ISOTOPE & RAD. RES., 46(1), 117-129 (2014)

Evaluation of the Dynamic Arc-Therapy in Comparison to Conformal Radiation Therapy in Radiotherapy Patients

Aliaa Mahmoud(1,4), Ehab M. Attalla(2,3), M..S. El-Nagdy(4) , Gihan Kamel(4)

(1) Radiation Physics Department, Oncology and Hematology Hospital, Maadi Armed Forces Medical Compound, Egypt;

(2) Children’s Cancer Hospital, Cairo, Egypt (3)National Cancer Institute, Cairo University, Egypt

(4) Physics Department , Faculty of Science, Helwan University, Cairo, Egypt

Abstract

This study aims to evaluate the dose distribution and the clinical applicability of the Dynamic Arc Therapy (DAT) in comparison to the Three-Dimensional Conformal Radiotherapy (3D-CRT). The work has been done over different types of tumors, locations and the dose distributions were evaluated through comparisons of dose-volumetric histograms. Twelve patients with prostate cancer with indicated to irradiate pelvic lymph node. Six patients with single tumors in the pituitary and Four patients in the bronchogenic treated with 3D-CRT, were retrospectively planned with DAT using Eclipse (V10) DAT plans for prostate showed a statistically comparable achievement of tumor conformity and dose sparing for bladder and rectum when compared to 3DCRT. Dose on femoral heads were similar to those achieved using 3DCRT .The mean volume percentages of rectum receiving 78Gy provide values of 11.6% and 17.8% for DAT and 3D-CRT, respectively. so DAT forward planning in prostate cancer decrease dose delivered to rectum than 3D-CRT. In pituitary adenoma 3D-CRT is observed to be better than the DAT because of the decreased dose delivered to organ at risk. In bronchogenic cases coverge tumor target and sparing normal organ in both techniques may be similar. Modulated factor (machine unite (MU)\Time) +1.3%in 3D-CRTthan DAT in prostate cancer but in bronchogenic carcinoma and pituitary adenoma similar. 3D-CRT is capable of providing conformal dose distributions to the targets accomplishing many of the DAT dose constraints simultaneously. Experimental dose-validation accuracy, ease of planning and reduced treatment times makes DAT for clinical use.

Key words: Dynamic Arc Therapy, 3D-Conformal Radiotherapy, Dose Validation, treatment time, external -beam radiotherapy.

Introduction

Three-dimensional Conformal Radiation Therapy (3D-CRT) is a technique in radiation therapy designed to deliver prescribed radiation doses to localized tumors with high precision, using a Multi-Leaf Collimator (MLC) to effectively exclude the surrounding normal tissues (Hugo GD, Agazaryan N, Solberg TD ,2003 and Orton NP and Tomé WA.,2004 ). In the forward planning process, as used in 3D-CRT, the MLC is set to shape the radiation fields using various angles (gantry angles) to conform to the tumor, or clinical target volume (CTV), and the dose weighting is then adjusted in a trial and-error fashion to refine the plan (Metwaly M, Awaad AM, El-Sayed el-SM, Sallam AS,2008). The only feasible way to change the radiation intensity in 3D-CRT is to use

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static or dynamic wedges (which achieve intensity modulation through a single line in the beam aperture plane) and compensating filters (metallic plates of non-uniform thickness, which achieve intensity modulation in the all points of the field aperture plane) (Hugo GD, Agazaryan N, Solberg TD ,2003 and Orton NP and Tomé WA.,2004). However, preparation and treatment delivery sessions with the use of compensating filters take a long time and bear a risk of human error (Lee C, Dong L, 2005).

