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ORIGINAL ARTICLE
Year : 2019  |  Volume : 15  |  Issue : 5  |  Page : 1035-1041

A comparative study for surface dose evaluation in conventional treatment of carcinoma breast patients irradiated with Co-60 and 6 MV radiation beam


1 Department of Radiotherapy, PGIMER; Department of Physics, Panjab University, Chandigarh, India
2 Department of Radiotherapy, PGIMER, Chandigarh, India
3 Department of Physics, Panjab University, Chandigarh, India
4 Department of Nuclear Medicine, PGIMER, Chandigarh, India

Date of Web Publication4-Oct-2019

Correspondence Address:
Rakesh Kapoor
Department of Radiotherapy, PGIMER, Chandigarh - 160 012
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jcrt.JCRT_789_17

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 > Abstract 


Aim: In the present study, surface doses within the target area and contralateral breast (CLB) received during conventional treatment of carcinoma breast are evaluate and compared for treatment on two different beam energies, i.e., Co-60 γ-ray and 6 MV X-ray beams with thermoluminescent dosimeter, LiF:Mg, Ti (TLD-100).
Materials and Methods: The study includes a group of 23 patients comprising 11 patients treated with Co-60 γ-ray beam and 12 patients by 6 MV X-ray beam.
Results and Discussion: The treatment using Co-60 γ-ray and 6 MV X-ray beams contributes an average percentage dose of 8.15% ± 0.56% and 4.73% ± 0.94%, respectively, to CLB in mastectomy patients. The contribution of tangential fields (mastectomy) to the CLB doses ranges between 12.71 and 16.40 cGy (5.45%–7.03%) for treatment with Co-60 γ-ray beam and 6.33–10.95 cGy (1.86–4.69%) for treatment with 6 MV X-ray beam. The supraclavicular field (SCF) contributes 1.45%–1.93% and 1.02%–1.43% for treatment with Co-60 γ-ray and 6 MV X-ray beams, respectively. The average surface dose (normalized with breast dose) 89.1% ± 8.5% for Co-60 beam in the SCF region differs significantly from the 60.2% ± 13.0% value for 6 MV X-ray beam.
Conclusion: The CLB doses for mastectomy patients are higher for Co-60 beam as compared to 6 MV X-ray beam, and better dose homogeneity is achieved within the irradiated breast from 6 MV X-ray beam. The CLB doses are slightly higher for patients treated with breast conservative radiotherapy or lumpectomy. The average surface dose to SCF decreases by ~30% of treated breast dose for treatment with 6 MV X-ray beam.

Keywords: Breast conservative radiotherapy, contralateral breast, lumpectomy, mastectomy, surface dose, thermoluminescent dosimeter-100


How to cite this article:
Singh R, Oinam AS, Trivedi G, Kainth HS, Shahi JS, Singh B, Kapoor R. A comparative study for surface dose evaluation in conventional treatment of carcinoma breast patients irradiated with Co-60 and 6 MV radiation beam. J Can Res Ther 2019;15:1035-41

How to cite this URL:
Singh R, Oinam AS, Trivedi G, Kainth HS, Shahi JS, Singh B, Kapoor R. A comparative study for surface dose evaluation in conventional treatment of carcinoma breast patients irradiated with Co-60 and 6 MV radiation beam. J Can Res Ther [serial online] 2019 [cited 2019 Nov 15];15:1035-41. Available from: http://www.cancerjournal.net/text.asp?2019/15/5/1035/244475




 > Introduction Top


The breast cancer is the most widespread cancer among women all over the world. According to the available breast cancer data for India for the year 2012, of 144,937 newly diagnosed women with breast cancer, 70,218 died, which corresponds to ~48%,[1] while in the other countries such as the United States of America and China, death rate was reported ~19% and ~25%, respectively, for breast cancer.[1],[2] The lack of awareness, hesitation, poor hygiene, and screening among women are the major reasons for late diagnosis and poor prognosis. Radiotherapy (RT) is one of the treatment modalities among surgery and chemotherapy for carcinoma breast. It plays an important part in the management of breast cancer, and better survival is achieved for early detection. The radiation therapy after breast-conserving surgery (BCS) for early as well as locally advanced tumor after neoadjuvant chemotherapy is now considered as an integral part of breast conservation, whereas postmastectomy radiation to the chest wall and regional area is considered beneficial for a select group of high-risk patients.[3],[4] The breast irradiation leads to both acute and late toxicities, for example, dry and wet desquamation and pain and heaviness in breast. Many studies show that these toxicities are due to inhomogeneity in dose distribution within the target volume.[5],[6] Therefore, dose homogeneity within treatment volume is the primary requirement for the treatment of carcinoma breast. However, the irregular contour of the breast makes it difficult to deliver uniform dose throughout the target volume. Although newer techniques, for example, intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT), improve dose uniformity and potentially reduce acute toxicity compared to that with conventional tangential whole-breast RT,[7],[8] still large population of carcinoma breast are treated with conventional treatment in most of the developing countries.

