The dosimetric effect of electron density overrides in 3DCRT Lung SBRT for a range of lung tumor dimensions

Evaluation of stereotactic VMAT lung treatment plans for small moving targets

PURPOSE

The aim of this study is to perform patient quality controls and end-to-end tests for stereotactic VMAT lung treatment plans and to investigate the influence of various parameters on the results.

METHOD

18 plans were defined by an experimental design methodology to cover a large variety of stereotactic VMAT lung treatments including different doses per fraction, target diameters, target movements and respiratory parameters. Plans were first controlled using portal dosimetry and a homogeneous static cylindrical phantom. End-to-end tests were then performed in a dynamic respiratory thorax phantom. Measurements were conducted with ionization chamber and films. Calculations were performed with the AcurosXB and AAA algorithms in 6 FFF.

RESULTS

Portal dosimetry gave excellent gamma pass rates (greater than 97.1 %) and dose deviations between measurement and calculations in a homogeneous static phantom were smaller than 2 %. The methodology followed for comparing calculated and measured doses in a moving target was validated in static fields (largest deviation smaller than  2 %). End-to-end tests showed mean deviations of 1.9 %, 3.3 % and 6.6 % for the 3, 2 and 1 cm diameter's target respectively. Deviations increased for larger movements for the 1 cm lesion.

CONCLUSION

End-to-end tests revealed that stereotactic VMAT lung treatment plans for moving targets can be delivered within 5 % for 3 and 2 cm diameter targets and amplitudes up to 1.5 cm. The AcurosXB and AAA algorithms however tend to underestimate the dose to the target. Even with satisfactory patient quality controls like portal dosimetry, extra care should be taken for GTV lesions smaller than 2 cm.

Planning target volume density impact on treatment planning for lung stereotactic body radiation therapy

BACKGROUND

To evaluate the impact of the planning target volume (PTV) density on treatment planning for lung Stereotactic Body Radiation Therapy (SBRT).

MATERIAL AND METHODS

The PTV coverage was analyzed in two groups of 40 lung SBRT patients. One group had PTV density <0.5 g/cm³, while the other group had PTV density >0.5 g/cm³. The treatments were planned in Pinnacle 9.10, using the collapsed cone convolution (CCC) algorithm. The prescribed dose was 60 Gy to the PTV in 4-8 fractions. Respecting constraint for the PTV coverage (D98% > 95%), we compared changes in the isodose line prescription, the number of monitor units (MU), maximum dose (D_max), irradiated volume covered with 30 Gy (V_30Gy), and the optimization planning volume (OPV).

RESULTS

For the same median values of the PTV coverage (98.3%), the differences are presented with median values between lower and higher density than 0.5 g/cm³. The isodose line prescription was 83 vs. 90% (p < 0.0001), the MUs were 2294 vs. 1655 MU (p < 0.0001), D_max was 74.26 vs. 68.09 Gy (p < 0.0001), V_30Gy was 117.03 vs. 104.81 cc (p = 0.04), and OPV was 28.48 vs. 39.35 cc (p < 0.001). By overriding the ITV density to 0.8 g/cm³, the isodose line prescription decreases. The D_max and MUs decrease by 7%, V_30Gy by 34%, and OPV by 70%.

CONCLUSION

To obtain similar PTV coverage for PTV which is <0.5 g/cm³, a larger margin irradiating a large OPV was used. More MUs and a higher maximum dose were delivered. For the PTV density of ≤0.36 g/cm^3, overriding is recommended to reduce the dose and irradiated volume.

Technical Note: A 3D-printed phantom for radiochromic film evaluation of moving lung tumor SBRT without dose convolution

PURPOSE

A common dosimetric quality assurance (QA) method in stereotactic body radiation therapy (SBRT) of lung tumors is to use lung phantoms with radiochromic film. However, in most phantoms, the film moves with the tumor, leading to the blurring effect. This technical note presents the QA performance of a novel phantom in which the film is fixed; this phantom can be used for both patient-specific QA and end-to-end testing.

METHODS

Lung tumor motion was simulated with the CIRS Model 008A phantom. A lung-equivalent insert that consisted of a fixed radiochromic film around which a 2-cm tumor moved in the inferior/superior direction (i.e., mimicking respiration-induced tumor motion) was generated by 3D printing. Two common SBRT plans [dynamic conformal arc (DCA) and volumetric-modulated arc therapy (VMAT)] were calculated on the average intensity projection (AIP) image set in Varian Eclipse using the dose calculation algorithm Acuros XB. The plans were delivered by a Varian TrueBeam STx accelerator using 6-MV flattening filter-free energy. EBT3 films were used for treatment-dose verification. The measured and planned dose distributions were compared by using the local gamma index at 3% and 2 mm.

RESULTS

Mean gamma pass rates of film and planned dose distributions were all ≥95%. DCA and VMAT plans did not differ in gamma pass rates. Planned and measured dose distributions agreed well, as did planned and measured gamma maps.

CONCLUSIONS

With this new insert, measured and planned dose distributions were very similar, which supports the current view in the field that dose calculations on AIP image sets account sufficiently for tumor motion during treatment. The phantom also performed well despite challenging breathing parameters (large tumor amplitude and slow breathing rate) and the application of a complex treatment technique (VMAT). This phantom could facilitate clinical and end-to-end film-based dosimetric QA for lung SBRT.

TAXONOMY

Twenty-seven TH- Radiation dose measurement devices. Eleven Phantoms for dosimetric measurement.