Journal of Cancer Research and Therapeutics Medknow Publications on behalf of the Association of Radiation Oncologists of India (AROI) ISSN: 0973-1482 EISSN: 1998-4138 Vol. 7, Num. 1, 2011, pp. 64-68

Journal of Cancer Research and Therapeutics, Vol. 7, No. 1, January-March, 2011, pp. 64-68

Brief Communication

MammoSite multilumen catheter: Dosimetry considerations

Akkamma Ravi¹, Susan Lee², Karen Karsif², Adrian Osian³, Dattatreyudu Nori¹

¹ Department of Radiation Oncology, New York Hospital Queens, Flushing, NY 11355; Department of Radiation Oncology, New York Presbyterian Hospital, Weill Cornell Medical College, New York, NY 10065, USA
² Department of Surgery, New York Hospital Queens, Flushing, NY 11355, USA
³ Department of Radiation Oncology, New York Hospital Queens, Flushing, NY 11355, USA

Correspondence Address: Akkamma Ravi, Department of Radiation Oncology, New York Hospital Medical Center of Queens, 56-45, Main Street, Flushing, NY 11355, USA, akr9001@med.cornell.edu

Code Number: cr11013

PMID: 21546745

DOI: 10.4103/0973-1482.80469

Abstract

Keywords: Accelerated partial breast irradiation, balloon catheter, brachytherapy, breast cancer, multilumen MammoSite

Introduction

Accelerated partial breast irradiation (APBI) is a treatment option in breast conservation therapy. The National Surgical Adjuvant Breast and Bowel Project (NSABP B-39) and The Radiation Therapy Oncology group 0413 (RTOG 0413) are evaluating the optimal target volume of whole breast versus partial breast irradiation in early breast cancer patients. Several techniques and applicators exist for APBI therapy, including multi-catheter interstitial brachytherapy, MammoSite device, multilumen catheters (namely, SAVI), Contura, ClearPath, and the more recently introduced MammoSite multilumen (ML) balloon catheter. Each of these devices has different capabilities for dose planning and treatment. The multi-catheter interstitial brachytherapy has shown a low cancer recurrence rate and favorable cosmesis. ^[1],[2],[3] Despite these benefits, the technique requires significant training and expertise. The single lumen (SL) MammoSite catheter balloon device (Hologic Inc, Bedford, MA, USA) is placed intraoperatively by the open cavity technique (OCT) or postoperatively by closed cavity technique. In comparison to the multi-catheter interstitial brachytherapy, this is a simpler technique and does not require elaborate training.

Several dosimetric parameters are used during treatment planning to optimize target coverage and minimize potential toxicity. Many of the initial parameters used to judge appropriateness for treatment are based on dosimetric plans from the SL MammoSite device; however, these dosimetry values are dependent on the location of the device within the breast, the symmetry of the balloon, and the fit of the inflated balloon within the surgical cavity relative to the skin and the rib cage. Often, treatment is not performed because the skin to balloon distance is less than 7 mm. When this occurs, the dosimetric parameters are compromised and there is less than optimal target coverage or a significantly higher dose to normal skin. Pooled multi-institutional investigations of local control and cosmesis of the MammoSite Radiation System have demonstrated favorable results. ^[4] However, these investigations have also revealed some complications associated with the device, including acute skin reactions, infections, seroma formation, telangiectasia, breast pain, edema, rib pain and fat necrosis. In addition, excessive rib doses can result in rib pain and fractures. These complications can be minimized or avoided by shifting the dose away from the ribs. The ML balloon may be advantageous over the SL plan because it is able to shift radiation dosages away from the skin, potentially reducing unwanted toxicity to the healthy skin, ribs, sternum and other subcutaneous structures. Utilization of the ML balloon may also aid in overcoming the present geometric restrictions of the SL balloon with respect to the optimum dose coverage and toxicity to adjacent normal structures. In theory, utilization of the multiple lumens could also assist in dose planning in patients where the skin to balloon distance is <7 mm.

The purpose of this study was to evaluate whether the multiple offset lumens and multiple dwell positions in an ML balloon plan provide superior dosimetry with respect to target (PTV_EVAL) coverage compared to the SL plan. Additionally, we sought to determine whether the ML provided lower doses to the normal structures of the skin and ribs by shifting dose distribution in cases where the skin distance was below 7 mm from the surface of the balloon, when compared to the SL plan.

Materials and Methods

Patients were deemed appropriate for APBI with MammoSite if they were of postmenopausal status, had either invasive ductal carcinoma or ductal carcinoma in situ, had tumors no larger than 3 cm with no evidence of nodal metastases or lymph vascular invasion with margins of 1 mm or greater following lumpectomy, had a balloon surface to skin distance of 7 mm or greater, and had air gaps that were 10% or less of the treatment volume. The NSABP B-39/RTOG 0413 Phase III trial requires that 90% of the PTV_EVAL is covered by 90% of the prescription dose. ^[5] The proximity of the balloon to skin could lead to excessive dose to the normal structures. For this reason, patients with <5 mm skin spacing were considered unacceptable candidates for APBI. At present, the SL single central source position of catheter does not have the capability to optimize the dose without excessively increasing hot spots (V150 or V200) to the skin or ribs. This can result in toxicity and damage to these structures.

