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Purpose: A higher minimum monitor unit (minMU) for pencil-beam scanning proton beams in intensity-modulated proton therapy is preferred for more efficient delivery. However, plan quality may be compromised when the minMU is too large. This study aimed to identify the optimal minMU (OminMU) to improve plan delivery efficiency while maintaining high plan quality., Methods: We utilized clinical plans including six anatomic sites (brain, head and neck, breast, lung, abdomen, and prostate) from 23 patients previously treated with the Varian ProBeam system. The minMU of each plan was increased from the current clinical minMU of 1.1 to 3-24 MU depending on the daily prescribed dose (DPD). The dosimetric parameters of the plans were evaluated for consistency against a 1.1-minMU plan for target coverage as well as organs-at-risk dose sparing. DPD/minMU was defined as the ratio of DPD to minMU (cGy/MU) to find the OminMU by ensuring that dosimetric parameters did not differ by >1% compared to those of the 1.1-minMU plan., Results: All plans up to 5 minMU showed no significant dose differences compared to the 1.1-minMU plan. Plan qualities remained acceptable when DPD/minMU ≥35 cGy/MU. This suggests that the 35 cGy/MU criterion can be used as the OminMU, which implies that 5 MU is the OminMU for a conventional fraction dose of 180 cGy. Treatment times were decreased by an average of 32% (max 56%, min 7%) and by an average of 1.6 min when the minMU was increased from 1.1 to OminMU., Conclusion: A clinical guideline for OminMU has been established. The minMU can be increased by 1 MU for every 35 cGy of DPD without compromising plan quality for most cases analyzed in this study. Significant treatment time reduction of up to 56% was observed when the suggested OminMU is used., (© 2024 The Author(s). Journal of Applied Clinical Medical Physics published by Wiley Periodicals LLC on behalf of American Association of Physicists in Medicine.)

Purpose/objective(s): With reports of CNS toxicity in patients treated with proton therapy at doses lower than would be expected based on photon data, it has been proposed that heavy monitor unit (MU) weighting of pencil beam scanning (PBS) proton therapy spots may potentially increase the risk of toxicity. We evaluated the impact of maximum MU weighting per spot (maxMU/spot) restrictions on PBS plan quality, prior to implementing clinic-wide maxMU/spot restrictions., Materials/methods: PBS plans of 11 patients, of which 3 plans included boosts, for a total of 14 PBS sample cases were included. Per sample case, a single dosimetrist created 4 test plans, gradually reducing the maxMU/spot in the plan. Test Plan 1, unrestricted in maxMU/spot, was the reference for all restricted plan comparisons (comparison sets 2 vs. 1; 3 vs. 1; and 4 vs. 1). The impact of MU/spot restrictions on plan quality metrics were analyzed with Wilcoxon signed rank test analyses. Treatment delivery time was modeled for a representative case., Results: A total of 14 PBS sample cases, 7 (50%) single-field optimized, 7 (50%) multi-field optimized, 9 (64%) delivering > 3500 cGy, 9 (64%) with 3 beams, and 7 (50%) without a range shifter were included. There were no differences in plan quality metrics of target coverage (V95% and V100% prescription), conformality and gradient indices, hot spot volume (V105% prescription), and dose to normal brain (V10%/30%/50%/70%/90%/100% prescription) with reductions of allowable maxMU/spot across all comparison sets (p > 0.05). Max MU/spot restrictions did not increase treatment delivery time when analyzed for a representative case., Conclusion: MaxMU/spot restrictions within the thresholds evaluated in this study did not degrade overall plan quality metrics. Future studies should evaluate spot weighting with linear energy transfer/relative biologic effectiveness-informed planning to determine if spot weighting manipulation impacts clinical outcomes and mitigates toxicity., (Copyright © 2021. Published by Elsevier B.V.)

Purpose/objective(s): Patients treated with proton beam therapy (PBT) undergo routine quality assurance CT (QACT) scans during their course of treatment, which determines whether the initial plan remains accurate for the whole course of treatment or needs to be re-planned based on the findings on their QACTs. The objective of this study is to optimize and reduce the QACT frequency, while maintaining the treatment quality. The benefits of reducing QACT frequency include reducing imaging dose and optimizing use of patient time and staff resources.Materials/methods: We performed a retrospective IRB-approved single-institution review of all patients of three anatomical sites (e.g., Brain, Prostate and Head and Neck [HN]), who were treated with PBT between July 2019 and July 2020. The number of patients treated for these three sites, number of QACTs performed, number of patients re-planned and number of re-plans are listed in the table. For each anatomic site, the QACT and re-planning pattern were analyzed to find the optimal frequency of QACTs needed.Results: Brain: Out of 204 QACTs, 6 (2.9 %) were used for adaptive planning. One patient was re-planned twice. It was found that 3 out of 5 patients were re-planned due to weight loss or anatomical changes. All revision plans were delivered within 7-12 fractions of the treatment. One QACT per patient at the beginning of the second week of the treatment was proposed for the brain patients. Prostate: Out of 236 QACTs, 3 (3.6 %) were used for adaptive planning. All three revision plans were delivered within 17-29 fractions of their treatments. Hence, one QACT per patient during the middle of the treatment course was proposed for this group. HN: Out of 97 patients, 32 (33 %) underwent adaptive planning on their QACTs. A total of 37 revision plans were made on 437 QACTs, where 8.5% QACTs were used for adaptive planning. Out of 32 patients re-planned, 26 of them were due to either weight loss or anatomy change and/or tumor shrinkage, 4 patients were due to need for a new mask and the remainder due to air density changes within sinuses. The average time between a QACT and the revision plan delivery was five days. Bi-weekly QACTs were proposed for the HN patients. We found no adaptive planning occurred in the last week of treatment.Conclusion: The above newly proposed quality assurance CT (QACT) patterns for these three anatomical sites are the optimal patterns, while maintaining the treatment quality. Following this analysis, we have adopted a new paradigm with decreased routine QACTs in these disease sites. To mitigate any risks due to the proposed QACT patterns, increasing the cone beam CT (CBCT) frequency during treatment is recommended. [ABSTRACT FROM AUTHOR]