Abstract
PurposeTo evaluate different strategies for proton lung treatment planning based on four-dimensional CT (4DCT) scans.Methods and MaterialsTwelve cases, involving only gross tumor volumes (GTV), were evaluated. Single image sets of (1) maximum intensity projection (MIP3) of end inhale (EI), middle exhale (ME) and end exhale (EE) images; (2) average intensity projection (AVG) of all phase images; and (3) EE images from 4DCT scans were selected as primary images for proton treatment planning. Internal target volumes (ITVs) outlined by a clinician were imported into MIP3, AVG, and EE images as planning targets. Initially, treatment uncertainties were not included in planning. Each plan was imported into phase images of 4DCT scans. Relative volumes of GTVs covered by 95% of prescribed dose and mean ipsilateral lung dose of a phase image obtained by averaging the dose in inspiration and expiration phases were used to evaluate the quality of a plan for a particular case. For comparing different planning strategies, the mean of the averaged relative volumes of GTVs covered by 95% of prescribed dose and its standard deviation for each planning strategy for all cases were used. Then, treatment uncertainties were included in planning. Each plan was recalculated in phase images of 4DCT scans. Same strategies were used for plan evaluation except dose-volume histograms of the planning target volumes (PTVs) instead of GTVs were used and the mean and standard deviation of the relative volumes of PTVs covered by 95% of prescribed dose and the ipsilateral lung dose were used to compare different planning strategies.ResultsMIP3 plans without treatment uncertainties yielded 96.7% of the mean relative GTV covered by 95% of prescribed dose (standard deviations of 5.7% for all cases). With treatment uncertainties, MIP3 plans yielded 99.5% of mean relative PTV covered by 95% of prescribed dose (standard deviations of 0.7%). Inclusion of treatment uncertainties improved PTV dose coverage but also increased the ipsilateral mean lung dose in general, and reduced the variations of the PTV dose coverage among different cases. Plans based on conventional axial CT scan (CVCT) gave the poorest PTV dose coverage (about 96% of mean relative PTV covered by 95% isodose) compared to MIP3 and EE plans, which resulted in 100% of PTV covered by 95% isodose for tumors with relatively large motion. AVG plans demonstrated PTV dose coverage of 89.8% and 94.4% for cases with small tumors. MIP3 plans demonstrated superior tumor coverage and were least sensitive to tumor size and tumor location.ConclusionMIP3 plans based on 4DCT scans were the best planning strategy for proton lung treatment planning.
Highlights
Proton treatment for lung cancer can spare or minimize dose to contralateral lung tissue
Plans based on conventional axial CT scan (CVCT) gave the poorest planning target volumes (PTVs) dose coverage compared to MIP3 and EE plans, which resulted in 100% of PTV covered by 95% isodose for tumors with relatively large motion
MIP3 planning showed the best overall coverage by the 95% isodose covering more than 95% of gross tumor volumes (GTV) for all cases from C to L, resulting in the best MGV95 of 96.7% and lowest standard deviation (5.7%)
Summary
Proton treatment for lung cancer can spare or minimize dose to contralateral lung tissue. We have been treating medically inoperable non-small-cell lung carcinoma (NSCLC) for more than ten years [1,2,3]. Bush et al [1] followed up 37 patients with early-stage NSCLC and reported better survival and local control using proton therapy with an updated review published in 2010 [2]. Moyers et al [4] studied the methodologies and tools for proton lung treatment planning, and pointed out that a proton beam design should include tumor motion and setup uncertainties instead of creating new targets. That study introduced aperture and compensator design using margins and smearing to take into account tumor motion and setup uncertainties. The smearing method was similar to the compensator design proposed by Urie et al [5] to account for setup uncertainties and tumor motion
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