The new MammoSite ML catheter is a single insertion, four-lumen balloon brachytherapy applicator [Figure - 1]. The multiple offset lumens can theoretically allow for dosimetric optimization to provide acceptable coverage to the target while minimizing the dose to skin surface and ribs. The three peripheral lumens are equidistant with a separation of 120° [Figure - 2].

In this study, two patients were treated with the ML balloon because the device has only recently become available. Plans using the single central lumen alone for dose distribution to the PTV_EVAL and surrounding normal structures consisting of skin and ribs were documented as per the NSABP B-39 guidelines. ^[5] In addition, rib-balloon-dose was also measured. This dosimetric exercise compared the ability to meet dosimetric goals between central lumen/single-dwell, and multilumen/multidwell treatment plans, using the new MammoSite ML catheter balloon.

Both patients underwent standard surgical lumpectomy to remove the tumors with negative surgical margins. The MammoSite Cavity Evaluation Device (CED), a simple 4-5 cm spherical SL, silicone device, was used in one patient. It was left in the lumpectomy cavity as a placeholder until it was exchanged for the MammoSite ML applicator. In the second patient, the ML balloon was placed under ultrasound guidance in the surgeon′s office at a later time. The balloon was inflated with saline with contrast material to visualize during computed tomography (CT) scan simulation.

Three-dimensional images were evaluated to assess the conformality, skin distance from surface of balloon, balloon-rib dose, air gaps and highest dose regions in the target area. Planning was performed using Eclipse Brachyvision planning system. ^[6] Prior to each treatment, imaging was performed using X-ray or fluoroscopy to assure integrity of the balloon. The high-dose-rate brachytherapy treatments were delivered using Iridium-192 as the source. A total dose of 3400 cGy in 10 treatments, delivered twice-daily 6 hours apart in 5 days, was prescribed at 1 cm from the surface of the balloon. The treatment and planning details using MammoSite balloon have been previously published. ^[6] To thoroughly evaluate the dosimetric advantages of the ML device, two separate plans were created using the same PTV_EVAL for each patient. In the ML plan, four additional lumens were used, each offset by 5 mm from the central catheter lumen [Figure - 1] and [Figure - 2].

At the time of CT for planning, appropriateness of balloon placement was assessed. Patient position and balloon rotational orientation (as indicated by the shaft orientation line) were documented via a picture of catheter and orientation line. Two separate three-dimensional conformal plans were performed for each patient. The first plan was done incorporating only the central SL of the ML balloon catheter and the second plan incorporated the ML to evaluate dose distribution. The following structures were contoured and/or created as a part of the treatment planning process: (a) balloon surface, (b) PTV_EVAL, (c) trapped air and/or fluid, (d) skin surface, and (e) aspect of the closest rib. The target volumes and normal tissue structures were outlined on all CT cuts. When determining the PTV_EVAL dose coverage, the volume of air/fluid percentage was subtracted.

The PTV_EVAL was defined and delineated as the breast tissue volume bounded by the uniform expansion of the balloon radius in all dimensions by 10 mm minus the balloon volume and limited to 5 mm from the skin surface and by the posterior breast tissue extent (chest wall and pectoralis muscles were not to be included). The SL plan was compared with the ML plan to evaluate PTV_EVAL coverage as well as doses to critical normal structures, ribs and skin. The dose covered by the volume of the breast receiving 100, 150 and 200% dose (V100, V150, V200) was documented in each plan.

The total prescribed dose (PD) was 34Gy, delivered to the (PTV_EVAL) in 10 fractions over 5 consecutive working days. The treatment fractions were delivered twice a day with at least 6 hours separating each fraction. Before delivery of each fraction, the patient′s position, balloon inflation, and rotational alignment status were confirmed to be identical with those at the time of initial planning CT. The decision to treat and the final treatment plan used for each patient were based on analyses of the dose volume histogram (DVH) PTV_EVAL coverage, the dose to normal tissues such as skin and ribs, and evaluation of V150 and V200. All treatments were completed using a high-dose-rate remote after-loader and Iridium-192 radioactive source. After completion of treatment delivery, the balloon was deflated and the applicator was removed.

Quality assurance of the dose distribution was recorded as per NSABP B-39 protocol. ^[5] Dosimetric goals were deemed "acceptable" when the DVH analysis of target coverage confirmed that 90% of the PD covered 90% of the PTV_EVAL. The maximum skin dose could not exceed 145% of the PD. The volume of breast tissue receiving 150% (V150) of the PD could not exceed 50 ml. The volume of breast tissue receiving 200% (V200) of the PD should be less than 10 ml. The maximum rib dose was unrestricted.

