Impact of Spot Size and Beam-Shaping Devices on the Treatment Plan Quality for Pencil Beam Scanning Proton Therapy

Published:December 29, 2015DOI:


      This study aimed to assess the clinical impact of spot size and the addition of apertures and range compensators on the treatment quality of pencil beam scanning (PBS) proton therapy and to define when PBS could improve on passive scattering proton therapy (PSPT).

      Methods and Materials

      The patient cohort included 14 pediatric patients treated with PSPT. Six PBS plans were created and optimized for each patient using 3 spot sizes (∼12-, 5.4-, and 2.5-mm median sigma at isocenter for 90- to 230-MeV range) and adding apertures and compensators to plans with the 2 larger spots. Conformity and homogeneity indices, dose-volume histogram parameters, equivalent uniform dose (EUD), normal tissue complication probability (NTCP), and integral dose were quantified and compared with the respective PSPT plans.


      The results clearly indicated that PBS with the largest spots does not necessarily offer a dosimetric or clinical advantage over PSPT. With comparable target coverage, the mean dose (Dmean) to healthy organs was on average 6.3% larger than PSPT when using this spot size. However, adding apertures to plans with large spots improved the treatment quality by decreasing the average Dmean and EUD by up to 8.6% and 3.2% of the prescribed dose, respectively. Decreasing the spot size further improved all plans, lowering the average Dmean and EUD by up to 11.6% and 10.9% compared with PSPT, respectively, and eliminated the need for beam-shaping devices. The NTCP decreased with spot size and addition of apertures, with maximum reduction of 5.4% relative to PSPT.


      The added benefit of using PBS strongly depends on the delivery configurations. Facilities limited to large spot sizes (>∼8 mm median sigma at isocenter) are recommended to use apertures to reduce treatment-related toxicities, at least for complex and/or small tumors.
      To read this article in full you will need to make a payment
      ASTRO Member Login
      ASTRO Members, full access to the journal is a member benefit. Use your society credentials to access all journal content and features.

      Purchase one-time access:

      Already a print subscriber? Claim online access
      Already an online subscriber? Sign in
      Institutional Access: Sign in to ScienceDirect