Dynamic arc therapy is a radiation therapy delivery technique based on forward-planning

dose calculation, which combines gantry rotation with MLC motion to conform to the beam eye-views of the CTV at various gantry angles (Zhu S, 2005 and Shiraishi K, 2006). The DAT technique is implemented mainly to acquire as steep as possible a dose gradient outside the tumor, particularly the risk organs. This approach avoids undesirable hot spots or localized high-dose regions outside the treatment volume delivering the total dose in as short a time period as possible is thought to be more effective (Morgan WF, Naqvi SA, Yu C, Smith LE, Rose M.,2002 andWang JZ, Li XA, D’Souza WD, Stewart RD,2003). For cases where static beams with no modulation are not sufficiently adequate to deliver dose to the tumor as well as sparing healthy organs, we shall present additional option of rotating the gantry while shaping the MLC (multi-leaf collimator) around the tumor. This may considerably improve the resulting dose distributions and achieve the desired concave dose distribution to the tumor, reduce treatment delivery time. Efficiency of the delivery and management of highly conformal treatment is also improved.

In the present work, we compared the 3D-CRT with the DAT to determine which of these

techniques may provide the desired benefits in sparing risk organs, and healthy tissues, with similar or better coverage of the prescribed dose to the tumor. The accuracy of the dose calculation algorithm of our planning system for 3D-CRT and DAT was determined by the comparison with the dose distribution on clinical target volume (CTV), sparing critical organs and time for planning and treatment (Lee C, Dong L, 2005).

Materials and Methods

Patients and Staging Evaluation:

All cases were selected with Prostate Carcinoma, Pituitary Adenoma and Bronchogenic Carcinoma and were treated using two both techniques DAT and 3D-CRT. The risk organs were identified for each site, in prostate cancer (bladder, Rectum, Right and left head of femur), in pituitary adenoma (two eyes, brain stem, chiasm and whole brain), and in Lung Cancer (bronchogenic) (heart, normal part of lung, spinal cord and esophagus). Therefore, as it is not easy to deliver high treatment dose to tumor and protect neighboring critical organs, these two techniques provide the dose distribution for treatment of target volume with optimal protection of organs at risk.

Treatment Planning System and Radiotherapy Machine:

An Eclipse External Beam Planning System (ver. 10: Varian Medical Systems, Palo Alto, CA) including the Pencil-beam algorithm was used for forward-planning dose calculation of dynamic arc plans with an 80-leaf (40 pairs) multi-leaf collimator (MLC) of 10 mm thickness at the isocenter. A Varian 23 EX-linear accelerator was used for treatment delivery sessions using the DAT and conformal treatment. The comparison includes parameters such as dose distribution on 118 

 

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target volume (CTV), sparing critical organ, and time for planning and treatment. The same contours for tumors and organs used in DAT were utilized to perform 3DCRT plans. The clinical DAT treatment plans were designed using the three treatment planning systems as step-and-shoot ARC, plans for delivery on the same machine. Arc fields have infinite number of static fields with movement MLCs to make fit target and shield organ at risk.

Computed Tomography Scans and Volume Definition:

For The twenty two patients of interest, we performed computed tomography (CT) in the supine position with slice spacing of 0.5cm in case of prostate. For bronchogenic and for pituitary adenoma a slice spacing of 2mm was chosen. In prostate patient, the scan borders were taken through the region from the lower end of the sacroiliac joint down to the penile urethra, in pituitary adenoma patient, they were fixed with Orfit (PTW) with 3-point, nose hole and the scan borders were taken from top of skull bone to cervical spine C1-C2. For cases of bronchogenic the CT extended from supraclav fossa up to suprarenal. The prostate (PO), Rectum (RC), and Bladder (BL), Femoral heads (FH), eyes right and left, brainstem, chiasm, optical nerve right and left, heart, lung, spinal cord and esophagus were contoured on 3D images. The margins for the CTV were taken to be 0.5cm in the superior, posterior, right-left direction and anterior reviewed in the CT images, followed by exporting the image to the Eclipse external beam planning system with Pencil-beam algorithm. This treatment planning system simplifies the modern radiation therapy planning for all kinds of treatments including the 3D-CRT and ARC and verified by the same radiation oncologist. (Metwaly M, Awaad AM, El-Sayed el-SM, Sallam AS,2008) Treatment Sites:

Prostate:

Twelve patients with localized prostate cancer with indicated to irradiate pelvic lymph node of had been treated with DAT using a typical 15 MV photon arrangement of coplanar beams (all patients with 2 field complete arc with angle from165˚ to 185˚) and two oblique wedged field (45˚) with angle 105˚ and 255˚). Each patient received clinical target volume (CTV) dose of 78Gy in 39 fractions (in DAT technique) 70GY in 35 fraction (in 3DCRT technique). A margin of 0.8cm had been delineated around the prostate gross target volume (CTV), except in the posterior direction where a 0.6cm margin was used to be reasonably away from the rectum.

