Dosimetric Impact of Inter-Fraction Variation in Interstitial HDR Brachytherapy

doi:10.4236/ijmpcero.2013.24015

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International Journal of Medical Physics, Clinical Engineering and Radiation Oncology, 2013, 2, 111-116

Published Online November 2013 (http://www.scirp.org/journal/ijmpcero)

http://dx.doi.org/10.4236/ijmpcero.2013.24015

Open Access IJMPCERO

Dosimetric Impact of Inter-Fraction Variation in

Interstitial HDR Brachytherapy

Saravanan Kandasam y1*, K. S. Reddy1, Vivekanandan Nagarajan2,

Parthasarathy Vedasoundaram1, Gunaseelan Karunanidhi1

1Department of Radiotherapy, Regional Cancer Centre, Jawaharlal Institute of Postgraduate Medical Education

and Research (JIPMER), Puducherry, India

2Medical Physics Department, Cancer Institute (WIA), Chennai, India

Email: *[email protected]

Received August 9, 2013; revised September 10, 2013; accepted October 1, 2013

License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ABSTRACT

Background: Patient setup errors in External Beam Radiotherapy (EBRT) are minimized to a great extent, due to recent

technological developments, but in contrary brachytherapy received less attention in inter-fraction catheter movement

and its impact in dose delivery. This article deals with inter-fraction interstitial catheter movement and its impact in

dose delivery to the target. An attempt is made to study the dosimetric impact of this variation. Objectives: The objec-

tive of the study is to evaluate the inter-fraction variation in the position of implanted interstitial applicators and to as-

sess the dosimetric impact in interstitial High Dose Rate (HDR) brachytherapy. Materials and Methods: 55 patients

treated for carcinoma tongue, breast, buccal mucosa, cervix, floor of the mouth and soft tissue sarcoma over a period of

2 years (December 2011-May 2013) were considered. All the patients underwent CT scan the next day of the implant-

ing and 3D planning was done either by Eclipse or Oncentra Master Plan Treatment Planning System (TPS). Patients

were treated by HDR brachytherapy remote after-loading units, either by Gamma Med iX plus or Microselectron. At the

end of the last fraction, CT scan was repeated and re-planning done. The variation in the position of the implanted ap-

plicators/catheters and its impact on dosimetric parameters was evaluated and analyzed. Results: The range of posi-

tional displacement of the interstitial catheters ranges from 4.5 mm to 6.8 mm. The maximum variation in prescribed

dose to D90 of Clinical Target Volume was 10.88%. Conclusions: If the total duration of the interstitial implant of HDR

brachytherapy extends for more than a week from the day of imaging, it is recommended to do CT imaging and re-plan

again. It is mandatory to suture the buttons of the implanting to the skin. Edema and movement of organs (e.g., tongue)

are the main cause for the positional variation of the catheters.

Keywords: HDR; Brachytherapy; Interstitial Implant; Inter-Fraction Variation; Dosimetric Variation

1. Introduction

The worldwide incidence of squamous cell carcinoma of

head and neck is more than 500,000 cases per year and

the management of patients with head and neck cancer is

complex [1]. The choice of treatment modality depends

on the stage and site of the disease. Brachytherapy plays

an integral role in the management of head and neck can-

cers and has been described as the first form of confor-

mal radiation [2]. Precise source placement enables very

high doses within the tumor and sufficient dose at the

margin between the tumor and normal tissue ensuring

high tumor control. At the same time, only small vol-

umes of normal tissue are irradiated thus decreasing the

normal tissue complications.

Brachytherapy involves the implantation of plastic

catheters (applicators) into tumors. In 1904, Wickham

and Derais used sharpened goose quills to perform intra-

tumoral implantations [3]. Abbe and Morton have re-

ported anecdotal reports of cure for cancers in the head

and neck with brachytherapy [4,5]. In 1909, Minet de-

scribed the first use of a radium tube placed in a catheter

to treat prostate cancer [6]. Traditionally treatment plan-

ning of brachytherapy was mainly based on radiographs

and point dosimetry [6]. The dose distribution was re-

lated to the geometry of the catheters. With the newer

three dimensional treatment planning together with CT

*Corresponding author.

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112

imaging, it is possible to get a 3D based dose distribution

with reconstruction of the tumor volume and the cathe-

ters [7].

