Breast irradiation with three conformal photon fields for patients with high lung involvement

Accelerated partial breast irradiation using external beam radiotherapy-A feasibility study based on dosimetric analysis

AIM

To investigate the feasibility of using External Beam radiotherapy for accelerated partial breast irradiation by a comparative tumour and normal tissue dose volume analysis with that of high dose rate interstitial brachytherapy.

BACKGROUND

Accelerated Partial Breast Irradiation (APBI) is more clinically appealing because of the reduced treatment course duration and the irradiated area. Brachytherapy application is more dependent on the clinician's expertise when it is practised free hand without image guidance and a template. It happens to be an invasive procedure with the use of local anaesthesia which adds patient discomfort apart from its cost compared to External Beam Radiotherapy. But APBI with brachytherapy is more commonly practised procedure compared to EBRT owing to its previous reults. Hence in this research study, we intend to explore the use of EBRT with the radiobiological corrections for APBI in the place of brachytherapy. It is done as a dosimetric comparison of Brachytherapy treatment plans with that of EBRT plans.

MATERIALS AND METHODS

The computed tomography images of 15 patients undergoing ISBT planning were simulated with conformal photon fields. Various dose volume parameters of each structure were obtained from the DVH generated in the brachytherapy and the simulated external beam planning which can correlate well with the late toxicity. The plan quality indices such as conformity index and homogeneity index for the target volume were computed from the dosimetric factors. The statistical p values for CI, HI and normal tissue dosimetric parameters were calculated and the confidence levels achievable were analysed. The dose prescribed in brachytherapy was 3400cGy in ten fractions. The equivalent prescription dose for the external beam radiotherapy planning was 3000cGy in five fractions applied with radiobiological correction.

RESULTS

All the fifteen patients were with complete lung data and six were with left sided tumours having complete cardiac data. The lung dosimetry data and the cardiac dosimetry data of the patients were studied. Lower percentages of lung and cardiac V 20 and V 5 volumes were obtained with conformal planning. The conformity of radiation dose to the tumour volume was akin to the interstitial brachytherapy planning. Moreover the external beam planning resulted in more homogenous dose distribution. For the sampled population, the statistical analysis showed a confidence level of 95% for using EBRT as an alternate to multi catheter ISBT.

CONCLUSION

The EBRT planning for Accelerated Partial Breast Irradiation was found to be technically feasible in the institution where the interstitial brachytherapy happens to be the only available technique as evident from the dose volume parameters and the statistical analysis.

Clinical experience in breast irradiation with intensity modulated photon beams

Since 2002, twenty-five patients affected by breast cancer, for whom extensive lung/heart involvement was expected from conventional techniques, were irradiated with intensity modulated photon beams (IMRT). For 10 patients the internal mammary lymphatic chain was included in the target, 6 patients received treatment of the thoracic wall after mastectomy and 9 patients were treated after breast conserving surgery. Dose prescription ranged between 48.6 and 50 Gy at 1.8 or 2 Gy per fraction. The target volume (PTV) ranged from 194 to 2121 cm3. For all cases a Dose Volume Histogram analysis has been conducted on omolateral lung, heart, contralateral lung and breast, healthy tissue and PTV. For PTV the volume receiving more than 90% of the prescribed dose (V90%) was 95.8+/-1.8% while V107% = 5.3+/-2.8%. The mean dose computed for the heart was 10.4+/-2.9 Gy, for the omolateral lung: 13.3+/-2.8 Gy, for the contralateral breast: 3.4+/-1.8 Gy, for the contralateral lung: 4.6+/-2.7 Gy. For the omolateral lung V20Gy = 23.1+/-7.0% and V45Gy = 0.8+/-0.9 Gy. With a short mean follow up of 10.4 months, no pulmonary or cardiac complications were observed. IMRT proved to be technically feasible on a clinical basis for the treatment of the whole breast, including internal mammary chain, or of the thoracic wall after mastectomy.

The impact of photon dose calculation algorithms on expected dose distributions in lungs under different respiratory phases

A planning study was carried out on a cohort of CT datasets from breast patients scanned during different respiratory phases. The aim of the study was to investigate the influence of different air filling in lungs on the calculation accuracy of photon dose algorithms and to identify potential patterns of failure with clinical implications. Selected respiratory phases were free breathing (FB), representative of typical end expiration, and deep inspiration breath hold (DIBH), a typical condition for clinical treatment with respiratory gating. Algorithms investigated were the pencil beam (PBC), the anisotropic analytical algorithm (AAA) and the collapsed cone (CC) from the Varian Eclipse or Philips Pinnacle planning system. Reference benchmark calculations were performed with the Voxel Monte Carlo (VMC++). An analysis was performed in terms of physical quantities inspecting either dose-volume or dose-mass histograms and in terms of an extension to three dimensions of the gamma index of Low. Results were stratified according to a breathing phase and algorithm. Collectives acquired in FB or DIBH showed well-separated average lung density distributions with mean densities of 0.27 +/- 0.04 and 0.16 +/- 0.02 g cm(-3), respectively, and average peak densities of 0.17 +/- 0.03 and 0.09 +/- 0.02 g cm(-3). Analysis of volume-dose or mass-dose histograms proved the expected deviations on PBC results due to the missing lateral transport of electrons with underestimations in the low dose region and overestimations in the high dose region. From the gamma analysis, it resulted that PBC is systematically defective compared to VMC++ over the entire range of lung densities and dose levels with severe violations in both respiratory phases. The fraction of lung voxels with gamma > 1 for PBC reached 25% in DIBH and about 15% in FB. CC and AAA performed, in contrast, similarly and with fractions of lung voxels with gamma > 1 in average inferior to 2% in FB and 4-5% (AAA) or 6-8% (CC) in DIBH. In summary, PBC proved to be severely defective in calculations involving lungs and particularly for cases where specific respiratory phases (e.g. DIBH) are assumed for treatment. In contrast, CC and AAA manifested a high degree of consistency against the Monte Carlo method and provided stable results over the entire range of clinically relevant densities.

Critical appraisal of a non-coplanar technique for radiotherapy of breast minimising lung involvement

BACKGROUND AND PURPOSE

To appraise the potential benefit of a conformal technique with non-coplanar fields to minimise lung irradiation in the radiation treatment of breast.

PATIENTS AND METHODS

A comparative study was carried out at planning level for six patients selected for their inadequate sparing of healthy lung tissue with the reference tangential technique. Plans were designed for the conventional tangential technique, for an alternative conformal approach with three beams and for the newly proposed technique with two non-coplanar beams.

RESULTS

In average for the new technique compared to the reference, mean lung dose dropped from approximately 16 to 10.5 Gy, V(20 Gy) from 29.5 to 18.2% and the dose delivered to 1/3 (1/4) of the lung volume dropped from 28.5% (67.3%) to 8.7% (13.4%). For PTV, the volume receiving at least 90% of the prescribed dose resulted 97.4% for the new and 97.3% for the reference. Conformity index improved significantly from 2.58 for the reference to 1.84 for the new technique.

CONCLUSIONS

For a subgroup population of breast cancer patients, where conventional techniques failed to achieve high conformal avoidance, a treatment modality with non-coplanar beams was developed and clinically tested for six patients. It resulted dosimetrically adequate, particularly when the risk of toxicity is relevant.