The seminal vesicles were contoured up to 1cm above the prostate superiorly. The rectum contour was drawn between the anal verge and the sigmoid flexure. (Cheung P, 2005 and van Tol-Geerdink, JJ.2006). The bladder contour was defined including its cavity. The left and right femoral heads were outlined in all cases. Following the Jereczek-Fossa et al. study, (Jereczek-Fossa et al., 2006) two-arc DAT coplanar beams (40°–140° and 220°–320°) and six-field 3DCRT plans were developed for each patient. Photon beams with 10 MV of energy were used in all of these plans. Brain Tumor:

Six patients with pituitary adenoma were treated with 3D was selected to be planned with DAT. The CTV was fairly spherical, 10.1 cm3, and treated using clinical tumor volumes (CTV). CTV surrounded with an added margin of 0.5 cm and 50.40Gy was prescribed to CTV in 28 fractions. Due to the spherical nature of the tumor, one whole arc (165°–185°) was used to develop the DAT plan.

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Lung Tumor:

Four patients with bronchogenic carcinoma treated with 3D was selected to be planned with DAT. The CTV was 383.3 cm3. DAT treated using clinical tumor volumes (CTV). CTV surrounded with an added margin of 0.5 cm. ctv treatment palliative and 56 Gy was prescribed to CTV in 28 fractions. Due to the spherical nature of the tumor, one whole arc (165°–185°) was used to develop the DAT plan.

Plan evaluation and comparisons:

The dose–volume histograms (DVHs) for all CTVs and organs at risk were used for plan evaluation and comparisons. All plans were normalized based on the DVHs to ensure that 95% of the PTV received 100% of the prescribed dose.

MLC (Multi Leaf Collimator): Static Treatment:

Static treatment holds all axes of motion (including the gantry angle, couch position, and MLC leaves) stationary during treatment.

Arc Dynamic Treatment:

For dynamic arc treatments, the MLC leaves are dynamically positioned as the gantry moves through an arc about the patient during treatment. With this treatment type, a larger dose builds up at the center of the rotation (isocenter) within the patient’s body than on any area of the skin. The MLC leaves move as a function of the gantry position in the arc.

Table 2:( Dose and Volume Comparison Metrics A Phase II Study of Concurrent Chemo- radiotherapy Using Three-Dimensional Conformal Radiotherapy (3D-CRT) , [Current Version Date 2011 Feb 16]).

Parameters Definitions

Dmax, Dmean, Dmin Maximum, mean and minimum to the volume of interest.

V40, V50, V60, V65 Percentage of the planning target volume covered by 40Gy, 50Gy, 60Gy, 65Gy

D10, D20, D95 Doses to 10%, 20%, and 95% of the volume of interest.

Results

Figure (1) shows the sagittal dose distribution for a typical prostate cancer case, while Figure (2) depicts the mean DVHs for prostate-CTV, bladder, rectum and left femoral head for the two different techniques. The mean, minimum and maximum dose values for prostate-CTV were better achieved with the DAT. Likewise, the doses received by 95% of the prostate-CTV volume (DPTV-95) for both techniques as shown in figure (2) indicate that the DAT presents the highest

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volume/dose slope for the prostate-CTV, although the two maximum slopes are very close to verify this.

Figure 1: Isodose distribution at the sagittal isocenter plane for dynamic arc therapy and five-field conformal

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Figure 2: Dose volume histograms showing maximum and minimum dose to CTV (Prostate case).

3D-CRT

DAT

The seminal vesicles included in CTV received a statistically similar range of doses (data not shown) for the DAT and the 3D-CRT plans. Thus, all targets received a comparable dose distribution from each of the two treatment techniques, allowing organs at risk dose comparison. Table 3 compares the prescription dose, mean dose (Dmean), the dose received by 95% of the volume (D95), and maximum dose (Dmax) to the CTVs to 6 cases of the pituitary adenoma treated with 3DCRT and DAT. Figures 1 and 2 show the corresponding isodose distributions and the DVHs, respectively, for 3DCRT and DAT.

