SBRT of lung cancer
Optimizing dose prescription in stereotactic body radiotherapy for lung tumours using Monte Carlo dose calculation

https://doi.org/10.1016/j.radonc.2009.11.008Get rights and content

Abstract

Purpose

To define a method of dose prescription employing Monte Carlo (MC) dose calculation in stereotactic body radiotherapy (SBRT) for lung tumours aiming at a dose as low as possible outside of the PTV.

Methods and materials

Six typical T1 lung tumours – three small, three large – were constructed centrally, peripherally in the lung, and nearby the thoracic wall, respectively. For each of these, five treatment plans employing dynamic conformal arc technique were made in which the dose was prescribed to encompass the PTV with the prescription isodose level (PIL) set in a range between 50% and 80% of the isocenter dose. Three shells of respectively 10 mm thickness around the PTV were constructed to assess the dose in the tissues directly adjacent to the PTV.

Results

The PTV was nicely covered (mean 98.8% ± 0.9%) with favourable conformity indices (mean 1.09 ± 0.1). Mean doses around the PTVs were 73% (±1.3%), 76% (±3.5%), and 85% (±5.1%) of the prescribed dose in shell 1 for PIL50%, PIL65%, and PIL80%, respectively; 40% (±2.6%), 44% (±5.1%), 54% (±9.3%) in shell 2; and 24% (±1.9%), 26% (±3.6%), 33% (±6.8%) in shell 3. All normal tissue doses including the integral dose were also consistently worst for PIL80%. Monitor units were 30% higher for PIL65%, and 70% higher for PIL50%, compared with PIL80%.

Conclusions

To improve normal tissue sparing the dose should be prescribed at an isodose lower than 80% of the isocenter dose in SBRT when using conformal arc technique with MC dose calculation.

Section snippets

Methods and materials

The four-dimensional planning CT acquired with free un-coached breathing of a patient treated with SBRT for a NSCLC stage T1N0M0 was entered into the planning software (i-plan-RT-dose 4.0, Brainlab, Feldkirchen, Germany). The patient had been positioned frameless in a vacuum-mattress with the arms raised above the head. In the planning CT, GTVs, ITVs en PTVs for two typical T1-tumours (small: diameter 15 mm, and large: diameter 30 mm, respectively) were constructed at three localizations: near

PTV coverage

The numeric planning parameters are displayed in Supplementary Table 1. For small and large lesions planned, coverage of the PTV by the PI (prescription isodose) was 96.5% or higher, and for 16 out of 30 of the plans, PTV coverage was >99%. Conformity indices (CI) were very low at <1.07 in 15 out of 30 plans, the worst three plans yielding conformity indices of 1.22–1.24 (all for PILs of 80%; Supplementary Table 1). In concordance with the steepness of the dose gradient (more rapid dose falloff

Discussion

Stereotactic body radiotherapy is an increasingly used treatment modality for indications including early-stage primary lung cancer and lung metastases. The essence of SBRT consists in administering very high doses per fraction to small target volumes, which in general only comprise gross tumour tissue with tight margins [7]. The tightness of the margin to the PTV in the millimetre range is made possible, firstly, because special methods such as 4D-CT for treatment planning [8], respiratory

Conclusion

In stereotactic radiotherapy for lung-lesions using MC dose calculation and dynamic conformal arc setup, dose prescription at a PIL (prescription isodose level) between 50% and 70% of the dose at the isocenter results in lower dose to surrounding tissues and lungs compared with employing a PIL of 80%. Except for large lesions adjacent to the thoracic wall and for lesions, where normal tissue lies within the PTV, dose should be prescribed at an isodose level considerably lower than 80% when

Reference (31)

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