Interstitial implant procedure involves in using a metal

needle or trocar. The skin is pierced at the planned entry

site and coursed along in the tumor volume, exiting at the

marked skin site at the other end of the target volume.

Once in place, a nylon after-loading catheter is then ad-

justed such that the wider portion of the after-loading

catheter encompasses the target volume. The trocar is

then removed along its original pathway while holding

the implanted catheter in place. A metal button along

with a half-moon-shaped plastic button is then threaded

over the exposed ends of the catheter and crimped in

placed over the skin entry sites. Silk ties can then be su-

tured to the skin through the available holes in the button

for further stabilization.

The exposed end of the catheter is then cut off leaving

at least several centimeters distal to the metal button.

These series of steps would then be repeated, resulting in

a parallel distribution of catheters along the tumor vol-

ume (Figure 1). Depending upon the site, a patient’s co-

morbidities and extent of implanting, the procedure can

be done under local or general anaesthesia. Since brachy-

therapy treatment is delivered over many days, the im-

planted catheters can move either due to oedema or move-

ment of the implanted organ.

The positional stability of the catheters and the resul-

tant dosimetric variation over a period of time is studied

and presented.

2. Objectives

The objective of the study is to quantify the variation in

the position of implanted applicators during treatment

and assess the dosimetric impact of this variation on

Clinical Target Volume (CTV) in interstitial HDR brachy-

therapy.

2.1. Materials and Methods

The remote after-loading HDR Brachytherapy treatment

unit GammaMed plus iX plus (Varian Medical Systems,

Palo Alto, CA) or Microselectron HDRV3 (Nucletron,

BV) using single sealed Iridium 192 radioactive source

was used. For treatment planning Eclipse (Varian Medi-

cal Systems, Palo Alto, CA) or Oncentra Master plan

(Nucletron, BV) was used. Images for planning were ac-

quired by CT Somatom spirit (Siemens). Fifty five pa-

tients were included in this study from December 2011 to

May 2013 (Tab le 1 ). The patients were treated after eva-

luation according to the stage of the disease.

2.2. Treatment Protocol

Patients were treated according to the institutional pro-

Figure 1. Patient with flexible interstitial implant.

Table 1. Patient characteristics.

Characteristics No of Patients (%)

Age

Mean 50.1

Standard Deviation 10.3

Median 48

Range 32 - 73

Gender

Male 26 (47.27%)

Female 29 (52.72%)

Diagnosis

Carcinoma Breast 13 (23.63%)

Carcinoma Buccal Mucosa 17 (30.91%)

Carcinoma Cervix 1 (1.82%)

Carcinoma Floor of Mouth 2 (3.64%)

Carcinoma Tongue 19 (34.55%)

Soft Tissue Sarcoma (Multiple Site) 3 (5.45%)

T Stage*

T1 6 (10.91%)

T2 20 (36.36%)

T3 29 (52.73%)

T4 0 (0%)

N Stage*

N0 42 (76.36%)

N1 10 (18.18%)

N2 3 (5.45%)

N3 0

*According to the 7th American Joint Commission on Cancer/Union for

International Cancer Control Staging system.

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Table 2. Institutional treatment protocol.

Diagnosis EBRT* HDR**

Carcinoma

Breast

200 cGy x

20 fractions

250 cGy x

6 fractions

Carcinoma

Buccal Mucosa

200 cGy x

25 fractions***

350 cGy x

6 fractions

Carcinoma

Cervix

200 cGy x

25 fractions

400 cGy x

5 fractions

Carcinoma

Floor of Mouth

200 cGy x

25 fractions***

350 cGy x

6 fractions

Soft Tissue

sarcoma

200 cGy x

20 fractions

250 cGy x

6 fractions

Carcinoma

Tongue

200 cGy x

25 fractions***

350 cGy x

6fractions

*Five fractions per week with one fraction per day. **Two fractions per

day with 6 hours gap between the two fractions. ***With Spinal shield

after 44 Gy.

tocol (Table 2).

2.3. Interstitial Implant Application

Under general anaesthesia, trocars and hollow needles

were inserted as guide tubes in and around the tumor 1

cm apart in single or multiple planes through which plas-

tic tubes were threaded. Theses tubes were then secured

by buttons. Similarly for rigid needle implant, the steril-

ized needles with the appropriate length were selected.