 

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Table 3: The prescription dose, mean dose (Dmean), the dose received by 95% of the volume (D95), and the maximum dose (Dmax) to the CTVs for the conformal and the dynamic arc therapy.

Technique Diagnoses Prescribed dose

(GY) Max dose

(GY) D95 (GY)

Mean dose (GY)

3D-CRT Prostate

70 71 64 68

DAT 78 85 68 76

3D-CRT Pituitary

50.4 53 50 49

DAT 50.4 54 49 40

3D-CRT Bronchogenic

56 61 53 57

DAT 56 58 53 55

The volume of the bladder covered by 40% and 60% of dose in 3DCRT were (V(40) = 39% from dose and V(60)=62%), in DAT(V(40)=21% and V(60)=12%), shown in Figure 3, the volume of the rectum covered by 40%, 50%, 60% and 65% of dose in 3DCRT (V(40)=39%, V(50)=21%, V(60)=6% and V(65)=4%, in DAT (V(40)=11%, V(50)=7%, V(60)=4% and V(65)=3%, shown in Figure 4. On the other hand, 3DCRT is much less effective in sparing these two organs than DAT is, provided the same target conditions. The volume of the left femoral head covered by 40% and 60% of dose in 3DCRT (V(40)=28% and V(60)=1%, in DAT V(40)=30% and V(60)=0%. The same behavior was found in right femoral head.

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3D-CRT

DAT

Figure 3: Delivered dose to bladder (Prostate cases)

 

ISOTOPE & RAD. RES., 46(1), 117-129 (2014)

Figure 4: Delivered dose to rectum (Prostate cases)

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Figure 5: Delivered dose to right femoral head (Prostate cases)

3D-CRT

DAT

3D-CRT

DAT

The DAT and the 3D-CRT are statistically similar in terms of their dose indicators. The

right femoral head (data not shown) provided similar results as the left femoral head, as shown in Figure (5). The maximum dose in the body was always outside the organs at risk in all the plans; the mean value for all the techniques is less than 107%. The mean numbers of monitor units (MU) needed to deliver the prostate plans were as follows: 319 with STD of 35MU for DAT, and 287 with STD 28MU for 3D-CRT. Therefore, with five beams in 3D-CRT, and two arcs in DAT, the dose-delivery times for treatment were considerably reduced using the DAT technique, show in table 4.

 

ISOTOPE & RAD. RES., 46(1), 117-129 (2014)

Figure 6: Distribution of dose in Plan conformal and plan dynamic arc therapy (Pituitary case)

DAT

3D-CRT

Figure 7: Evaluation of3D -CRT and DAT plans in the dose volume histograms (Pituitary adenoma cases)

From 165° to 185° DAT plan delivered higher maximum doses for the brain stem and the optic nerves, as shown in Table 5. Likewise, the accumulated number of monitor units, MU, for the four-field 3DCRوplan is 54 MU for each field from four-field, while the one-arc DAT plan takes only 213 MU , relation between MU and GY (modulated factor), the treatment delivery time and planning time using two treatment techniques show in table 4. Table 3 compares the prescription dose, mean dose (Dmean), the dose received by 95% of the volume (D95), and the maximum dose (Dmax) to the CTVs for the 3DCRT, DAT for the pituitary-brain tumor treated with 3DCRT and planned with DAT. Figures 8 and 9 show the corresponding isodose distributions and the DVHs, respectively, for 3DCRT and DAT.

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Figure 8: Dose distribution in 3D-CRT and DAT (Bronchogenic cases)

3D-CRT

DAT

Figure 9: Evaluation of the 3D-CRT and the DAT plans in the dose volume histograms to (Bronchogenic cases)

Table 4: Planning delivery time, treatment and modulated factor for the DAT and the 3D-CRT for all the investigated cases.

