Evaluation of the surface radiation dose and dose gradient in early stage breast cancer using high-dose-rate brachytherapy MammoSite™ applicator

Skin dose assessment with treatment planning system (TPS) and skin reaction evaluation of early breast cancer patients treated via an intraoperative radiation therapy (IORT) device

Purpose

To assess skin dose and incidence of skin reactions in early breast cancer patients treated via Intrabeam™ intraoperative radiation therapy (IORT) device.

Materials and methods

In total, 250 breast cancer patients treated with a single fraction of 20 Gy using 50 kV photon were recruited. The applicator to skin distance (ASD) was measured before the initiation of the radiation and the skin dose in each patient was accordingly calculated based on the treatment planning system (TPS).

Results

The average skin doses calculated were equal to 7·91, 5·83, 3·96 and 2·14 Gy for 6–10, 10–15, 15–20 and 20–30 mm ASD values, respectively. It is noticeable that the skin doses could be lower than the TPS measurements up to 45%, mostly due to lack of backscatter radiation in breast tissue compared with the full scatter condition in the Zeiss water phantom. Finally, only three patients showed low-grade skin reactions 1 week after IORT. A review of the related literature also revealed the incidence of lower skin complications among patients treated via Intrabeam™ compared with MammoSite™ machine.

Conclusions

The Intrabeam™ TPS did not seem to be very reliable for accurate skin dosimetry. However, breast cancer treatment using Intrabeam™ could result in fewer incidences of skin reactions than MammoSite™ machine.

Present state and issues in IORT Physics

Literature was reviewed to assess the physical aspects governing the present and emerging technologies used in intraoperative radiation therapy (IORT). Three major technologies were identified: treatment with electrons, treatment with external generators of kV X-rays and electronic brachytherapy. Although also used in IORT, literature on brachytherapy with radioactive sources is not systematically reviewed since an extensive own body of specialized literature and reviews exists in this field. A comparison with radioactive sources is made in the use of balloon catheters for partial breast irradiation where these are applied in almost an identical applicator technique as used with kV X-ray sources. The physical constraints of adaption of the dose distribution to the extended target in breast IORT are compared. Concerning further physical issues, the literature on radiation protection, commissioning, calibration, quality assurance (QA) and in-vivo dosimetry of the three technologies was reviewed. Several issues were found in the calibration and the use of dosimetry detectors and phantoms for low energy X-rays which require further investigation. The uncertainties in the different steps of dose determination were estimated, leading to an estimated total uncertainty of around 10-15% for IORT procedures. The dose inhomogeneity caused by the prescription of electrons at 90% and by the steep dose gradient of kV X-rays causes additional deviations from prescription dose which must be considered in the assessment of dose response in IORT.

Dose estimation for different skin models in interstitial breast brachytherapy

PURPOSE

Skin is a major organ at risk in breast-conserving therapy (BCT). The American Brachytherapy Society (ABS) recommendations require monitoring of maximum dose received, however, there is no unambiguous way of skin contouring provided. The purpose of this study was to compare the doses received by the skin in different models.

MATERIAL AND METHODS

Standard treatment plans of 20 patients who underwent interstitial breast brachytherapy were analyzed. Every patient had a new treatment plan prepared according to Paris system and had skin contoured in three different ways. The first model, Skin 2 mm, corresponds to the dermatological breast skin thickness and is reaching 2 mm into an external patient contour. It was rejected in a further analysis, because of distinct discontinuities in contouring. The second model, Skin 4 mm, replaced Skin 2 mm, and is reaching 2 mm inside and 2 mm outside of the External contour. The third model, Skin EXT, is created on the External contour and it expands 4 mm outside. Doses received by the most exposed 0.1 cc, 1 cc, 2 cc, and the maximum doses for Skin 4 mm and Skin EXT were compared.