With the guidance of templates the needles were inserted

into the tissue. The template helped to maintain a proper

geometry of the needle placement. The needles were se-

cured by stainless steel buttons. Table 3 gives the details

of the interstitial implants.

2.4. Imaging and Planning

On the second day of implantation, the patients under-

went CT scan of the involved region with a slice thick-

ness of 1 mm. The applicator reconstruction was done

and at the tip of all the applicators a reference point was

inserted (Figure 2). The source dwell positions and step

size were identified and accordingly the fine tuning of

dose optimization was performed by changing the dwell

time and dwell weight age for individual dwell positions.

In most cases, dwell time was changed to reduce the hot

spot or to remove the cold spot. Graphical optimization

was never used. It was ensured that at least 90% of the

CTV receives the prescribed dose. The dose distribution

was generated by TPS using the AAPM TG-43 dose

formalism [5]. Treatment was delivered using the HDR

remote after-loading system. On the last fraction a repeat

CT and re-planning was done and the catheters were re-

moved.

For each patient planning was done on both Pre HDR

and Post HDR images. Pre HDR and Post HDR images

were fused and matched using a prominent anatomical

land mark. The step size, dwell position and dwell time

was maintained the same in both the plans and only the

Table 3. Institutional treatment protocol.

Type No of Patients (%)

Rigid Needle Implants 13 (23.64%)

Flexible Catheter Implants 42 (76.36%)

Number of Planes

Single Plane 17 (30.91%)

Double Plane 34 (61.82%)

Triple Plane 4 (7.27%)

(a)

(b)

Figure 2. (a) Reconstructed rigid needle implant with ref-

erence points—Cancer Right Breast; (b) Reconstructed

flexible implant with reference points—Cancer Left Tongue.

catheter position was updated in the post HDR brachy-

therapy plan. The tip of the catheters where the referen-

cepoints were inserted gave the co-ordinates in x, y and z

axis. The variation in the reference points between the

two plans were estimated which gave the actual dis-

placement in the catheter position in 3D axis. Using the

Dose Volume Histogram (DVH) the dosimetric parame-

ters were studied.

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3. Result

A total of 55 patients were treated between December

2011 to May 2013. 110 brachytherapy plans were gener-

ated. The demographic details of the patients included in

the study are listed in Table 1. In three patients, brachy-

therapy was the sole modality of treatment. All other

patients were treated with both external beam radiation

and brachytherapy.

Table 4 gives the details of the mean dose variation

(%) to D90 of CTV. Rigid implant was used for patients

with carcinoma breast and cervix (25.45%). For all the

other patients flexible catheter implants were used

(74.55%). Table 5 gives the variation in dose to the vol-

ume receiving 150% of the prescribed dose.

For patients with carcinoma breast the displacement in

catheter position is shown in Figure 3. For all the pa-

tients the catheter displacement and D90 dose to CTV is

less than 3 mm and 3% respectively.

For patients with carcinoma buccal mucosa the cathe-

ter displacement for 35.29% of the patients was more

than 5 mm (Figure 4). In 29.41% of the patients D90

dose to CTV was more than 3%.

For patients with carcinoma tongue the displacement

in catheter position is shown in Figure 5. The catheter

displacement for 42.11% of patients was more than 5mm.

As per DVH in 26.32% of the patients D90 dose to CTV

was more than 3%.

Figure 6 shows the catheter displacement in patients

with soft tissue sarcoma (3 patients), carcinoma floor of

mouth (2 patients) and carcinoma cervix (1 patient).

In Figure 7 the maximum dosimetric variation (D90) to

CTV is shown with respect to the time interval in days

between the Pre HDR and Post HDR plans. In 15 patients

the duration between the two plans was 5 days, in 10

patients it was 6 days. In 5 patients the time interval was

7 days and in 8 patients it was 8 days. 5 patients (9.09%)

had 9 days and 2 patients (3.67%) had 10 days interval

between the plans. The variation in intervals between

plans was because of treatment protocol, holidays and

machine down time.

4. Discussion

Feng Xu et al. evaluated interfractional and intrafrac

Table 4. Dosimetric variation in percentage to D90 of CTV.