Diagnoses Planning Delivery Time (sec) Treatment Delivery Time (sec)MU/CGY

Technique ARC 3D-CRT ARC 3D-CRT ARC 3D-CRT

Prostate 660 64.8 306.6 258 2.71 1.44

Pituitary 240 49.2 72 240 1.2 1.4

Bronchogenic451 30 82 155 1.3 1.4

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The conformity of the CTV coverage as well as the spinal cord sparing were found to be better for the DAT technique. However, from 165° to 185°, the arc DAT plan delivered higher maximum doses for heart, as shown in Table 2. Likewise, the accumulated number of monitor units

 

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for the three-field 3D-CRT plan is 81MU for each field , while the one-arc DAT plan takes only 231MU. Relation between MU and GY (modulated factor), the treatment delivery time, and the planning time using the two treatment techniques are shown in Table 4.

Discussion

This study shows that targets at different sites can be adequately treated with 3DCRT, providing a cost-effective advantage in treatment time when compared to DAT, yet keeping a similar dose uniformity of the target and an acceptable dose tolerance to the organs at risk. We showed that, by choosing appropriate DAT based on tumor shape and relative location to organs at risk. Ultimately, selection of a DAT plan versus a 3DCRT one should be based on some flexibility regarding DAT constraints which, in turn, depends on each specific patient. DAT has become the regular choice for radiation therapy of prostate cancer since modulation reduces the rectum dose considerably.

The results presented here closely matched the earlier findings of this study. In a recent clinical study involving treatment of 12 patients with prostate cancer using two fields of arc with anglesfrom165 to 185º wide arc DAT, Jereczek -Fossa et al( Jereczek-,2008).demonstrated that the dose escalation from 78Gy/3 fractions in DAT and 70 Gy/35 in 3DCRT fractions does not increase toxicity; unfortunately, their results have not produced better tumor outcome. From the bladder DVHs in Figure 3, below 40Gy (“low dose”), DAT seems to be the best technique in sparing low doses, while above 40Gy (“high dose”). At full prescribed dose, the mean volume percentages of bladder receiving 78Gy are 9.3% and 17.3% for DAT and 3D-CRT. Figure 4 shows that for rectum, the threshold for “low” and “high” doses to distinguish between higher DAT or 3DCRT toxicity is lower than that of bladder; yet.

At higher dose range, the mean volume percentages of rectum receiving 78Gy provide values of 11.6% and 17.8% for DAT and 3DCRT, respectively. How a lower or a higher dose-volume distribution of toxicity in the bladder or rectum using this radiation-fractionation scheme for prostate cancer ultimately affects these organs is still a topic for further investigations.

Jereczek-Fossa, and Sasaoka et al. showed that a combination of five-field 3DCRT and DAT can reduce the rectal dose compared to a five-field 3DCRT or DAT only(Jereczek-Fossa,2008 and Sasaoka et al.2009 ) .Shiraishi et al., showed that two dynamic-arc therapies with full rotation around two isocenters provided equivalent sparing of normal structures to 3DCRT although with inferior target-dose uniformity(Shiraishi K, Nakagawa K, Yamashita H, Nakamura N, Tago M, Ohtomo K,2006) . Alternatively, Metwaly et al., have demonstrated that a combination of dynamic arcs and two lateral-oblique conformal fields may produce further protection to the rectum(Metwaly M, Awaad AM, El-Sayed el-SM, Sallam AS. J, 2007) . All these studies, including this investigation, reveal the flexibility of DAT forward planning in low-risk prostate cancer as well as its suitability for dose escalation. Modulated factor (MU\time) +1.3% in 3D-CRT than DAT in prostate cancer.

The 3DCRT plan for the pituitary-brain tumor satisfies most of the corresponding DAT con-straints as presented in figure 7. Pituitary adenoma shows the conformity of the CTV coverage as well as t. By choosing different arcs and appropriate angles, the dose exposure on the eyes can be reduced considerably, if required. We developed one -arc coplanar, which deliver a dose reduction in these organs but at the expense of a dose increase in the optic chiasm, optic nerves and brainstem. Thus, if the priority is to protect these latter organs, the one-from 165 to 185° arc plan is

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the best option, but 3DCRT is better than DAT because decrease dose deliver to organ at risk. Modulated factor in two techniques the same.

