Use of peripheral dose data from uniform dynamic multileaf collimation fields to estimate out-of-field organ dose in patients treated employing sliding window intensity-modulated radiotherapy

Abstract

Peripheral doses (PD) from uniform dynamic multileaf collimation (DMLC) fields were measured for 6 MV x-rays on a Varian linear accelerator using a 0.6 cc ionization chamber inserted at 5 cm depth into a 35 x 35 x 105 cm3 plastic water phantom. PD measurements were also carried out under identical conditions for seven patients treated for head and neck and cervical cancer employing sliding window intensity-modulated radiotherapy (IMRT). The measured PD from these patient-specific intensity-modulated beams (IMBs) were compared with the corresponding data from uniform DMLC fields having similar jaws setting. The measured PD per monitor unit (PD/MU) decreases almost exponentially with out-of-field distance for all uniform DMLC and static fields. For the same strip field width of 1.2 cm, uniform DMLC fields with a larger size of 14 x 22 cm2 deliver an average of 3.51 (SD = 0.51) times higher PD/MU at all out-of-field distances compared to 6 x 6 cm2. Similar to uniform DMLC fields, PD/MU measured from different patient-specific IMBs was found to decrease almost exponentially with out-of-field distance and increase with increase in field dimension. PD per MU from uniform DMLC fields and patient-specific IMBs having similar jaws setting shows good agreement (+/-7%) except at the most proximal distance, where a variation of more than 10% (maximum 15%) was observed. Our study shows that PD data generated from uniform DMLC fields can be used as baseline data to estimate out-of-field critical organ or whole-body dose in patients treated employing sliding window IMRT if an appropriate correction factor for field dimension is applied. The whole-body dose information can be used to estimate the possible increase in risk of fatal secondary malignancy in patients treated employing sliding window IMRT.

Evaluation of the peripheral dose in stereotactic radiotherapy and radiosurgery treatmentsa)

“…1, [8][9][10][11][12][13][14][15] Pre-estimation of peripheral dose during a given treatment is difficult because it depends on myriad factors and variables.…”

Experimental Determination of Peripheral Doses for Different IMRT Techniques Delivered by a Siemens Linear Accelerator

“…Existing literature on peripheral doses (PD) is, however, limited and the number of IMRT publications dealing with radiation safety aspects [5,6,10,21,22] is not comparable to other IMRT-related topics such as theoretical treatment-planning exercises that demonstrate the potential clinical benefit of IMRT [18,28].…”

“…Die Feldgröße des offenen Feldes und die maximal mit dem Primärstrahl bestrahlte Fläche bei den Introduction Secondary doses from radiation therapy have been investigated for many decades [24,25], but the interest in this field of research has increased in the light of intensity-modulated radiotherapy (IMRT). Existing literature on peripheral doses (PD) is, however, limited and the number of IMRT publications dealing with radiation safety aspects [5,6,10,21,22] is not comparable to other IMRT-related topics such as theoretical treatment-planning exercises that demonstrate the potential clinical benefit of IMRT [18,28].Measurements and calculations of out-of-field doses in IMRT are difficult to perform for several reasons. There is a vast number of IMRT delivery techniques [3], ranging from compensators [17,19] to multileaf collimator-(MLC-)based techniques (segmental and dynamic MLC delivery) and from serial to spiral tomotherapy [1,4].…”

Peripheral dose measurements for 6 and photon beams on a linear accelerator with multileaf collimator

“…The scattered dose beyond the field edge from radiation fields shaped using MLC has also been measured. [26][27][28][29][30][31][32][33][34][35] Five of the above studies presented data from intensity modulated radiation therapy. [26][27][28]33,34 Out-of-field dose measurements have been performed on linear accelerators equipped with a MLC system added below the upper and lower collimator jaws.…”

“…[26][27][28][29][30][31][32][33][34][35] Five of the above studies presented data from intensity modulated radiation therapy. [26][27][28]33,34 Out-of-field dose measurements have been performed on linear accelerators equipped with a MLC system added below the upper and lower collimator jaws. 29,30,32,35 The introduction of MLCs significantly reduced PD compared to that measured with the MLC leaves fully retracted due to the decrease in scatter from primary and secondary collimator and head leakage.…”