SU-D-304-03: Small Field Proton Dosimetry Using MicroDiamond and Gafchromic Film

Abstract

Purpose: Certain dosimetric characteristics continue to make proton beam therapy an appealing modality for cancer treatment. The proton Bragg peak allows for conformal radiation dose delivery to the target while reducing dose to normal tissue and organs. As field sizes become very small the benefit of the Bragg peak is diminished due to loss of transverse equilibrium along the central beam axis. Furthermore, aperture scattering contributes additional dose along the central axis. These factors warrant the need for accurate small field dosimetry. In this study small field dosimetry was performed using two different methods. Methods: Small field dosimetry measurements were performed using a PTW microdiamond detector and Gafchromic EBT2 film for aperture sizes ranging from 0.5cm to 10cm and a proton range in water of 10cm to 27cm. The measurements were analyzed and then compared to each other and to reference dosimetry data acquired with a Markus chamber. Results: A decrease in normalized output is observed at small field sizes and at larger ranges in water using both measurement methods. Also, a large variation is observed between the output measurements by microdiamond and film at very small field sizes. At the smallest aperture, normalized output ranged from 0.16 to 0.72 and the percent difference between both measurement methods ranged from 36% to 70% depending on proton range. At field sizes above 5cm the film and microdiamond agree within 3%. Conclusion: Although both measurement methods exhibit a general decrease in output factor at small field sizes, dosimetric measurements for small fields using these two methods can vary significantly. Dosimetry under standard conditions is not sufficient to correctly model the dose distributions and outputs factors for small field sizes, additional small field measurements should be performed.

Comprehensive investigation of lateral dose profile and output factor measurements in small proton fields from different delivery techniques

“…Perturbations in output factor measurements of collimated fields have already been described in the past. Andersen et al 2 measured dose output in proton beams collimated with apertures between 5 and 100 mm using radiochromic film and a microDiamond detector, and found differences between both detectors of up to 70%. 2 Hoehr et al 12 identified differences in output factor measurements for collimated 74 MeV scattered proton fields down to 5 mm diameter.…”

“…Andersen et al 2 measured dose output in proton beams collimated with apertures between 5 and 100 mm using radiochromic film and a microDiamond detector, and found differences between both detectors of up to 70%. 2 Hoehr et al 12 identified differences in output factor measurements for collimated 74 MeV scattered proton fields down to 5 mm diameter. Their investigation included a scintillation detector, a diamond detector oriented perpendicular to the beam direction and a silicon diode detector, of which the latter had the largest crosssectional area of 2.65 mm × 2.65 mm and showed a relative signal difference of 2% for the smallest field size, which the authors attributed to larger volume averaging.…”

“…Dosimetry of small or steep radiation fields can be challenging and recent works have emphasized the need for higher spatial accuracy of detectors in proton therapy 1 as well as investigations of small proton field dosimetry. [2][3][4] While the dosimetry of small photon fields has been studied intensively in the past and international guidelines were published as a report by the AAPM Task Group 155 5 and as TRS 483 by the IAEA, 6 there are no national or international standards for dosimetry of small proton fields which may require a separate treatment due to the different nature of dose deposition processes of protons from that of photons.…”

“…Nevertheless, the commissioning and quality assurance tasks involved are similar to photon beams including measurements of lateral dose profiles and output or field size factors, that is, the change of the dose per monitor unit as a function of field size. [7][8][9] Such measurements are usually acquired using point detectors with extended sensitive volume that may be subject to perturbations 1,2,[10][11][12] related to the so-called volume effect. 13,14 Although several studies found that the detector's volume effect does not noteworthy perturb measurements of proton fields with Gaussian shaped profiles or large fields with broad lateral penumbra as, for example, occurring in pencil beam scanning (PBS) fields, 13,[15][16][17] volume effect perturbations have been identified in some studies and especially for proton beam delivery techniques leading to reduced penumbras.…”

“…13,14 Although several studies found that the detector's volume effect does not noteworthy perturb measurements of proton fields with Gaussian shaped profiles or large fields with broad lateral penumbra as, for example, occurring in pencil beam scanning (PBS) fields, 13,[15][16][17] volume effect perturbations have been identified in some studies and especially for proton beam delivery techniques leading to reduced penumbras. 1,2,10,12 While small proton fields are used in ocular treatments, treatments of pediatric patients, minibeam irradiations or stereotactic treatments, 4,10,[18][19][20] the smallest possible lateral penumbra is generally desired as this allows for better sparing of adjacent organs at risk. [20][21][22][23] As a consequence, several studies presented novel proton irradiation techniques utilizing small proton fields with reduced lateral penumbras.…”