Dosimetric evaluation of breast radiotherapy in a dynamic phantom

Hybrid Treatment Planning for Chest Wall Irradiation Utilizing Three-Dimensional Conformal Radiotherapy (3DCRT), Intensity-Modulated Radiation Therapy (IMRT), and Volumetric Modulated Arc Therapy (VMAT): A Systematic Review

Novel hybrid approaches for chest wall irradiation show promising outcomes regarding target coverage and sparing organs at risk (OARs). In this systematic review, we compared hybrid volumetric modulated arc therapy (H-VMAT) or hybrid intensity-modulated radiotherapy (H-IMRT) techniques with non-hybrid techniques, such as three-dimensional conformal radiation therapy (3DCRT), field-in-field (FIF), intensity-modulated arc therapy (IMRT), and volumetric modulated arc therapy (VMAT), for breast cancer patients with mastectomy. Our focus was the plan quality and dose distribution to the OARs. Using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) checklist, we performed a systematic review and quality appraisal of primary studies evaluating hybrid therapy to the chest wall and the OARs. An extensive online search of PubMed and Scopus databases was conducted using appropriate keywords. The dose to the OARs (lung, heart, and contralateral breast), planning target volume (PTV), homogeneity index (HI), and conformity index (CI) were extracted. The data were then tabulated and compared for the outcomes between modalities among the studies. Nine studies that met the search criteria were selected to evaluate the PTV coverage and dosimetric results of hybrid and non-hybrid techniques. In terms of 95% PTV coverage, among nine reviewed studies, the largest difference between the two techniques was between VMAT (47.6 Gy) and H-VMAT (48.4 Gy); for the conformity index, the largest difference was noted between 3DCRT (0.58) and H-VMAT (0.79). In both cases, differences were statistically significant (P < 0.005). Two studies showed dose homogeneity improvement within the treatment target in H-VMAT (0.15 and 0.07) compared with 3DCRT (0.41 and 0.12), with a P value of <0.001. Two studies did not report on the homogeneity index, and three others observed no statistical difference. Regarding OARs, in the comparison of H-VMAT and VMAT, the largest significant change was in the volume receiving 5 Gy (V5Gy) of the ipsilateral lung and the V10Gy of the contralateral lung. For the ipsilateral lung, V5Gy was 90.7% with VMAT versus 51.45% with H-VMAT. For the contralateral lung, V10Gy was 54.9% with VMAT versus 50.5% with H-VMAT. In six studies, the mean dose of the contralateral breast was lower in hybrid techniques than in single modalities: VMAT (4.2%, 6.0%, 1.9%, 7.1%, 4.57%) versus H-VMAT (1.4%, 3.4%, 1.8%, 3.5%, 2.34%) and IMRT (9.1%) versus H-IMRT (4.69%). Although most studies did not report on monitor units and treatment time, those that included them showed that hybrids had lower monitor units and shorter treatment times. Hybrid techniques in radiotherapy, such as combining two modalities, can indeed facilitate lower doses to OARs for patients with a high risk of toxicities. Prospective clinical studies are needed to determine the outcomes of breast cancer treated with hybrid techniques.

Verification of the junctional dose for irradiation of the chest wall and supraclavicular regions under the circumstances of advanced technologies

A single-isocenter half-beam technique is commonly used when irradiating the chest wall and supraclavicular regions in patients with high-risk breast cancer. However, several studies have reported that underdosage can occur at the junction of the chest wall and supraclavicular regions due to a "tongue-and-groove" effect. This study verified the efficacy of an open leaf technique (OL-tech) that involves placing a multileaf collimator 5 mm outside from the beam central axis to remove the effect of the multileaf collimator in a single-isocenter half-beam technique. We compared the junction doses of the OL-tech with those of a conventional technique (C-tech) in square and clinical plans, using 4 and 10 MV x-rays in the Clinac iX and 6 and 10 MV x-rays in the Trilogy accelerators (Varian Medical Systems, Palo Alto, CA). EBT3 radiochromic films were used for measurements. Measurements were performed at a depth of 3 cm when verifying field matching. The EBT3 films in the square plan indicated junction doses for the C-tech of 78.3% with the Clinac iX accelerator and 73.6% with the Trilogy accelerator. By contrast, the corresponding doses for the OL-tech were 107.2% and 99.8%, respectively. In the clinical plan, the junction doses for the C-tech were 76.5% with the Clinac iX accelerator and 72.6% with the Trilogy accelerator; the corresponding doses for the OL-tech were 108.3% and 101.7%, respectively. As with the square plan, variations in the junction dose were much smaller using the OL-tech than using the C-tech. Our results suggest that the OL-tech can be useful for improving dose homogeneity at the junction of the chest wall and supraclavicular regions.