RESULTS

Mean, median, maximum, and standard deviation of percentage dose difference between Skin EXT and Skin 4 mm for the most exposed 0.1 cc (D0.1cc) of skin were 18.01%, 17.20%, 27.84%, and 4.01%, respectively. All differences were statistically significant (p < 0.05).

CONCLUSIONS

Monitoring of doses received by skin is necessary to avoid complications and obtain a satisfactory cosmetic effect. It is difficult to assess the compatibility of treatment plans with recommendations, while there is no unambiguous way of skin contouring. Especially, if a mean difference of doses between two models of skin contouring is 18% for the most exposed 0.1 cc and can reach almost 28% in some cases. Differences of this magnitude can result in skin complications during BCT.

Brachytherapy in accelerated partial breast irradiation (APBI) - review of treatment methods

Breast conserving surgery (BCS) with following radiotherapy (EBRT) of the conserved breast became widely accepted in the last decades as the treatment of early invasive breast cancer. In an early stage of breast cancer, research has shown that the area requiring radiation treatment to prevent cancer from local recurrence is the breast tissue that surrounds the area where the initial cancer was removed. Accelerated partial breast irradiation (APBI) is an approach that treats only the lumpectomy bed with 1-2 cm margin, rather than the whole breast and as a result allows accelerated delivery of the radiation dose in four to five days. Published results of APBI are very promising. It is evident that APBI will play a role in the management of a selected group of early breast cancer. We discuss current status, indications, technical aspects and recently published results of APBI using different brachytherapy techniques.

Determination of exit skin dose for 192Ir intracavitary accelerated partial breast irradiation with thermoluminescent dosimeters

PURPOSE

Intracavitary accelerated partial breast irradiation (APBI) has become a popular treatment for early stage breast cancer in recent years due to its shortened course of treatment and simplified treatment planning compared to traditional external beam breast conservation therapy. However, the exit dose to the skin is a major concern and can be a limiting factor for these treatments. Most treatment planning systems (TPSs) currently used for high dose-rate (HDR) 192Ir brachytherapy overestimate the exit skin dose because they assume a homogeneous water medium and do not account for finite patient dimensions. The purpose of this work was to quantify the TPS overestimation of the exit skin dose for a group of patients and several phantom configurations.

METHODS

The TPS calculated skin dose for 59 HDR 192Ir APBI patients was compared to the skin dose measured with LiF:Mg,Ti thermoluminescent dosimeters (TLDs). Additionally, the TPS calculated dose was compared to the TLD measured dose and the Monte Carlo (MC) calculated dose for eight phantom configurations. Four of the phantom configurations simulated treatment conditions with no scattering material beyond the point of measurement and the other four configurations simulated the homogeneous scattering conditions assumed by the TPS. Since the calibration TLDs for this work were irradiated with 137Cs and the experimental irradiations were performed with 192Ir, experiments were performed to determine the intrinsic energy dependence of the TLDs. Correction factors that relate the dose at the point of measurement (center of TLD) to the dose at the point of interest (basal skin layer) were also determined and applied for each irradiation geometry.

RESULTS

The TLD intrinsic energy dependence for 192Ir relative to 137Cs was 1.041 +/- 1.78%. The TPS overestimated the exit skin dose by an average of 16% for the group of 59 patients studied, and by 9%-15% for the four phantom setups simulating treatment conditions. For the four phantom setups simulating the conditions assumed by the TPS, the TPS calculated dose agreed well with the TLD and MC results (within 3% and 1%, respectively). The inverse square geometry correction factor ranged from 1.023 to 1.042, and an additional correction factor of 0.978 was applied to account for the lack of charged particle equilibrium in the TLD and basal skin layer.

CONCLUSIONS

TPS calculations that assume a homogeneous water medium overestimate the exit skin dose for intracavitary APBI treatments. It is important to determine the actual skin dose received during intracavitary APBI to determine the skin dose-response relationship and establish dose limits for optimal skin sparing. This study has demonstrated that TLDs can measure the skin dose with an expanded uncertainty (k = 2) of 5.6% when the proper corrections are applied.