Type of Tumor Mean Value + 1 Stand ard Deviation

Carcinoma Breast 0.423 ± 1.72 (13 Patients)

Carcinoma Buccal Mucosa 1.16 ± 3.88 (17 Patients)

Carcinoma Tongue 1.22 ± 2.71 (19 Patients)

Soft Tissue Sarcoma 1.53 ± 4.66 (3 Patients)

Carcinoma Floor of Mouth 4.6 ± 5.37 (2 Patients)

Carcinoma Cervix (MUPIT) 5.72 (1 Patient)

D90-Dose Received by at Least 90% of the Volume.

Table 5. Dosimetric variation to volume receiving 150% of

dose.

Type of Tumor Variation in Volume Receiving

More than 150% of Dose

Carcinoma Breast 1.8 cm3 (1.76%)

Carcinoma Buccal Mucosa −6.42 cm3 (−2.14%)

Carcinoma Tongue 1.04 cm3 (7.91%)

Soft Tissue Sarcoma 1.3 cm3 (2.31%)

Carcinoma Floor of Mouth 2.8 cm3 (2.85%)

Carcinoma Cervix (MUPIT) 1.2 cm3 (1.35%)

Figure 3. Catheter displacement for carcinoma breast.

Figure 4. Catheter displacement for carcinoma buccal mu-

cosa.

tional setup error in external beam radiotherapy using

cone beam computed tomography as per their result a

total of 1934 CBCT scans were performed on 51 patients,

the setup errors were ≤2 mm on all axes [8] similarly

Slow T. R. et al. has estimated interfraction prostate mo-

tion during intensity modulated radiotherapy for prostate

cancer of 157 patients with prostate cancer the mean

setup errors 21 mm [9], whereas in interstitial HDR Bra-

chytherapy we have estimated that the catheter positional

variation is >6 mm for few cases which was not

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Figure 5. Catheter displacement for Carcinoma Tongue.

Figure 6. Catheter displacement for soft tissue sarcoma (1 -

3), carcinoma floor of mouth (4,5) and carcinoma cervix (6).

Figure 7. Dosimetric variation with time.

taken into account seriously. Velumurugan et al. evalu-

ated MUPIT implantation in patients with carcinoma

cervix. In their study they recommended to repeat CT

and re-plan before each fraction [10]. In our study imag-

ing and planning was done before the first fraction. This

plan was used for treatment delivery. At the end of the

last fraction imaging and re-planning was done again and

this plan was used only for analysis. Velumurugan et al.

observed that the CTV mean dose varies from +9.8% to

−13.3%. Out of 10 patients the mean dose was more than

the prescribed dose in seven patients and less in three

patients [10]. The maximum variation to CTV dose was

estimated to be around 13%. In majority of the cases the

variation was around 1%. However in their study the

physical movement of the needles was not estimated.

Similarly Rey F., et al. in their study estimated the day 1

plan on the day 2 and day 3. CT and re-plan was done

before each treatment, with updated catheter positions

the treatment was delivered [8]. The mean CTV D90 was

reduced from 93.4% on day1 to 89.3% on day 2 and to

87.7% on day 3. Re-planning on day 2 and day 3 com-

pensated for catheter movement. They have also not es-

timated the positional variation of catheters. In our study

we have estimated the positional variation of catheters

and its dosimetric implant and we have also extended the

study to different sites of implants and given the com-

parison. Similarly we have also given the comparison

between rigid and flexible implants. As per International

Commission on Radiation Units and Measurements

(ICRU 58) the dosimetric variation should be limited to

3% of D90 to CTV [11]. As per our results it has been

identified that for carcinoma tongue the variation in

catheter dislocation is relatively more when compared to

other sites because of the movement of the organ. Hence,

dosimetric variation is more for implants of carcinoma

tongue. Our data also shows those rigid needle implants

are more stable compared to flexible implant with less

dosimetric variation for the former.

5. Conclusion

An inter-fraction error is reported frequently in HDR-

brachytherapy. Rigid needle implants are dosimetrically

superior to flexible implants. Suturing of the buttons in

flexible implanting is not a solution for arresting the

catheter movement, since it immobilizes the button alone

and the catheter movement is still possible. Adequate

care should be taken while connecting the catheters to the

HDR brachytherapy unit with transfer tubes to prevent

physical displacement of the catheters. With an increase

in duration of treatment, the inter-fraction error increases

and it is recommended to repeat CT and re-plan on the

fifth day from the day of implanting.

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