Plans for lung cancer shown in figure 9 dose delivery to heart ,spinal cord and oseghegus less in developed one arc by angle from 165° to 185° DAT than 3DCRT ,coverage of CTV in two techniques may similar and Modulated factor(MU\time) in bronchogenic carcinoma similar in two technique. This study also evaluated the variation of the tumor dose coverage, by moving the isocenter to a distance of 2mm away from the one in the original plan. Comparing the shifted and the non-shifted tumor DVHs, DAT dose shows more sensitivity than the corresponding 3DCRT dose. This result was expected, given the higher dose gradients in DAT plans, which depends heavily on steep dose gradients between the tumor and organs at risk. (Jiang R, 2007). Thus, the DAT plans offer more chances for deviation from the prescribed dose due to setup errors and/or internal motion of the organs than 3DAT. Given that the number of monitor units, MU, for the 3DCRT is considerably smaller than that in the DAT for all the plans presented in this study, the treatment delivery is fast. By the same token, chances of internal organ motion during delivery are reduced, and the achievability of the on-board imaging can be, therefore, more surely assessed. The MU factors between the 3DCRT and the DAT plans were 3.03 for prostate (average), 3.41 for brain. Considering that the dosimetry of these plans may be clinically acceptable, the significant reduction in the number of monitor units for 3D-CRT make these plans attractive as an alternative to DAT, for both the comfort of the patients during treatment and the associated reduced costs in treatment time.

Conclusions

In this work, we have shown that DAT has the potential to be conventional planning technique for tumors, where there is flexibility in dose constraints to organs at risk. DAT plans with satisfactory dose limitations were developed for prostate cancer. To validate the dose distribution and absolute values in these plans, this study indicates that the implementation of DAT for the treatment of patients with prostate cancer and bronchogenic carcinoma, but in cases of a brain tumor (pituitary adenoma) , is clinically viable 3DCRT is better than DAT.

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Table 5. A summary of the diagnosis, dose prescriptions, and clinical objectives (CObj) for for organs at risk (OAR) of the investigated cases.

Diagnosis

Number of

patients

Technique Radiotherapy dose

prescription

Target volume

CTV

Beam arrangement

Organs at risk dose objectives

Prostate

12

3D-CRT 70GY 94.9 cmз 5 field with angle 0˚,90˚ (wedge 45out),270˚(wedge 45in),45˚(wedge 45out), 315˚(wedge 45in)

rectum max dose=67GY bladder max dose 67GY femoral head=42GY

DAT 78GY 94.9 cmз 2 field (complete arc with angle from165˚ to 185°and complement) and two oblique wedged field(45°) with angle 105˚ and 255˚

rectum max dose=65GY bladder max dose=82GY femoral head=44GY

Pituitary

6

3D-CRT 50.40GY 10.15 cmз 4 field with angle 0˚,90˚(wedge 30 out),270˚(wedge 30in),superior(couch=90 , gantry=90,collimator=90)

Right eye mean dose=.39GY Lt eye mean dose=.44GY Rt optical nerve average max dose=28GY lt optical nerve max dose=32GY brain stem max dose=3869 GY whole brain mean dose=8.27GY

DAT 50.40GY 10.15 cmз 1field (complete arc with angle from165˚ to 185˚)

Right eye mean dose=5.59GY Lt eye mean dose=6.21GY Rt optical nerve average max dose=20GY lt optical nerve max dose=32GY brain stem max dose=4397GY whole brain mean dose=11GY

bronchogenic

4

3D-CRT 56GY 247.1 cmз 3 fields wedged (60,20) with angle 53.2˚, 114.4 ,197.3˚ oblique field

Spinal cord max dose=50GY lung mean dose 97GY v20%=16GY heart max dose=37GY esophagus mean dose=41GY

DAT 56GY 247.1 cmз 1 field (complete arc with angle from165˚ to 185˚)

Spinal cord max dose=37GY lung mean dose 12GY v20%=21GY heart max dose=12GY esophagus mean dose=40GY

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