In RT, the conventional breast treatment involves two parallel opposed tangential fields followed by a supraclavicular field (SCF).[9],[10],[11] The parallel opposed tangential fields allow adequate coverage to the breast tissue while minimizing the dose to adjacent normal structures, i.e., contralateral breast (CLB), ipsilateral lung, and heart. Sometimes, depending on the contour and the beam energy, modifying devices, for example, wedges and bolus were used to reduce the dose inhomogeneity within the entire treatment volume of breast. The surface dose measurement by radiations dosimeters recommended for in vivo dosimetry, i.e., thermoluminescent dosimeters (TLDs), optical stimulated luminescence dosimeters, radiographic films, diodes, and metal-oxide field-effect transistor provides best estimation of radiation doses received by the target volume. The in vivo surface dosimetry is essential to predict the occurrence of unwanted skin reactions at different regions within the target volume and also act as an important tool to assess the accuracy of delivered dose. The TLDs are widely used in medical practice as a radiation dosimeter due to their small energy dependence, small size, cost-effectiveness, wide dose range, and reusability.[12],[13],[14]

In the treatment of primary breast by external beam therapy, evaluation of surface doses for the patient underwent RT is very much helpful in predicting the dose homogeneity within the target volume.[15] The dose inhomogeneity within the target volume leads to skin toxicity, and in vivo dosimetry is useful in the early prediction of acute skin reaction during the course of treatment. In the treatment of carcinoma breast, the CLB receive scattered radiation dose not only from primary irradiation site but also from collimator jaws, flattening filter in linac, backscattered from patient body, high energy electrons produced from interaction of X-rays/gamma rays with air and any shielding structures in the close proximity to patient. The CLB tissues are radiation sensitive, and the risk of second malignancy increases with increase in received dose.[16] Hence, due to the carcinogenic effect of ionizing radiation, doses to the CLB should be quantified to estimate the risks. In the present study, LiF: Mg, Ti (TLD-100) is used to evaluate the surface doses of whole irradiated breast and doses to the CLB during conventional treatment of a group of patients diagnosed with carcinoma breast at two different photon energies (Co 60 γ-rays and 6 MV X-rays). The dose homogeneity within the target volume was accessed for both beam energies.


 > Materials and Methods Top


The samples used in the present study were TLD-100, cuboid shaped with dimension 1 mm × 1 mm × 6 mm (Rexon TLD Systems and Components, Ohio, USA). The radiation dose was delivered by γ-rays from Co-60 telecobalt machine (Theratronics, Kirloskar Pvt. Ltd., Canada) and photon beam from 6 MV linear accelerator (Clinac-2300 CD, Varian, CA). Rexon UL-300 readout system (Rexon TLD Systems and Components, Ohio, USA) interfaced to PC was used for recording and analyzing the thermoluminescent spectra from TLDs after irradiation. Heat treatment was given to all TLDs by microprocessor-controlled heating oven, to erase any residual information before irradiation. It involves both preirradiation and postirradiation annealing. In preirradiation annealing, all TLDs are heated at 400°C for 1 h followed by 105°C for 2 h.[17] The postirradiation annealing includes the heating of TLDs at 105°C for 15 min immediately after irradiation.

The radiation dose was measured using 0.6 cc ionization chamber (type 30013, PTW Freiburg, Germany), traceable to Reference Standard Laboratory of Radiation Standardization Section (Bhabha Atomic Research Centre, Mumbai, India), RW3 solid water phantom (area 30 cm × 30 cm, thickness range 0.1–1 cm, density 1.045 g/cm 3), and UNIDOS E Electrometer (PTW, Freiburg, Germany). After postirradiation annealing, readout was carried out at linear heating rate of 5°C/s in the presence of nitrogen gas in the planchet chamber.

