Abstract
PurposeWe developed a 4D interplay effect model to quantitatively evaluate breathing-induced interplay effects and assess the feasibility of utilizing spot-scanning proton arc (SPArc) therapy for hypo-fractionated lung stereotactic body radiotherapy (SBRT). The model was then validated by retrospective application to clinical cases.Materials and MethodsA digital lung 4DCT phantoms was used to mimic targets in diameter of 3cm with breathing motion amplitudes: 5, 10, 15, and 20 mm, respectively. Two planning groups based on robust optimization were generated: (1) Two-field Intensity Modulated Proton Therapy (IMPT) plans and (2) SPArc plans via a partial arc. 5,000 cGy relative biological effectiveness (RBE) was prescribed to the internal target volume (ITV) in five fractions. To quantitatively assess the breathing induced interplay effect, the 4D dynamic dose was calculated by synchronizing the breathing pattern with the simulated proton machine delivery sequence, including IMPT, Volumetric repainting (IMPTvolumetric), iso-layered repainting (IMPTlayer) and SPArc. Ten lung patients’ 4DCT previously treated with VMAT SBRT, were used to validate the digital lung tumor model. Normal tissue complicated probability (NTCP) of chestwall toxicity was calculated.ResultTarget dose were degraded as the tumor motion amplitude increased. The 4D interplay effect phantom model indicated that motion mitigation effectiveness using SPArc was about five times of IMPTvolumetric or IMPTlayer using maximum MU/spot as 0.5 MU at 20 mm motion amplitude. The retrospective study showed that SPArc has an advantage in normal tissue sparing. The probability of chestwall’s toxicity were significantly improved from 40.2 ± 29.0% (VMAT) (p = 0.01) and 16.3 ± 12.0% (IMPT) (p = 0.01) to 10.1 ± 5.4% (SPArc). SPArc could play a significant role in the interplay effect mitigation with breathing-induced motion more than 20 mm, where the target D99 of 4D dynamic dose for patient #10 was improved from 4,514 ± 138 cGy [RBE] (IMPT) vs. 4,755 ± 129 cGy [RBE] (SPArc) (p = 0.01).ConclusionSPArc effectively mitigated the interplay effect for proton lung SBRT compared to IMPT with repainting and was associated with normal tissue sparing. This technology may make delivery of proton SBRT more technically feasible and less complex with fewer concerns over underdosing the target compared to other proton therapy techniques.
Highlights
Lung cancer remains a leading cause of cancer mortality in the world [1]
We developed a 4D interplay effect model to quantitatively evaluate breathinginduced interplay effects and assess the feasibility of utilizing spot-scanning proton arc (SPArc) therapy for hypo-fractionated lung stereotactic body radiotherapy (SBRT)
Two planning groups based on robust optimization were generated: [1] Two-field Intensity Modulated Proton Therapy (IMPT) plans and [2] Scanning Proton Arc (SPArc) plans via a partial arc. 5,000 cGy relative biological effectiveness (RBE) was prescribed to the internal target volume (ITV) in five fractions
Summary
Lung cancer remains a leading cause of cancer mortality in the world [1]. Compared to conventional radiotherapy, hypofractionated stereotactic body radiotherapy (SBRT) has been proved to improve local tumor control and survival rate for stage I non-small cell lung cancer (NSCLC) patients [2,3,4,5,6]. With the development of pencil beam scanning (PBS) technology, intensity modulated proton therapy (IMPT) offers the potentials to spare the adjacent normal tissues further while maintaining similar or superior target coverage in a more efficient way without using beam specific blocks or compensators compared to passive scatter proton therapy (PSPT) [8,9,10,11]. Such scanning technique is susceptible to the interplay effect between proton spot scanning and respiratory induced motion during dose delivery. An alternative approach is called iso-layered repainting, in which first delivered several rescans within one energy plane before switching to the plane with the dose per spot being limited by a maximal MU value [17]
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