In vivo and phantom measurements of the secondary photon and neutron doses for prostate patients undergoing IMRT

Abstract

For intensity modulated radiation therapy (IMRT) treatments 6 MV photons are typically used, however, for deep seated tumors in the pelvic region, higher photon energies are increasingly being employed. IMRT treatments require more monitor units (MU) to deliver the same dose as conformal treatments, causing increased secondary radiation to tissues outside the treated area from leakage and scatter, as well as a possible increase in the neutron dose from photon interactions in the machine head. Here we provide in vivo patient and phantom measurements of the secondary out-of-field photon radiation and the neutron dose equivalent for 18 MV IMRT treatments. The patients were treated for prostate cancer with 18 MV IMRT at institutions using different therapy machines and treatment planning systems. Phantom exposures at the different facilities were used to compare the secondary photon and neutron dose equivalent between typical IMRT delivered treatment plans with a six field three-dimensional conformal radiotherapy (3DCRT) plan. For the in vivo measurements LiF thermoluminescent detectors (TLDs) and Al2O3 detectors using optically stimulated radiation were used to obtain the photon dose and CR-39 track etch detectors were used to obtain the neutron dose equivalent. For the phantom measurements a Bonner sphere (25.4 cm diameter) containing two types of TLDs (TLD-600 and TLD-700) having different thermal neutron sensitivities were used to obtain the out-of-field neutron dose equivalent. Our results showed that for patients treated with 18 MV IMRT the photon dose equivalent is greater than the neutron dose equivalent measured outside the treatment field and the neutron dose equivalent normalized to the prescription dose varied from 2 to 6 mSv/Gy among the therapy machines. The Bonner sphere results showed that the ratio of neutron equivalent doses for the 18 MV IMRT and 3DCRT prostate treatments scaled as the ratio of delivered MUs. We also observed differences in the measured neutron dose equivalent among the three therapy machines for both the in vivo and phantom exposures.

Neutron spectra and dose equivalents calculated in tissue for high‐energy radiation therapy

“…9 Generally, in an evaluation of the total dose equivalent, these secondary gammas are accounted for through photon measurements carried out using a separate, photon-specific dosimeter. 5,7,8 Because a photon-specific dosimeter does not distinguish between the secondary gammas arising from neutron capture and photons due to head leakage or patient scatter, all photon doses are generally bundled together. For such circumstances, it would be incorrect to bias the quality factor with photons that were already accounted for as part of the photon measurements.…”

“…These neutrons pose a health risk for any patient undergoing high-energy radiation therapy because they increase the out-of-field radiation dose and, thus, the corresponding risk of secondary malignancies. [5][6][7][8][9] Studies have measured or used Monte Carlo to calculate neutron fluence and energy spectrum, but the majority of these studies have evaluated such neutron properties in air or on the patient's surface rather than within a phantom or patient where organs at risk are actually found. [5][6][7][10][11][12][13] This observation is not surprising because measurements within a patient or even a phantom are very difficult due to substantial challenges in neutron detection.…”

The use of LiF (TLD‐100) as an out‐of‐field dosimeter

“…In contrast, the out‐of‐field neutron dose at that energy has previously been determined in a phantom to be in the range of 0.1 – 1 μGy/MU (assuming a radiation weighting factor of 10 for the dose equivalents determined in the literature). ( 1 , 8 , 9 ) The TLD‐100 response to neutrons is not known for the system used at the RPC, nor is there any simple technique for separating the photon and neutron contributions when reading the TLD. This issue is further complicated because most of the neutron dose equivalent is delivered by fast neutrons, but most of the TLD signal originates from thermal neutrons.…”

A review of the impact of photon and proton external beam radiotherapy treatment modalities on the dose distribution in field and out-of-field; implications for the long-term morbidity of cancer survivors

“…A comparison of published data [1,14,18,27,28] on peripheral dose as a function of the distance from an 18 MV photon field is shown in Figure 7. Data for conventional radiotherapy and IMRT are presented.…”