Treatment_Planning_and_Dose_Calculation for_External_Beam_Radiation_Therapy

Question 1

What is the main aim of external beam radiation therapy?

Answer 1

To apply the dose as conformal as possible to the tumor while delivering a homogeneous dose within the target volume and sparing healthy tissue.

Question 2

What are the key target volumes defined in external beam radiation therapy?

Answer 2

Gross Tumor Volume (GTV), Clinical Target Volume (CTV), Internal Target Volume (ITV), and Planning Target Volume (PTV).

Question 3

What does GTV stand for and what does it represent?

Answer 3

GTV stands for Gross Tumor Volume and represents the visible region of the tumor on imaging studies.

Question 4

How is the Clinical Target Volume (CTV) defined?

Answer 4

CTV is the volume that includes GTV and any microscopic spread of tumor cells that need to be eradicated by ionizing radiation.

Question 5

What is the purpose of the Planning Target Volume (PTV)?

Answer 5

PTV accounts for variations in patient positioning and organ motion to ensure the prescribed dose covers the entire target area.

Question 6

Why is dose prescription important in radiation therapy?

Answer 6

It defines the total dose to the PTV, fractionation scheme, and dose limits for organs-at-risk.

Question 7

How are organs-at-risk (OARs) categorized in radiation therapy?

Answer 7

OARs are categorized as either serial organs, where the maximum dose is critical, or parallel organs, where the mean dose is more important.

Question 8

What are the ICRU 50 requirements for dose homogeneity within the PTV?

Answer 8

The minimal dose within the PTV must be greater than 95% of the prescribed dose, and the maximum dose must be lower than 107%.

Question 9

What is adaptive radiotherapy?

Answer 9

A technique where treatment is modified in real time based on changes in the patient’s anatomy or tumor during the course of treatment.

Question 10

Why is it important to consider patient positioning in radiation therapy?

Answer 10

Accurate patient positioning ensures consistent treatment delivery and reduces the risk of radiation exposure to healthy tissues.

Question 11

What is the role of CT in modern radiation therapy?

Answer 11

CT provides 3D anatomical information and material properties, which are crucial for accurate treatment planning.

Question 12

What is the relationship between Hounsfield units and electron density?

Answer 12

Hounsfield units are converted into relative electron density, which is used in dose calculations for treatment planning.

Question 13

What factors influence the shape of the depth dose curve in radiation therapy?

Answer 13

Attenuation, inverse square law, and scattering.

Question 14

What is the significance of the Tissue-Air-Ratio (TAR) in radiation therapy?

Answer 14

TAR is used to calculate the dose to the tumor in isocentric irradiations and is independent of the source surface distance (SSD).

Question 15

How is the Peak Scatter Factor (PSF) related to field size in radiation therapy?

Answer 15

PSF increases with increasing field size due to increased scattering.

Question 16

What does TMR stand for and how is it used?

Answer 16

Tissue-Maximum-Ratio (TMR) is used to calculate the dose at a given depth in the patient and is derived from TAR.

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Question 17

What is the Off-Axis-Ratio (OAR) in radiation therapy?

Answer 17

OAR is the ratio of the dose at a point off the central axis to the dose at the central axis, used to study lateral dose distribution.

Question 18

What is the impact of inhomogeneities on dose distribution in medical physics?

Answer 18

Inhomogeneities affect both the primary and scatter contribution of the radiation, causing deviations in the dose distribution from the calculated distribution in a homogeneous water medium.

Question 19

What is the purpose of inhomogeneity correction factors in dose calculation?

Answer 19

Inhomogeneity correction factors are used to adjust the calculated dose in a water box to account for the effects of patient tissues that differ in density from water.

Question 20

Name some algorithms used for inhomogeneity corrections in dose calculations.

Answer 20

Algorithms include correction according to effective pathlength, Power-Law, Batho, Modified Batho, Equivalent tissue air ratio (ETAR), Differential Scatter Air Ratio, and Delta Volume.

Question 21

Why is electronic disequilibrium a concern in high energy photon beams?

Answer 21

For high energy photon beams, the range of scattered electrons can be several centimeters, leading to electronic disequilibrium, which causes inaccuracies in dose calculations when using simplified correction-based algorithms.

Question 22

How does the effective SSD approach correct depth dose curves?

Answer 22

The effective SSD approach corrects the depth dose curve by applying the inverse square law and assuming that the curve does not change significantly for small variations in SSD.

Question 23

What is the equivalent tissue-air ratio (ETAR) method?

Answer 23

The ETAR method uses TARs, an effective depth, and an effective beam radius to model differences in scattered radiation when correcting for inhomogeneities in dose calculations.

Question 24

How is dose calculation performed for irregular fields using the Clarkson method?

Answer 24

The Clarkson method divides the irregular field into circular sectors and calculates the dose by combining the primary photon contribution and scatter contribution using TARs.

Question 25

What is TERMA in the context of dose calculation?

Answer 25

TERMA (Total Energy Released Per Unit Mass) represents the total energy released per unit mass due to photon interactions, considering both energy transferred to secondary charged particles and energy taken by scattered photons.

Question 26

How are energy deposition kernels used in dose calculations?

Answer 26

Energy deposition kernels describe the energy distribution deposited by secondary particles after photon interactions. They are computed using Monte Carlo simulations or derived from measurements.

Question 27

What is the role of the superposition/convolution method in dose calculations?

Answer 27

The superposition/convolution method calculates dose by convolving the TERMA with energy deposition kernels, taking into account the inhomogeneities by scaling the kernels according to electron density.

Question 28

How does the Collapsed Cone Convolution algorithm improve dose calculation efficiency?

Answer 28

The Collapsed Cone Convolution algorithm integrates the point kernel along the beam direction, reducing the computation to a 2D convolution, which speeds up the calculation while maintaining reasonable accuracy.

Question 29

What are the limitations of pencil beam algorithms in dose calculation?

Answer 29

Pencil beam algorithms are faster but less accurate, as they integrate the point kernel along the beam direction and lose information about lateral scattering, leading to potential inaccuracies in dose distribution.