Abstract
The purpose of this study was to investigate the impact of Monte Carlo (MC) calculations and optimized dose definitions in stereotactic body radiotherapy (SBRT) for lung cancer patients. We used a retrospective patient review and basic virtual phantom to determine dose prescriptions. Fifty‐three patients underwent SBRT. A basic virtual phantom had a gross tumor volume (GTV) of 10.0 mm with equivalent water density of 1.0 g/cm3, which was surrounded by equivalent lung surrounding the GTV of 0.25 g/cm3. D95 of the planning target volume (PTV) and D99 of the GTV were evaluated with different GTV sizes (5.0 to 30.0 mm) and different lung densities (0.05 to 0.45 g/cm3). Prescribed dose was defined as 95% of the PTV should receive 100% of the dose (48 Gy/4 fractions) using pencil beam (PB) calculation and recalculated using MC calculation. In the patient study, average doses to the D95 of the PTV and D99 of the GTV using the MC calculation plan were 19.9% and 10.2% lower than those by the PB calculation plan, respectively. In the phantom study, decreased doses to the D95 of the PTV and D99 of the GTV using the MC calculation plan were accompanied with changes GTV size from 30.0 to 5.0 mm, which was decreased from 8.4% to 19.6% for the PTV and from 17.4% to 27.5% for the GTV Similar results were seen with changes in lung density from 0. 45 to 0.05 g/cm3, with doses to the D95 of the PTV and D99 of the GTV were decreased from 12.8% to 59.0% and from 7.6% to 44.8%, respectively. The decrease in dose to the PTV with MC calculation was strongly dependent on lung density. We suggest that dose definition to the GTV for lung cancer SBRT be optimized using MC calculation. Our current clinical protocol for lung SBRT is based on a prescribed dose of 44 Gy in 4 fractions to the GTV using MC calculation.PACS number: 87.55.D‐, 87.55.K‐
Highlights
39 Miura et al.: MC treatment planning for l u n g c a n c e r method available, Monte Carlo (MC) calculation, models the actual physical process, leading to a dose deposition, including secondary electron distribution.[6]The many studies of dosing to the target using MC calculation have found that this results in significant dose differences in clinical radiotherapy in regions with inhomogeneous materials, for lung cancer.[7,8,9] These differences occur because the simple algorithms such a pencil beam (PB) calculation merely account for decreased attenuation of the primary photon beam in low-density lung tissue
dose to 95% of the PTV (D95) of the planning target volume (PTV) and dose to 99% of the GTV (D99) of the gross tumor volume (GTV) using the MC calculation plan were on average 19.9% and 10.2% lower than those with the PB calculation plan, respectively
In the Japan Clinical Oncology Group (JCOG) 0403 protocol, dose prescription is defined as the point dose at the isocenter of the PTV with inhomogeneous correction, such as the pencil beam convolution with Batho power-law and Clarkson with effective path length correction, but this prescription is not accurate for dose calculations of lung cancer.[10]. In contrast, the Radiation Therapy Oncology Group (RTOG) 0236 protocol defines dose prescription as the volume dose at the periphery of the PTV without inhomogeneous correction.[11]. The dose maximum within the PTV should preferably not be less than 110% or exceed 140% of the prescribed dose, similar to the criteria formulated in RTOG protocol 0618.(12) comparing the actual delivered doses among these trials is difficult
Summary
39 Miura et al.: MC treatment planning for l u n g c a n c e r method available, Monte Carlo (MC) calculation, models the actual physical process, leading to a dose deposition, including secondary electron distribution.[6]The many studies of dosing to the target using MC calculation have found that this results in significant dose differences in clinical radiotherapy in regions with inhomogeneous materials, for lung cancer.[7,8,9] These differences occur because the simple algorithms such a pencil beam (PB) calculation merely account for decreased attenuation of the primary photon beam in low-density lung tissue. 39 Miura et al.: MC treatment planning for l u n g c a n c e r method available, Monte Carlo (MC) calculation, models the actual physical process, leading to a dose deposition, including secondary electron distribution.[6]. Our institution uses PB calculation for all treatments, we recently considered changing our treatment planning calculation algorithm in SBRT for lung cancer patients from PB to MC calculations. An important consideration in this decision was that MC calculation planning makes use of clinical knowledge already gained from PB calculation treatment planning. We investigated the impact of a change in calculation methods from PB to MC calculations in SBRT for lung cancer patients, and determined changes in doses delivered to patients. Our protocol for lung SBRT is 48 Gy in 4 fractions to the planning target volume (PTV) using PB calculation. We used a retrospective patient review and basic virtual phantom to determine dose prescriptions
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