Review of MammoSite brachytherapy: advantages, disadvantages and clinical outcomes

BACKGROUND

The MammoSite radiotherapy system is an alternative treatment option for patients with early-stage breast cancer to overcome the longer schedules associated with external beam radiation therapy. The device is placed inside the breast surgical cavity and inflated with a combination of saline and radiographic contrast to completely fill the cavity. The treatment schedule for the MammoSite monotherapy is 34 Gy delivered in 10 fractions at 1.0 cm from the balloon surface with a minimum of 6 hours between fractions on the same day.

MATERIAL AND METHODS

This review article presents the advantages, disadvantages, uncertainties and clinical outcomes associated with the MammoSite brachytherapy (MSB).

RESULTS

Potential advantages of MSB are: high localised dose with rapid falloff for normal tissue sparing, minimum delay between surgery and RT, catheter moves with breast, improved local control, no exposure to staff, likely side-effects reduction and potential cost/time saving (e.g. for country patients). The optimal cosmetic results depend on the balloon-to-skin distance. Good-to-excellent cosmetic results are achieved for patients with balloon-skin spacing of > or =7 mm. There have been very few published data regarding the long term tumour control and cosmesis associated with the MSB. The available data on the local control achieved with the MSB were comparable with other accelerated partial breast irradiation techniques. The contrast medium inside the balloon causes dose reduction at the prescription point. Current brachytherapy treatment planning systems (BTPS) do not take into account the increased photon attenuation due to high Z of contrast. Some BTPS predicted up to 10% higher dose near the balloon surface compared with Monte Carlo calculations using various contrast concentrations (5-25%).

CONCLUSION

Initial clinical results have shown that the MammoSite device could be used as a sole radiation treatment for selected patients with early stage breast cancer providing good local control, minimal complication rate and excellent cosmesis.

Placement of MammoSite brachytherapy catheter under computed-tomography scan guidance

Placement of the MammoSite breast brachytherapy catheter is most commonly performed either intraoperatively or under ultrasound-guided technique. Below, we present a case report of an alternate approach utilizing CT-scan guidance. This is the first reported case of a balloon brachytherapy catheter placement with this technique.

Monte Carlo-derived TLD cross-calibration factors for treatment verification and measurement of skin dose in accelerated partial breast irradiation

Monte Carlo simulation was employed to calculate the response of TLD-100 chips under irradiation conditions such as those found during accelerated partial breast irradiation with the MammoSite radiation therapy system. The absorbed dose versus radius in the last 0.5 cm of the treated volume was also calculated, employing a resolution of 20 microm, and a function that fits the observed data was determined. Several clinically relevant irradiation conditions were simulated for different combinations of balloon size, balloon-to-surface distance and contents of the contrast solution used to fill the balloon. The thermoluminescent dosemeter (TLD) cross-calibration factors were derived assuming that the calibration of the dosemeters was carried out using a Cobalt 60 beam, and in such a way that they provide a set of parameters that reproduce the function that describes the behavior of the absorbed dose versus radius curve. Such factors may also prove to be useful for those standardized laboratories that provide postal dosimetry services.

Accelerated partial breast irradiation as part of breast conserving therapy of early breast carcinoma: a systematic review

New strategies for adjuvant radiotherapy of early breast cancer are being investigated in several phase III randomised trials at the present time. Accelerated partial breast irradiation (APBI) is a way to offer an early breast cancer patient, who has had breast conservative surgery, an adjuvant radiotherapy of short duration aimed at the tumour bed with a certain margin. The rationale of this strategy is that most local recurrences appear close to the tumorectomy cavity and a wish to spare the patient late radiation morbidity. This review discusses the background for APBI, the different techniques, and we highlight possible pitfalls using these techniques. A systematic overview of all phase I and II studies is provided. Patient selection for this therapy is pivotal and based on evidence from previous studies on patient/tumour characteristics and pattern of local recurrences we propose inclusion criteria for patients in APBI protocols.