000 0.910

29.300 ⫾ 17.600 12.910 ⫾ 11.000 0.028n

Right lung

Left lung

V4.5 Gy

V2.5 Gy

V5 Gy

V15 Gy

V45 Gy

12.910 ⫾ 11.000 7.210 ⫾ 7.000 0.016n

16.560 ⫾ 14.500 0.000 ⫾ 0.000 0.001n

43.720 ⫾ 17.200 11.730 ⫾ 10.800 0.001n

14.330 ⫾ 11.900 4.280 ⫾ 4.800 0.069

14.250 ⫾ 3.300 16.880 ⫾ 5.100 0.226

By Mann-Whitney U test with statistical significance defined as p o 0.05.

Left breast

M.-J. Kim et al. / Medical Dosimetry 38 (2013) 327–331

Treatment with H-TOMO could be damaging to surrounding tissues because of low dose delivery as H-TOMO operates with a rotative motion around the patient’s body. Especially, low radiation doses to the heart could lead to the worsening of preexisting cardiovascular lesions caused by radiation-induced pneumonitis.19–22 This has been reported after treatment in early-stage breast cancer patients with left-sided tumors. Radiation therapy for inner-quadrant breast cancer is associated with an increase in cardiovascular mortality, possibly caused by high irradiation of the heart.19–22 Therefore, the goal of reducing the irradiated volume for the lungs and heart is vitally important when treating breast cancer. To confirm these results, further research was performed at 10 different tumor locations (LIQ-S an LIQ-D subsections) using CT data sets of early-stage breast cancer patient with the 3D-CRT and the ET modalities. These extra planning sets complied with the methods of classification for tumor location, the planning techniques, and the evaluation methods presented in this study. Based on this work, evaluation of OARs according to tumor location and anatomical structure could be investigated. The results further confirmed that ET was the preferable method to treat early-stage breast cancer patient’s tumor located in LIQ-S and LIQ-D regions. All dosimetric planning results showed that target coverage was more than 99%. The average irradiated volumes for each surrounding organ were greater with 3D-CRT than with ET, and resulting p values from the Mann-Whitney U test were considerably lower than 0.05, with the exception of the left breast (Table 4). These results suggest that ET is highly advisable for the treatment of early-stage breast tumors located in the LIQ-S and LIQ-D sections as opposed to 3D-CRT, owing to the acceptable target coverage and the much lower dose applied to the surrounding tissue.

Conclusions An investigation of PBI using 3D-CRT, ET, and H-TOMO treatment modalities for breast tumors of various locations was performed in this study. The ET method was effective for early-stage breast cancer patient’s tumor located in the lower and inner sections (LIQ-S and LIQ-D). The H-TOMO method delivered high levels of radiation to surrounding healthy tissue, such as the heart and lungs, with generally the same degree of target coverage as the other methods. Further exploration of approaches that measure dosimetric effects on breast tumors based on location is recommended to determine clinically active applications of PBI methods. Acknowledgements This research was supported by Leading Foreign Research Institute Recruitment Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) of Korea (Grant No.2009-00420), and the Korea Heavy-ion Medical Accelerator project grant funded by MEST of Korea (Grant No. 2012K001141).

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