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1、Ch3.Depth dose distribution,深度剂量分布,Introduction,Why we should know depth dose distribution?What will affect the depth dose distribution?How can we describe the depth dose distribution?How can we calculate depth dose distribution?,Section1 Basic concepts in physics of radiation therapy,放射源(source,S)：
2、没有特别说明的情况下一般规定为放射源前表面的中心，或产生辐射的靶面中心。射野中心轴(central axe)：射线束的中心对称轴线。临床上一般用放射源穿过照射野中心的连线作为射野中心轴。,Section1 Basic concepts in physics of radiation therapy,照射野(field size)：射线束经准直器后垂直通过模体的范围，用模体表面的截面大小表示照射野的面积(几何射野）。临床剂量学中规定模体内50%同等剂量曲线的延长线交于模体表面的区域定义为照射野的大小。,Section1 Basic concepts in physics of radiation
3、 therapy,参考点(reference point)：规定模体表面下射野中心轴上某一点作为剂量计算或测量参考的点，表面到参考点的深度记为d0。400KV以下X射线，参考点取在模体表面（d0=0），对高能X射线或射线，参考点取在模体表面下最大剂量点位置（d0=dm），该位置随能量变化并由能量确定。源皮距（source-surface distance,SSD）：放射源到模体表面照射野中心的距离。,Section1 Basic concepts in physics of radiation therapy,源瘤距（source-tumor distance,STD）：放射源沿射野中心轴到肿
4、瘤内所考虑点的距离。源轴距（source-axis distance,SAD）：放射源到机架旋转轴或机器等中心点的距离。,Section1 Basic concepts in physics of radiation therapy,Section2.Percentage Depth Dose and its Distribution,1.The concept of PDD1.1 the definition:The quantity percentage depth dose may be defined as the quotient,expressed as a percentage,
5、of the absorbed dose at any depth d to the absorbed dose at a fixed reference depth d0,along the central axis of the beam.,Section2.Percentage Depth Dose Distribution,Section2.Percentage Depth Dose Distribution,1.2 The property:For orthovoltage(up to 400KVp)and lower-energy x-rays,the reference dept
6、h is usually at the surface(d0=0).For higher energies,the reference depth is taken at the position of the peak absorbed dose(d0=dm).In clinical practice,the peak absorbed dose on the central axis is sometimes called the maximum dose,the dose maximum,or simply the Dmax.,Section2.Percentage Depth Dose
7、 Distribution,Percentage depth dose curves in water for a 1010 cm2 field at an SSD of 100 cm for various megavoltage photon beams ranging from cobalt-60 gamma rays to 25 MV x rays.,最大剂量点深度dm随射线能量增加而增加，对于Co-60 射线，最大剂量点深度在5mm处，对于 4 MV X 射线，最大剂量点深度在1cm处，对于 8 MV X 射线，最大剂量点深度在2cm处，,Section2.Percentage De
8、pth Dose Distribution,对于半价层在12mmCu的低能X射线，当射野很大时，最大剂量点略在表面下，此时参考点仍在表面，故最大吸收剂量点处的PDD大于100%。如HVL=2.0mmCuX射线，SSD=50cm，射野400cm2，在0cm、1cm、2cm深度处，PDD分别为100%、102.4%、99%。这主要是由于大照射野造成的过量散射。但是能量小于400KV X射线，参考点仍放在表面上。,Section2.Percentage Depth Dose Distribution,1.3 The influencing factors A.Dependence on beam q
9、uality and depth B.Effect of field size and shapeC.Dependence on source-surface distance,Section2.Percentage Depth Dose Distribution,Section2.Percentage Depth Dose Distribution,A.Dependence on beam quality and depthThe PDD(beyond dm)increases with the beam energy.Higher-energy beams have greater pen
10、etrating power and thus deliver a higher PDD.If the effects of inverse square law and scattering are not considered,the PDD variation with depth is governed approximately by exponential attenuation.,Section2.Percentage Depth Dose Distribution,A.Dependence on beam quality and depth,Section2.Percentag
11、e Depth Dose Distribution,A.1 Initial dose buildupThere is an initial buildup of dose which becomes more and more pronounced as the energy is increased.In the case of the orthovoltage or lower-energy x-rays,the dose builds up to a maximum on or very close to the surface.But for higher-energy beams,t