To measure the element correction coefficient (ECC) for each TLD (Si), all TLDs were irradiated for a known dose of 100 cGy and were calculated by dividing individual TL counts with the average TL counts of all TLDs. This process was repeated thrice, and the ECC [18] of each TLD was evaluated as , where Cavg is the average counts of all TLDs irradiated with the same exposed dose and Ci is the counts from individual TLD, respectively. [Figure 1] shows the plot of average ECC (average over three consecutive readings) of all TLDs (80 in numbers) irradiated with 6 MV X-ray beam. Only TLDs within ±10% variation were included and others were discarded. The TLD materials were divided in large number of batches (each containing 5 TLDs) and irradiated with radiation absorbed dose ranging from 0.4 to 1000 cGy. Before irradiation, the radiation doses from Co-60 γ-rays and 6 MV X-rays were measured using 0.6 cc ionization chamber, and calibration curves were plotted [Figure 2]. The uncertainty in dose measurement was found to be within ±1.96% and ±2.27%, respectively, for 6 MV X-ray beam and Co-60 beam.
Figure 1: The variation in element correction coefficient for all 80 thermoluminescent dosimeters (LiF: Mg, Ti) used in the present work

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Figure 2: Calibration curve for thermoluminescent dosimeter-100 for the Co-60 ϒ-ray and 6 MV X-ray beams. The readout of the irradiated thermoluminescent dosimeters was carried out at 5°C/s

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In the present study, the radiation doses at 16 different points (20 TLDs for each patient) comprising 14 points within the treated volume (breast and supraclavicular region) and 2 points on the normal breast (one at the nipple and other at 3 cm superior to CLB's nipple) were measured using TLDs. The treatment was carried out with two different machine modalities: linac (6 MV X-rays) and telecobalt 780C (Co-60 γ-rays). In the present study, 23 patients (11 patients on Co-60 beam [Table 1] and 12 patients by 6 MV X-ray beam) [Table 2] were treated with the conventional RT treatment on two different treatment beam energies. In patients of mastectomy, bolus (tissue equivalent material) was used for primary breast irradiation on every alternate day. For such patients, TL dosimetry was performed for both consecutive days, and average radiation doses at all points were calculated. The patient was treated in supine position on the angled breast board with one or both arms stretched over the head. The gantry angle of medial tangential field varies between 50° and 66° with respect to transverse planes on either side. The doses to the CLB, dose homogeneity, and surface doses were evaluated and recorded.
Table 1: Site of breast, mode of surgery, age, gantry angle of medial tangential field, tangential separation, total prescribed doses, and dose per fraction for patients treated with Co-60 ϒ-beam

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Table 2: Site of breast, mode of surgery, age, gantry angle of medial tangential field, tangential separation, total prescribed doses, and dose per fraction for patients treated with 6 MV photon beam

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 > Results Top


The mean dose delivered with photon to the primary breast for patients treated on 6 MV linear accelerator was 37.2 ± 2.7 Gy (range: 34–40 Gy) with a mean single-fraction dose of 2.58 ± 0.27 Gy (range: 2.33–3.40 Gy). The mean dose delivered for patients treated on Co-60 machine to the primary breast was 35.4 ± 1.5 Gy (range: 35–40 Gy) with single-fraction dose of 2.35 ± 0.05 Gy (range: 2.33–2.50 Gy). The average values of percentage dose measured at the nipple and 3 cm superior to the CLB for the mastectomy patient were 8.15% ± 0.56% (range: 6.93%–8.97%) and 4.73% ± 0.94% (range: 3.03%–5.86%) for treatment with Co-60 γ-rays and 6 MV X-ray beam, respectively [Table 3]. The contribution of tangential field (mastectomy) to the CLB doses ranges between 5.45% and 7.03% (average: 6.42% ± 0.44%) for treatment with Co-60 γ-rays and 1.86%–4.69% (average: 3.86% ± 1.00%) for treatment with 6 MV X-ray beam. The percentage CLB doses for breast conservative RT (BCR) by 6 MV X-ray beam (30° wedges for tangential fields) were found in the range of 5.94%–8.19% (average: 6.66% ± 0.82%). The SCF contributes 1.45%–1.93% and 1.02%–1.43% for treatment with Co-60 γ-rays and 6 MV X-ray beam, respectively.
Table 3: Contribution of doses from different fields used in conventional treatment of carcinoma breast to the contralateral breast for different patient