Accounting for respiratory motion in partial breast intensity modulated radiotherapy during treatment planning: a new patient selection metric

PURPOSE

External beam partial breast irradiation intensity modulated radiotherapy (PBI IMRT) plans experience degradation in coverage and dose homogeneity when delivered during respiration. We examine which characteristics of the breast and seroma result in unacceptable plan degradation due to respiration.

METHODS

Thirty-six patient datasets were planned with inverse-optimised PBI IMRT. Population respiratory data were used to create a probability density function. This probability density function (PDF) was convolved with the static plan fluences to calculate the delivered dose with respiration. To quantify the difference between static and respiratory plan quality, we analysed the mean dose shift of the target dose volume histogram (DVH), the dose shift at 95% of the volume and the dose shift at the hotspot to 2 cm(3)of the volume. We explore which patient characteristics indicate a clinically significant degradation in delivered plan quality due to respiration.

RESULTS

Dose homogeneity constraints, rather than dosimetric coverage, were the limiting factors for all patient plans. We propose the dose evaluation volume-to-planning target volume (DEV-to-PTV) ratio as a delineating metric for identifying patient plans that will be more degraded by respiratory motion. The DEV-to-PTV ratio may be a more robust metric than ipsilateral breast volume because the seroma volume is contoured more consistently between physicians and clinics.

CONCLUSIONS

For patients with a DEV-to-PTV ratio less than 55% we recommend either not using PBI IMRT or employing motion management. Small DEV-to-PTV ratios occur when the seroma is close to inhomogeneities (i.e. air/lung), which exacerbates the dosimetric effect of respiratory motion. For small breast sizes it is unlikely that the DEV-to-PTV ratio will meet these criteria.

A two-point scheme for optimal breast IMRT treatment planning

We propose an approach to determining optimal beam weights in breast/chest wall IMRT treatment plans. The goal is to decrease breathing effect and to maximize skin dose if the skin is included in the target or, otherwise, to minimize the skin dose. Two points in the target are utilized to calculate the optimal weights. The optimal plan (i.e., the plan with optimal beam weights) consists of high energy unblocked beams, low energy unblocked beams, and IMRT beams. Six breast and five chest wall cases were retrospectively planned with this scheme in Eclipse, including one breast case where CTV was contoured by the physician. Compared with 3D CRT plans composed of unblocked and field-in-field beams, the optimal plans demonstrated comparable or better dose uniformity, homogeneity, and conformity to the target, especially at beam junction when supraclavicular nodes are involved. Compared with nonoptimal plans (i.e., plans with nonoptimized weights), the optimal plans had better dose distributions at shallow depths close to the skin, especially in cases where breathing effect was taken into account. This was verified with experiments using a MapCHECK device attached to a motion simulation table (to mimic motion caused by breathing).

External respiratory motion analysis and statistics for patients and volunteers

We analyzed a large patient and volunteer study of external respiratory motion in order to develop a population database of respiratory information. We analyzed 120 lung, liver, and abdominal patients and 25 volunteers without lung disease to determine the extent of motion using the Varian Real‐Time Position Management system. The volunteer respiratory motion was measured for both abdominal and thoracic placement of the RPM box. Evaluation of a subset of 55 patients demonstrates inter‐ and intrafraction variation over treatment. We also calculated baseline drift and duty cycle for patients and volunteers. The mean peak‐to‐peak amplitude (SD) for the patients was 1.0 (0.5) cm, and for the volunteers it was abdomen 0.8 (0.3) cm and thoracic 0.2 (0.2) cm. The mean period (SD) was 3.6 (1.0) s, 4.2 (1.1) s, and 4.1 (0.8) s, and the mean end exhale position (SD) was 60% (6), 58% (7), and 56% (7) for patient, volunteer abdomen, and volunteer thoracic, respectively. Baseline drift was greater than 0.5 cm for 40% of patients. We found statistically significant differences between the patient and volunteer groups. Peak‐to‐peak amplitude was significantly larger for patients than the volunteer abdominal measurement and the volunteer abdominal measurement is significantly larger than the volunteer thoracic measurement. The patient group also exhibited significantly larger baseline drift than the volunteer group. We also found that peak‐to‐peak amplitude was the most variable parameter for both intra‐ and interfraction motion. This database compilation can be used as a resource for expected motion when using external surrogates in radiotherapy applications.

PACS number: 87.19.Wx, 87.55.Km