12、he point of maximum dose lies deeper into the tissue or phantom.The region between the surface and the point of maximum dose is called the dose build-up region.,Section2.Percentage Depth Dose Distribution,A.1.1The physics of dose build-up may be explained as follow:(a).As the higher-energy photon be
13、am enters the patient or the phantom,high-speed electrons are ejected from the surface and the subsequent layers.(b).These electrons deposit their energy a significant distance away from their site of origin.(c).Because of a and b,the electron fluence and hence the absorbed dose increase with depth
14、until they reach a maximum.However,the photon energy fluence continuously decreases with depth and,as a result,the production of electrons also decreases with depth.The net effect is that beyond a certain depth the dose eventually begins to decrease with depth.,Section2.Percentage Depth Dose Distrib
15、ution,B.Effect of field size and shapeFor a sufficiently small field one may assume that the depth dose at a point is effectively the result of the primary radiation,i.e.,the photons which have traversed the overlying medium without interacting.The contribution of scattered photon to depth dose in t
16、his case is negligibly small or 0.But as the field size is increased,the contribution of the scattered radiation to the absorbed dose increases.,Section2.Percentage Depth Dose Distribution,B.Effect of field size and shapeSince this increase in scattered dose is greater at larger depths than at the d
17、epth of Dmax,the PDD increases with increasing field size.The increase in PDD caused by increase in field size depends on beam quality.Since the scattering probability or cross-section decreases with energy increase and the higher-energy photons are scattered more predominately in the forward direct
18、ion,the field size dependence of PDD is less pronounced for the higher-energy than for the lower-energy beams.,Section2.Percentage Depth Dose Distribution,Section2.Percentage Depth Dose Distribution,B.Effect of field size and shapePDD data for radiotherapy beams are usually tabulated for square fiel
19、ds.Since the majority of the treatments encountered in clinical practice require rectangular and irregularly shaped(blocked)fields,a system of equating square fields to different field shapes is required.,Section2.Percentage Depth Dose Distribution,Section2.Percentage Depth Dose Distribution,B.1.1 e
20、quivalent field射野等效的物理意义：如果使用的矩形或不规则形射野在其射野中心轴上的PDD与某一方形野的相同时，该方形野叫做所使用的矩形或不规则形射野的等效射野。临床上经常使用面积/周长比法。如果使用的矩形野和某一方形野的面积/周长比值相同，则认为这两种射野等效，即射野中心轴上PDD相同。面积/周长比法虽没有很好的物理基础，只不过是一经验公式，但在临床上得到广泛的应用。,Section2.Percentage Depth Dose Distribution,Section2.Percentage Depth Dose Distribution,Section2.Percentage
21、 Depth Dose Distribution,C.Dependence on source-surface distanceThe variation of PDD is governed by three effects:Inverse square lawExponential attenuation(primary radiation)Scattering(scattered radiation),Section2.Percentage Depth Dose Distribution,Section2.Percentage Depth Dose Distribution,照射野大小不
22、变，深度相等，忽略由于散射锥度改变引起的散射份额变化,Section2.Percentage Depth Dose Distribution,C.Dependence on source-surface distance当f2f1时，F1，即延长SSD，PDD增加当f2f1时，F1，即缩短SSD，PDD减少PDD随着SSD的增加而增加对低能X射线和大野，一般用（F+1）/2因子代替F，可近似完成不同源皮距的PDD的转换PDD随SSD增加的程度始终小于F，因为较短SSD深度d处的射野比较长SSD深度d处的射野要大，散射条件不同。,Section2.Percentage Depth Dose Distribution,Example:设Co-60 射线，SSD=80cm，15cm15cm射野，其百分深度剂量PDD=58.4。求当源皮距拉长到100cm，相同射野和深度时的百分深度剂量？,Section2.Percentage Depth Dose Distribution,