        • Goitein M.
        Magical protons?.
        Int J Radiat Oncol Biol Phys. 2008; 70: 654-656
        • Steneker M.
        • Lomax A.J.
        • Schneider U.
        Intensity modulated photon and proton therapy for the treatment of head and neck tumors.
        Radiother Oncol. 2006; 80: 263-267
        • Efstathiou J.A.
        • Paly J.J.
        • Lu H.M.
        • et al.
        Adjuvant radiation therapy for early stage seminoma: Proton versus photon planning comparison and modeling of second cancer risk.
        Radiother Oncol. 2012; 103: 12-17
        • Olsen D.R.
        • Bruland O.S.
        • Frykholm G.
        • et al.
        Proton therapy: A systematic review of clinical effectiveness.
        Radiother Oncol. 2007; 83: 123-132
        • Kandula S.
        • Zhu X.
        • Garden A.S.
        • et al.
        Spot scanning beam proton therapy vs intensity-modulated radiation therapy for ipsilateral head and neck malignancies: A treatment planning comparison.
        Med Dosim. 2013; 38: 390-394
        • Yock T.I.
        • Tarbell N.J.
        Technology insight: Proton beam radiotherapy for treatment in pediatric brain tumors.
        Nat Clin Pract Oncol. 2004; 1: 97-103
        • Hoffman K.E.
        • Yock T.I.
        Radiation therapy for pediatric central nervous system tumors.
        J Child Neurol. 2009; 24: 1387-1396
        • Yock T.I.
        • MacDonald S.M.
        • Safai S.
        • et al.
        Proton radiotherapy for ependymoma: Initial clinical outcomes and dose comparisons for intensity modulated radiation with photons, proton radiation, and intensity modulated proton therapy.
        Int J Radiat Oncol Biol Phys. 2007; 69: S575-S576
        • Yock T.I.
        • Yeap B.Y.
        • Pulsifer M.B.
        • et al.
        Results from a prospective trial of proton radiotherapy for medulloblastoma: Clinical outcomes including hearing and neurocognitive.
        Int J Radiat Oncol Biol Phys. 2011; 81: S113
        • Armstrong G.T.
        • Stovall M.
        • Robison L.L.
        Long-term effects of radiation exposure among adult survivors of childhood cancer: Results from the Childhood Cancer Survivor Study.
        Radiat Res. 2010; 174: 840-850
        • Hudson M.M.
        • Mulrooney D.A.
        • Bowers D.C.
        • et al.
        High-risk populations identified in Childhood Cancer Survivor Study investigations: Implications for risk-based surveillance.
        J Clin Oncol. 2009; 27: 2405-2414
        • Meadows A.T.
        • Friedman D.L.
        • Neglia J.P.
        • et al.
        Second neoplasms in survivors of childhood cancer: Findings from the Childhood Cancer Survivor Study cohort.
        J Clin Oncol. 2009; 27: 2356-2362
        • Neglia J.P.
        • Robison L.L.
        • Stovall M.
        • et al.
        New primary neoplasms of the central nervous system in survivors of childhood cancer: A report from the Childhood Cancer Survivor Study.
        J Natl Cancer Inst. 2006; 98: 1528-1537
        • Oeffinger K.C.
        • Mertens A.C.
        • Sklar C.A.
        • et al.
        Chronic health conditions in adult survivors of childhood cancer.
        N Engl J Med. 2006; 355: 1572-1582
        • Meeske K.A.
        • Patel S.K.
        • Palmer S.N.
        • et al.
        Factors associated with health-related quality of life in pediatric cancer survivors.
        Pediatr Blood Cancer. 2007; 49: 298-305
        • Diller L.
        • Chow E.J.
        • Gurney J.G.
        • et al.
        Chronic disease in the Childhood Cancer Survivor Study cohort: A review of published findings.
        J Clin Oncol. 2009; 27: 2339-2355
        • Duffner P.K.
        Risk factors for cognitive decline in children treated for brain tumors.
        Eur J Paediatr Neurol. 2010; 14: 106-115
        • Zacharatou Jarlskog C.
        • Paganetti H.
        Risk of developing second cancer from neutron dose in proton therapy as function of field characteristics, organ, and patient age.
        Int J Radiat Oncol Biol Phys. 2008; 72: 228-235
        • Paganetti H.
        • Athar B.S.
        • Moteabbed M.
        • et al.
        Assessment of radiation-induced second cancer risks in proton therapy and IMRT for organs inside the primary radiation field.
        Phys Med Biol. 2012; 57: 6047-6061
        • Athar B.S.
        • Bednarz B.
        • Seco J.
        • et al.
        Comparison of out-of-field photon doses in 6 MV IMRT and neutron doses in proton therapy for adult and pediatric patients.
        Phys Med Biol. 2010; 55: 2879-2891
        • Kooy H.M.
        • Clasie B.M.
        • Lu H.M.
        • et al.
        A case study in proton pencil beam scanning delivery.