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The percentage surface doses within the irradiated breast region of all patients treated with Co-60 beam range from 53.9% to 147.6%. Similarly, the percentage surface doses measured for mastectomy patient treated with 6 MV X-ray beam range from 56.5% to 133.2% [Table 4]. The BCR and lumpectomy patients treated with 6 MV X-ray beam reported percentage surface doses between 60.6% and 78.1%. The surface dose (normalized with treated breast dose) to the SCF region changes significantly from 89.1% ± 8.5% for Co-60 beam to 60.2% ± 13.0% for 6 MV X-ray beam.
Table 4: Surface doses evaluated within the irradiated regions of treated breast and supraclavicular for patients treated with Co-60 and 6 MV photon beam

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The quality of conventional treatment (two-dimensional planning) of carcinoma breast was best examined by the central lung distance (CLD) of intact breast which is evaluated during imaging. CLD is the perpendicular distance from the posterior tangential edge to the posterior part of the anterior chest wall at the center of the field. In the present study, the maximum and minimum CLD were 1.3 cm and 2.5 cm (mean: 2.0 cm) for patient treated on telecobalt machine and 1.8 cm and 3.5 cm (mean: 2.3 cm), respectively, for patient treated on 6 MV linear accelerator. A CLD of 3 cm results in irradiation of 10%–23% of ipsilateral lung.[19],[20],[21] According to the EORTC [22] guidelines, the CLD exceed 3 cm results in the greater risk of radiation pneumonitis.


 > Discussion Top


In the present study, 23 patients (11 patients on Co-60 beam and 12 patients by 6 MV X-ray beam) were treated with the conventional RT.

The patient treatment comprised mastectomy, lumpectomy, and BCR. A large variation in the dose was observed at the borders of breast field for treatment on both energies because of penumbra region, reduced scattered contribution, and uncertainty in placing TL dosimeter exactly at the field markings due to patient breathing motion and dosimeter size. The doses to the CLB during treatment with Co-60 beam from tangential fields range 5.45%–7.03%, whereas the contributions from the SCF range 1.49%–1.94%. The total dose from all the three fields was found to be in the range of 6.94%–8.97%, and the contribution of medial tangential field is quite significant (~50%). This is because of two reasons (i) shorter perpendicular distance from the CLB surface to the geometric beam edge and low beam energy and (ii) collimator contribution which results in increased surface dose to the opposite breast.

