Pencil-Beam Algorithm Based Dose Prediction in Small Proton Fields: Accuracy and Dosimetric Comparison to Photon Plans

• Main Author: DePew, Kyle David
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2018
• Subjects: Oncology; Physics
• ISBN: 0355993074; 9780355993073

Dose calculation accuracy of the Varian Eclipse treatment planning system (TPS) is empirically assessed for small-aperture fields using the Mevion S250 double scattering proton therapy system. In addition, Brainlab intensity-modulated radiation therapy (IMRT) plans for small brain tumors are compared to proton plans in terms of target coverage and healthy tissue sparing. In order to investigate accuracy of Eclipse pencil beam algorithm for small fields, proton treatment plans were created for five spherical pseudo-targets in an Alderson RANDO head phantom using apertures of 1, 2, 3, 4 and 5 cm diameter. Three plans of one, two, and three beams for each target were made with a fractional dose of 180 cGy to the target. Each small beam planned was characterized with a depth dose scan in water and a lateral profile in Gafchromic EBT3 film compared to Eclipse computation. Clinical implications of the small beam plans were evaluated by irradiating film inserted into the phantom. Film quenching was also considered in this study. For the plan comparison study, ten patients treated with hypofractionated Brainlab IMRT were selected. Brainlab plans with 10 to 15 beams were transferred to the Eclipse TPS for recalculation and corresponding proton plans of 2 or 3 beams were made. The clinical target volume was covered by 99% of prescribed dose (20 Gy to 40 Gy in 3 to 5 fractions) for both plans. The plans were compared by homogeneity index (HI), conformity index (CI), CTV dose (mean and maximum), critical organ (brainstem, brain, optic chiasm, optic nerve, and cochlea) dose (mean and maximum), and dose at 3 mm depth. Depth dose scans in water showed a range pullback (up to 2.0 mm), widening of modulation (up to 9.5 mm), and dose escalation in the proximal end and sub-peak region (up to 23.1%) when compared to the Eclipse calculations for small fields. Measured full width at half maximums (FWHMs) and penumbrae for lateral profiles were generally smaller than Eclipse predictions, but differed less than 1.0 mm from calculations for most of the comparisons. In the phantom study, one-beam plans did not generally provide acceptable target coverage whereas three-beam plans did. Gamma test results with 50% threshold (evaluation of target coverage) improved with each additional beam added to the plan. The Eclipse TPS did not accurately predict the surface dose and dose in cranium. More pronounced range pullback and degradation of modulation than water scans were observed due to film quenching. The quenching effect became more noticeable as the target size increased. In the plan comparison study, the HI was smaller by 3.0 ± 3.4 in the Brainlab plans versus the proton plans, while the CI was larger by 0.7 ± 0.9. Dose at 3 mm water depth is 11.4 ± 19.9% higher for the proton plans versus Brainlab. Proton plans in general spared healthy tissue better than Brainlab plans for all the critical structures compared. Proton treatments in general increased surface dose owing to the smaller number of beams in each treatment plan. In conclusion, Eclipse TPS underestimates range and entrance dose and overestimates modulation for small proton beams. Even though each small beam produces a large dosimetric discrepancy from Eclipse calculation, plans with multiple proton fields may produce clinical acceptable target coverage and mitigate dose increases at the proximal end of spread-out Bragg peaks. Plan quality for small brain targets heavily depends on target location, target size, and organ proximity to target which should be considered for more desirable treatment technique.