        Int J Radiat Oncol Biol Phys. 2010; 76: 624-630
        • Clasie B.
        • Depauw N.
        • Fransen M.
        • et al.
        Golden beam data for proton pencil-beam scanning.
        Phys Med Biol. 2012; 57: 1147-1158
        • Lomax A.J.
        • Bohringer T.
        • Bolsi A.
        • et al.
        Treatment planning and verification of proton therapy using spot scanning: Initial experiences.
        Med Phys. 2004; 31: 3150-3157
        • Clasie B.
        • Wroe A.
        • Kooy H.
        • et al.
        Assessment of out-of-field absorbed dose and equivalent dose in proton fields.
        Med Phys. 2010; 37: 311-321
        • Wang D.
        • Dirksen B.
        • Hyer D.
        • et al.
        Impact of spot size on plan quality of spot scanning proton radiosurgery for peripheral brain lesions.
        Med Phys. 2014; 41: 121705
        • van de Water T.A.
        • Lomax A.J.
        • Bijl H.P.
        • et al.
        Using a reduced spot size for intensity-modulated proton therapy potentially improves salivary gland-sparing in oropharyngeal cancer.
        Int J Radiat Oncol Biol Phys. 2012; 82: e313-e319
        • Chanrion M.A.
        • Ammazzalorso F.
        • Wittig A.
        • et al.
        Dosimetric consequences of pencil beam width variations in scanned beam particle therapy.
        Phys Med Biol. 2013; 58: 3979-3993
        • Dowdell S.J.
        • Clasie B.
        • Depauw N.
        • et al.
        Monte Carlo study of the potential reduction in out-of-field dose using a patient-specific aperture in pencil beam scanning proton therapy.
        Phys Med Biol. 2012; 57: 2829
        • Hyer D.E.
        • Hill P.M.
        • Wang D.
        • et al.
        Effects of spot size and spot spacing on lateral penumbra reduction when using a dynamic collimation system for spot scanning proton therapy.
        Phys Med Biol. 2014; 59: N187-N196
        • Kooy H.M.
        • Grassberger C.
        Intensity modulated proton therapy.
        Br J Radiol. 2015; 88: 20150195
        • Craft D.L.
        • Hong T.S.
        • Shih H.A.
        • et al.
        Improved planning time and plan quality through multicriteria optimization for intensity-modulated radiotherapy.
        Int J Radiat Oncol Biol Phys. 2012; 82: e83-e90
        • Feuvret L.
        • Noël G.
        • Mazeron J.J.
        • et al.
        Conformity index: A review.
        Int J Radiat Oncol Biol Phys. 2006; 64: 333-342
        • Niemierko A.
        A generalized concept of equivalent uniform dose (EUD).
        Med Phys. 1999; 26 ([Abstract]): 1100
        • Gay H.A.
        • Niemierko A.
        A free program for calculating EUD-based NTCP and TCP in external beam radiotherapy.
        Phys Med. 2007; 23: 115-125
        • Oinam A.S.
        • Singh L.
        • Shukla A.
        • et al.
        Dose volume histogram analysis and comparison of different radiobiological models using in-house developed software.
        J Med Phys. 2011; 36: 220-229
        • Fuss M.
        • Poljanc K.
        • Miller D.W.
        • et al.
        Normal tissue complication probability (NTCP) calculations as a means to compare proton and photon plans and evaluation of clinical appropriateness of calculated values.
        Int J Cancer. 2000; 90: 351-358
        • Merchant T.E.
        • Rose S.R.
        • Bosley C.
        • et al.
        Growth hormone secretion after conformal radiation therapy in pediatric patients with localized brain tumors.
        J Clin Oncol. 2011; 29: 4776-4780
        • Geng C.
        • Moteabbed M.
        • Xie Y.
        • et al.
        Assessing the radiation-induced second cancer risk in proton therapy for pediatric brain tumors: The impact of employing a patient-specific aperture in pencil beam scanning.
        Phys Med Biol. 2016; 61: 12-22
        • Hong L.
        • Goitein M.
        • Bucciolini M.
        • et al.
        A pencil beam algorithm for proton dose calculations.
        Phys Med Biol. 1996; 41: 1305-1330


      Commenting Guidelines

      To submit a comment for a journal article, please use the space above and note the following:

      • We will review submitted comments as soon as possible, striving for within two business days.
      • This forum is intended for constructive dialogue. Comments that are commercial or promotional in nature, pertain to specific medical cases, are not relevant to the article for which they have been submitted, or are otherwise inappropriate will not be posted.
      • We require that commenters identify themselves with names and affiliations.
      • Comments must be in compliance with our Terms & Conditions.
      • Comments are not peer-reviewed.