"Unacceptable" quality was defined as any of the following: 1) dose-volume analyses of the target volume confirming <90% of the prescribed dose and/or <90% coverage of the PTV_EVAL, 2) a maximum skin dose exceeding 145% of the PD, 3) the volume of breast tissue receiving 150% (V150) of the PD exceeded 50 ml or 4) the volume of breast tissue receiving 200% (V200) of the PD exceeded 10 ml. The maximum rib dose was unrestricted.

Plans using the single central lumen alone for dose distribution to the PTV_EVAL and surrounding normal structures consisting of skin and ribs were documented as per the NSABP B-39 guidelines and rib-balloon-dose was also measured. ^[5] The ability to meet NSABP B-39 protocol "acceptable" dosimetric goals was compared between central lumen/single-dwell, and ML/multidwell treatment plans, using the new MammoSite ML catheter balloon in both patients.

Results

For patients 1 and 2, the fill volumes of the balloon were 54.6 and 48.7 ml, respectively. The use of the MLB MammoSite enabled >95% coverage in all plans. Compared to the SL plan, the ML plan resulted in D95 coverage that was 5.3% greater in the first patient and 0.8% greater in the second patient [Table - 1]. For the first patient where the skin-balloon distance was greater than 1 cm and the rib-balloon distance was 1.2 cm, the difference between the SL and ML plans was small. However, while keeping the skin dose of 96% of prescription dose, a higher PTV_EVAL dose of 100.7% was achieved with ML plan. The rib dose was 71% of the PD in both the SL and ML plans. In the second patient, the skin-balloon distance was 7 mm from the surface of the balloon and the dose was 107 and 112% of PD for ML and SL plans, respectively. Additionally, the rib-balloon dose was lower (108% versus 112% of PD) in the ML plan.

The skin dose and rib dose were reduced using the ML plan for patient 2, without excessively increasing hot spots (V150 or V200) [Table - 1]. However, dose homogeneity within the PTV was worse for the ML plan. The volume covered by 150% dose (V150) was increased in the first patient by 3.8 ml and by 2.2 ml in the second patient for the ML plan when compared to SL plan.

The V200 was marginally increased in the first patient by 0.1 ml using ML plan compared to the SL plan, whereas in the second patient the V200 increased by 3.6 ml using the ML plan [Table - 1], revealing that the dose in-homogeneity was higher in the PTV_EVAL area using ML plan.

Discussion

Using an SL balloon is often confounded by excessive skin doses (secondary to inadequate skin spacing), potentially necessitating the removal of the device and abandoning the procedure. A ML balloon has the potential to reduce these complications, particularly in patients where there is a need to optimize the radiation dose relative to skin distance and rib chest wall proximity, as observed in the patient in this report. By utilizing the multiple offset lumens in the ML device, the excessive dose can be directed away from the skin, chest wall and ribs, potentially permitting the use of balloon brachytherapy in cases where the skin-balloon distance is less than optimal. As observed in patient 2, the ML plan reduced the skin and rib doses while allowing acceptable PTV_EVAL doses.

Rib dose limitations have not been traditionally outlined in the inclusion criteria for patients undergoing APBI using MammoSite devices. Currently, the NSABP B-39 protocol does not restrict the maximum rib dose. ^[5] The late toxicities of rib discomfort, skin fibrosis and fat necrosis are a concern following balloon brachytherapy and continue to be reported and are likely related to excessive rib doses during treatment. Cuttino et al. reported a rib dose tolerance of 125% of 340 Gy prescription dose using APBI via ML Contura balloon compared to SL MammoSite balloon. ^[7]

Improved cosmetic results have consistently been correlated with increased balloon surface-to-skin distance. ^[8] Using the single central lumen device, increased distances between the balloon surface and the skin resulted in lower maximum skin doses. It is important to avoid excessive rib dose, as it can lead to rib fractures or chest wall discomfort, in addition to potential toxicity from excessive hot spots (V150 and V200). ^[9],[10]

Based on our experiences and the results of these two patients, superior dosimetric goals can be achieved by using the ML MammoSite balloon. In cases where the skin and ribs are close to PTV_EVAL with a distance of ≤7 mm and a rib distance of <1 cm, the ML plan allows to shift doses away from the skin and rib and maintain a dose coverage of 95% (D95) to 95% of PTV_EVAL.

Conclusions

Long-term toxicities leading to rib discomfort, skin fibrosis and fat necrosis are a concern following balloon brachytherapy. Optimizing the dose to the target and minimizing the excessive hot spots in the normal structures can reduce these complications. In our initial experience, it appears that the dosimetric goals are be better achieved by use of ML MammoSite balloon instead of a SL balloon. The new MammoSite ML balloon catheter appears to be technically advantageous due to multiple lumens that facilitate acceptable dose distribution to the target and normal structures compared to the SL device. This device may be particularly useful for patients who were previously considered unsuitable for accelerated partial breast irradiation due to skin distance of <7 mm. Additional clinical data need to be accrued to elucidate the theoretical dosimetric advantage of the ML MammoSite balloon catheter.

Acknowledgment

The authors thank Samyukta Ravi for the help in the preparation of the manuscript.

References

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