Similarly, the treatment using 6 MV X-ray beam contributes 3.45%–9.89% from tangential fields and 1.05%–3.87% from SCF of the total dose to the CLB. The contributions from tangential field for patient with mastectomy and BCS (or lumpectomy) were found to be in the range 3.03%–4.73% (average: 4.73% ± 0.94%) and 7.00%–7.64% (average: 7.34% ± 0.29%), respectively. The surface doses were significantly varied with the use of beam-modifying devices, for example, wedges. The 30° physical wedges were used in the BCR or lumpectomy and increased the doses to the CLB because of scattered radiation, although the surface doses to the primary irradiating breast were decreased. The decrease in the surface doses of the primary breast is due to beam hardening because of physical wedge in the path of beam. The lateral tangential field contributes approximately 40%–50% and 50%–55%, respectively, for patient with mastectomy and BCS or lumpectomy (6 MV X-ray beam). The CLB doses received for patient with mastectomy and tangential separation are plotted in [Figure 3]. It is evident that the contralateral doses are significantly higher for Co-60 beam as compared to the 6 MV X-ray beam and increases with breast size/area for both the energies. The surface radiation doses of the primary breast were nonuniform due to the irregular contour of the breast. The use of bolus on every alternate day in patients with mastectomy did not contribute dose to the normal breast. The use of half beam block instead of asymmetric collimator in Co-60 machine increased the dose to CLB dose. The surface doses within the irradiated region of affected breast show significant variations. The maximum values of standard deviation obtained were ±21.45% and 22.18% for Co-60 and 6 MV X-ray beams, respectively [Table 3]. The standard deviation values for Co-60 are in general higher as compared to that for the 6 MV X-rays for patients with mastectomy. Many studies have been carried out to measure CLB doses for different treatment modalities, beam energies, and beam-modifying devices. Faaruq et al.[23] carried out measurements on 60 patients and evaluated both contralateral and chest wall doses. The chest wall doses were 3.5%–12.5% and 2.5%–9.0% higher than prescribed dose (2 Gy) for treatment through Co-60 γ-ray and 6 MV X-ray machines, respectively. The average CLB doses were found to be 43% higher for Co-60 as compared to 6 MV linac. The conventional half beam block of Co-60 machine was the main cause for significantly higher contribution. The dose to the CLB is mainly due to scatter and transmission of radiation through half beam blocker. Rather et al.[10] carried out the study on 40 patients, and the dose contribution from different fields used in conventional RT treatment for carcinoma breast was evaluated using rainbow detector. The surface doses recorded for CLB for tangential fields range 5.34%–6.40% and 1.2%–1.75% for SCF. The advanced treatment modalities, for example, IMRT, image-guided radiation therapy, and VMAT reduce the dose to the CLB and also improve the dose distribution within the target area. Bhatnagar et al.[24] evaluated the effect of breast size and measure radiation doses to the CLB using TLDs placed at 4 cm from the geometrical field border during primary breast irradiation using Intensity modulated radiotherapy (IMRT). They concluded that the size of primary breast has a significant effect on scattered dose reaching the normal breast and its volume is positively correlated with doses to CLB. Another study carried out by Bhatnagar et al.[25] measures the radiation dose to CLB by placing TLDs at 4 and 8 cm from the center of the medial border of the tangential field. At the 4 cm contralateral position, the mean percentage of prescribed dose was 7.19% ± 2.28% and 11.22% ± 2.73%, respectively, for IMRT and conventional tangential field technique. The mean percentage of prescribed dose was found to be 4.63% ± 2.12% and 10.70% ± 3.44%, respectively, for patients treated with IMRT as compared to conventional technique. Similar results were obtained by Hong et al.[26] and Chang et al.[27] and they reported a significant reduction in the CLB doses with IMRT as compared to that of conventional standard wedge tangential technique. The doses to the coronary arteries, ipsilateral lung, and surrounding soft tissues were also reduced.[26] Saur et al.[28] evaluated doses to normal breast for three different tangential techniques applied on female thorax phantom using GafChromic EBT film. A highly inhomogeneous dose distribution within the CLB and skin doses due to the medial fields was obtained with film dosimetry which was several times higher than the interior dose (lateral tangent field). The results were compared for doses obtained by commercially available treatment algorithms, i.e., pencil beam and collapsed cone. All tangential techniques were found to give a mean CLB dose of approximately 0.5 Gy. The wedged fields resulted in higher CLB doses as compared to all open fields, and the lateral open fields resulted in higher CLB doses than the medial open fields. The physical and virtual wedges result in increased CLB dose by 35% and 16% for the medial and lateral fields, respectively. Although lead shielding results in significant dose reduction (~50%), mean CLB doses were reduced only by ~11%.
Figure 3: The variation of doses to contralateral breast with the tangential separation for patient (mastectomy) treated on both Co-60 ϒ-rays and 6 MV photon beams

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 > Conclusion Top


In the present work, LiF: Mg, Ti (TLD-100) dosimeters were used for in vivo dosimetry of 23 breast patients treated on two different energy beams (Co-60 γ-rays and 6 MV X-ray beam). The doses to the CLB and dose inhomogeneity within the treated breast are slightly higher for treatment with Co-60 beam as compared to that of 6 MV X-ray beam. The main contribution to the CLB dose during treatment with Co-60 beam is from the medial tangential field. While the lateral tangential field has a major contribution to the CLB for treatment with 6 MV X-ray beam. The doses to the CLB increase both with patient thickness and size/area of the primary breast to be irradiated. The CLB doses are found to be slightly higher for BCR or lumpectomy as that of patient with mastectomy. The increase and decrease of surface doses in the primary breast with the introduction of bolus, i.e., mastectomy and wedges, i.e., lumpectomy or BCR are also verified with TLD-100. The average surface dose to SCF decreases by ~30% of the treated breast dose for treatment by 6 MV X-rays as compared to the Co-60 γ-rays.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
 > References Top

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Venables K, Miles EA, Aird EG, Hoskin PJ, START Trial Management Group. The use of in vivo thermoluminescent dosimeters in the quality assurance programme for the START breast fractionation trial. Radiother Oncol 2004;71:303-10.  Back to cited text no. 14
    
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Stovall M, Smith SA, Langholz BM, Boice JD Jr., Shore RE, Andersson M, et al. Dose to the contralateral breast from radiotherapy and risk of second primary breast cancer in the WECARE study. Int J Radiat Oncol Biol Phys 2008;72:1021-30.  Back to cited text no. 16
    
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